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A Specialist Periodical Report

Photochemistry Volume 8

A Review of the Literature published between July 1975 and June 1976

Senior Reporter D. Bryce-Smith, Department of Chemistry, University of Reading Reporters M. D. Archer, The Royal lnsfifufion, London G. Beddard, The Royal lnsfifufion, London H. A. J. Carless, Birkbeck College, Universify of London A. Gilbert, University of Reading W. M. Horspool, University of Dundee J. M. Kelly, University of Dublin D. Phillips, Universify of Soufhampfon S. T. Reid, Universify ofKent K. Salisbury, University of Soufharnpfon M. A. West, The Royal Institution, London

The Chemical Society Burlington House, London, W i V oBN

ISBN :0 85186 075 3 ISS N :0556-3860 Library of Congress Catalog Card No. 73-17909

Copyright @ 1977 The Chemical Society A II Rights Reserved No part of this book may be reproduced or transmitted in any form or by any means - graphic, electronic including photocopying, recording, taping or information storage and retrieval systems - without written permission from The Chemical Society

Organic formulae composed by Wright's Symbolset method

PRINTED IN GREAT BRITAIN BY JOHN WRIGHT AND SONS LTD., AT THE STONEBRIDGE PRESS, BRISTOL BS4 5NU

Introduction and Review of the Year

Volume 8 of ‘Photochemistry’ undergoes a slight shift towards the biological end of the spectrum with the inclusion for the first time of a section dealing with chemical aspects of photobiology (Part VI). This has been contributed by Dr. Godfrey Beddard, whom we are pleased to welcome to the team of Reporters. Although photobiology has not been totally neglected in previous Volumes, we felt that its growing chemical interest justified the emphasis of separate treatment. We are also glad to welcome back Dr. M. A. West who provides a two-year review of developments in instrumentation and techniques. We may for an experimental period continue to review these aspects on a biennial basis. On the other hand, the spectroscopic and theoretical aspects, which have hitherto been covered on an annual basis, are not included this year, and it is intended that they will form the subject of a biennial review in Volume 9. It is not only photochemists who will have felt that one of the most important reports to appear in the chemical literature during the year has been the preliminary announcement by Whitten and his co-workers of the efficient photoproduction of molecular hydrogen and oxygen from water using near-u.v. and visible light in the presence of monolayer-bound ruthenium(1x) bipyridyl complexes. Although the reported findings appear to be unquestionably authentic, it is necessary to add a cautionary note, for Professor Whitten has informed the writer that he and his colleagues have been unable to reproduce the original results using more highly purified reagents. Some unrecognized impurity may have been involved, so it is greatly to be hoped that further work will lead to the identification of this, and indeed to the development of improved systems which are capable of functioning in homogeneous solution. A successful development along these lines would be of the greatest importance in relation to the ‘energy crisis’. In this connection, it is interesting that [Ru(bipy),12+has been shown to act as a photocatalyst for the air oxidation of Fe2+ to Fe3+, possibly via an 0,-complex (Winterle, Kliger, and Hammond). Schrauzer and Guth have also reported that U.V. irradiation promotes the evolution of hydrogen from ferrous hydroxide gels, apparently via disproportionation to iron metal (see also reports by Crosby, Juris, and their co-workers). Continuing with developments in inorganic photochemistry, we may note that several reports have stressed the importance of electron-transfer as a general mechanism for quenching the excited states of co-ordination compounds. Nonradiative processes usually occur very rapidly in such excited states, and picosecond laser flash photolysis is proving a very useful technique for investigations in this area (Kirk et al.). On the theoretical side, Burdett’s molecular orbital procedure for calculating the course of non-dissociative photoisomerizations of transition-metal compounds appears to have predictive value.

iv

Introductory Review

There is growing evidence that photoaddition reactions of metal-co-ordinated dienes and mono-enes do not generally follow a concerted pathway, so application of orbital symmetry arguments may be less generally justified than has hitherto been believed. Attention is drawn to an important paper by Endicott and Ferraudi on photosubstitution reactions in Co"' and Rh"' complexes which occur via ligand-field excitation. The mechanism of quenching by ferrocene now appears clearer following independent studies by Herkstroeter, and Farmilo and Wilkinson. The production of Si=C compounds has long been a goal in organosilicon chemistry, so it is interesting that Boudjouk and co-workers have obtained evidence for the transitory formation of Me2Si=CH2 by the vacuum U.V. photolysis of Me,SiCMe,. Unstable Si=C intermediates may also be involved in the novel C( 1)C(4)/C(9)C(10) intramolecular dimerization of organosilicon-substituted anthracene rings reported by Felix and co-workers. Parker and Sommer have generated the highly reactive silaimines R,Si=NR by photolysis of the corresponding azides R3SiN3,and have trapped these as t-butanol adducts. Irradiation of ozone co-deposited with sulphur trioxide has led to the first spectroscopic characterization of the species SO4 (Kugel and Taube). On the physical side of the subject, an increasing trend may be noted towards studies of small molecules and atoms of interest in the atmospheric photochemistry of Earth and other planets following developments noted in previous Volumes. Among medium-small molecules, benzene and glyoxal continue to command an enthusiastic following. Formosinho and Dias da Silva have had considerable success in calculating the rate constants for Sl-+s,, and Sl-+ transitions in benzene and 2[H],benzene by application of an empirical tunnelling theory. This treatment indicates that the rate constant for the former process is more energy-dependent than that for the latter. Time- and wavelength-resolved emission spectra are proving very useful in studies of the relaxation of excited aromatic hydrocarbons and other species (D. Phillips and co-workers, among others), and a promising molecular beam technique has also been briefly reported (Sander et a2.): this latter appears to avoid problems from the complexity of emission spectra which result from the normally unavoidable excitation of hot U-ZI bands in the absorption step. See also Smally et al. for a related elegant study of 12. Concern continues to be expressed about possible effects of chlorofluorocarbons (Freons), which are widely used as aerosol propellants, on depletion of the ozone layer in the upper atmosphere. The rate constant for the key reaction C1* O3 C10. + O2has been measured independently by Razumovskii et al., and Kurylo and Braun. The results are in fair agreement, and indicate the possibility that previous estimates of ozone loss may have been too high. Likewise the half-lives of various chlorofluoromethanes in the troposphere have been calculated to be 1-2 years, or less, whereas periods of about 50 years have been estimated by some previous workers. There has been a continued upsurge of academic and other interest in isotope separation by use of high-power i.r. lasers selectively to excite low-lying vibrational transitions by multiphotonic absorption in isotopic mixtures of small molecules. Isotope separation via electronic excitation has also been described.

+

--f

Introductory Review

V

Carroll and Quina have described a new method for determination of intersystem-crossing quantum yields. In methylbenzenes, it is interesting to note that the values show some tendency to increase with the number of methyl groups and with increasing symmetry of the substitution pattern, The vacuum-u.v. irradiation of benzene in argon and nitrogen matrices can lead to thermoluminescence, i.e. phosphorescence after irradiation as the matrix warms up (Hellner and Vermeil). It is well known that the acidity of phenols in the S, state is much higher than in the Sostate, but a previous report of supposed enhancement of a reaction rate due to this phenomenon (nitrosation of 2-naphthol) has been severely questioned by Chandross. Exciplexes and ‘encounter complexes’ continue to attract a good deal of attention, and triple exciplexes (D . . . A . . . D) have been reported (Saltiel et d ; Mimura and Itoh). A study of pyrene and NN-diethylaniline has provided the first direct evidence for solvent-induced changes in the electronic structure of a polar exciplex. In highly polar solvents, the absorption spectrum becomes identical with that of the separated radical-ions (Orbach and Ottolenghi; see also Gupta and Basu, and Mataga et al.). Slifkin and Al-Chalabi have used flash photolysis of donor-acceptor complexes (e.g. perylene-chloranil) to obtain triplet-triplet spectra of the donors. The technique appears promising, but ambiguities may complicate interpretation of the results. The importance of the stereochemical configuration of ground-state and excited-state complexes in determining the stereochemistry of photoaddition reactions is being increasingly recognized. Hochstrasser and King have reported some interesting isotopically selective photochemistry in molecular crystals. Lahav et aZ. have described an ingenious new method for purification of enantiomers based on topochemical control of photodimerization of anthracene units, as with esters of 9-anthroic acid. The presence of cyanobenzenes (which promote charge-transfer processes) can profoundly modify the course of some photoreactions, e.g. dimerization of styrenes This interesting phenomenon merits wider study. Yang and (Asanuma et d,). his co-workers have observed fluorescence emission from a substituted dieneanthracene system which they attribute to an ‘encounter complex’, a species previously more often proposed than identified. The technique of using xenon to detect singlet and triplet processes by efficient enhancement of intersystem crossing rates is becoming more widely employed : see, for example, Morrison, Nylund, and Palensky. The section on Instrumentation and Techniques covers the period July 1974 to June 1976 inclusive. Although 724 references are cited, the review is to some extent restricted to what are considered to be the most significant advances. Some interesting developments in the rather neglected field of U.V. lasers has been stimulated by requirements for isotope separation, as noted above (and many new examples of this have been reported), and thermonuclear fusion. Hoffman, Hays, and Tisone have described devices based on electron-beam pumped halides such as XeBr and KrF: these offer promise as powerful tunable U.V. lasers. Since nitrogen-lasers (at 337 nm) are the most common types now used in photochemistry; it is interesting to note that addition of SF, to a nitrogenlaser can double the power output (Judd).

vi

Introductory Review

Applications of photoacoustic spectroscopy are multiplying rapidly, particularly for the measurement of absorption spectra of biological materials and atmospheric pollutants. Schwarz et al. have described fluorescence techniques whereby sulphur dioxide and nitric oxide can be rapidly and continuously monitored, and Tucker et al. have developed a laser-based procedure for nitrogen dioxide which is even more sensitive. Bradley and Sibbett have described a new type of streak camera which makes possible the achievement of sub-picosecond time resolution from the vacuum-u.v. to the near-i.r. The section on Chemical Aspects of Photobiology deals mostly with photochemical and photophysical aspects of primary processes in photosynthesis and vision. A new examination of highly purified chlorophyll a in EPA has shown that fluorescence previously attributed to a ‘hot’ band is due to an impurity, and that no dimer emission occurs at concentrations up to mol 1-1 (Mau). Among the developments concerning vision processes during the year, one may particularly note Salem and Bruckmann’s proposal of a mechanism whereby twisting about the 11-cis-ethylenic bond in the S 1 m *state of an N-retinylidene chromophore triggers an electrical signal which, inter alia, changes the permeability of the disc membrane to Na+. Downer and Englander have reported hydrogen-exchange studies which indicate that the action of light in vision processes promotes hydrogen exchange which in turn may cause conformational changes in a protein, and thereby distort a ‘plug’ in the cell membrane which opens a channel for passage of an as yet unidentified transmitter molecule. Another ingenious proposal by Warshel invokes a type of ‘bicycle pedal’ motion in photoisomerization of retinal when both ends of the molecule are restrained. In contemplating such developments in our groping attempts to understand photobiological processes, one is left with feelings of profoundest humility. These immensely subtle, complex, efficient, and robust ‘natural’ photosystems far transcend as constructions the human artefacts with which these annual Volumes are largely concerned. It is becoming increasingly difficult to deny that a better photochemist has gone before. We may now turn to some of the significant developments in more conventional photochemistry. The carbonyl group continues to exert its perennial fascination. Thus Yates and Tam have provided useful evidence on the structural features which promote the ring-enlargement of cyclic ketones to oxacarbene intermediates. Rather controversially, Wagner and Thomas have invoked fluorine hyperconjugation to account for the photochemical behaviour of certain fluoroketones. Hydrogen-abstractions by carbonyl and azomethine systems have been compared by Alexander and Jackson. These systems can show a similar degree of reactivity towards primary hydrogen atoms, but the carbonyl systems are much the more chemically reactive towards secondary hydrogen atoms. In a study of photoenolization, Wagner and Chen have obtained evidence for the formation of two triplet species derived from the syn- and anti-conformers of o-methylacetophenone: the former has the shorter lifetime. Some photoenolization occurs from the S1state of the syn-conformer. hv Deshayes et al. have described examples of the useful process ROAc RH in the steroid series: the yields are high.

-

Introductory Review

vii

Upper triplet states continue to be proposed in carbonyl photochemistry. Thus Bellobono et al. have invoked a T2state in the photocyclization of a furanone to a coumarin. The formation of cyclic enones having trans C=C has previously been suggested, so it is interesting that examples have now been isolated and trapped as furan adducts (Hart and Suzuki). In view of the frequent use of t-butanol as a solvent for photoreactions, it should be noted that Stille and his co-workers have observed this solvent to be photochemically reactive, giving products suggestive of the formation of free methyl radicals, possibly via fragmentation of intermediate t-butoxyl radicals : Wubbels et al. have actually obtained a t-butanol/anthraquinone photoadduct in the presence of ammonia. Barton and his co-workers have continued to develop and exploit photochemical procedures in the natural products field, most recently for synthesis of the antibiotic bikaverin, and in an improved synthesis of aldosterone. Maier, Hartan, and Sayrac have reported some interesting studies on the photoproduction of cyclobutadiene, and have observed the possible formation of a cyclobutadiene-CO, complex. The short-lived tetrafluorocyclobutadiene has been obtained (Gerace, Lemal, and Ertl). Courtot and his co-workers have reported an interesting series of studies in the field of hexatriene photochemistry: the conformation of the ground-state species has emerged as an important factor. Some new studies by Bender and Brooks on the photoisomerization of barrelenes to cyclo-octatetraenes appear to provide results in conflict with previous proposals. Interesting developments continue to be reported in the field of aromatic photochemistry. Barltrop and Day have provided further applications of their procedure for analysing ring transposition reactions of aromatic molecules in terms of ring permutation patterns, and Chambers et al. have presented an interesting paper on the pyridazine-pyrazine conversion. The first examples of photochemical cine-substitution have been reported (Bryce-Smith, Gilbert, and Krestonosich). Two separate reports of the photochemical cleavage of a benzene ring have appeared (Saito, Takami, and Matsuura; Hasselmann and Laustrial). Katritzky and Wilde have shown that 3-oxido-1-phenylpyridinium undergoes photodimerization and photoisomerization by 2,s-bonding. The latter process is without precedent in pyridinium chemistry: the former is rare, but Nagano et nl. have provided a further example. Muszkat et al. have presented a notable analysis of mechanistic factors in stilbene photocyclizations. These and related reactions continue to find numerous useful applications in synthesis, e.g. of helicenes, apolignans, and berberine alkaloids. Anderson et al. have used a photochemical route to provide the first example of an isolated 1,Zdiazetidine : cyclization of a lH-2,3-benzodiazepine is involved, and the extrusion of N2 which might have been expected does not occur. There has been considerable interest in nitrile ylides during the year, for example the formation from azirines, and intra- and inter-molecular addition reactions to carbonyl and other groups. Barltrop, Day, and co-workers have reported that the photoisomerization of 2- to 3-cyanopyrroles appears to involve initial 2,s-bonding in the pyrrole ring, followed by ‘walk’ reactions of the NH group.

viii

Introductory Review

Scattered reports appear in the literature that various inorganic ions can act as photocatalysts of reactions involving organic species. An interesting example to appear during the year has been the observation that Cu2+salts strongly promote the photoisomerization of pyridine N-oxides to 2-formylpyrroles (Bellamy, Martz, and Streith). Hata has reported the use of eosin adsorbed on alumina as a photosensitizer for reactions of heterocyclic N-oxides using visible light. Sat0 et al. have described a remarkable new regioselective synthesis of mediumto-large ring azathiocyclols by irradiation of N-alkylphthalimides bearing -SR groups as remote substituents in the alkyl group, e.g. at C-5. Cyclization occurs on to a carbonyl group in the phthalimide ring via a-C to the S. The photoelimination of N2 from pyrazoles, triazolines, triazines, etc. continues to provide valuable procedures for synthesis of benzcyclopropenes, aziridines, azetes, and other strained systems. Likewise, the long known photoelimination of N2 from diazo-compounds to give carbenes continues to find new synthetic applications. The photolysis of 3-diazobenzofuranone provides an interesting new source of benzyne which involves two successive photochemical steps (Chapman et d).The first case of reversible photochromic valence isomerization between a diazo-compound and a diazirine has been described by Voight and Meier. Cadogan et al. have reported the first example of the addition of an a-oxocarbene to benzene. Oda et al. have generated the highly strained allenic 1,2,4,6-cyclo-octatetraeneby a chemical procedure involving the generation of an intermediate carbene, and have trapped it as an adduct with cyclopentadiene. Mykytka and Jones have used an intramolecular carbene-acetylene addition to generate the highly strained dibenzobicyclo[4,1 ,O]heptatriene (1) and have trapped this by cycloaddition of butadiene to the cyclopropene moiety. Few photochemists normally look for effects of light intensity on quantum yields, but the observation of such effects in the photoreduction of a cyclohexadienone by Schuster et nl. may stimulate investigations of this parameter in other photochemical systems, especially those involving free-radical intermediates.

H. Fischer and his co-workers have used a CIDNP procedure to show that enols having lifetimes of a few seconds at room temperature can be formed during the photoreduction of aliphatic alcohols and ketones, being derived from ketyl radicals. For example, acetone and isopropanol form the acetone enol MeC(OH)=CH, (together with pinacol, of course). At -70 "C, the lifetime of the enol is ca. 5000 s, and addition of this enol to acetone gives the oxetan (2) as the major product. Interest continues in the photodegradation of polychloroaromatics, doubtless under the stimulus of environmental concern, and several interesting studies have appeared, e.g. photoreduction by borohydride and by methanol. It is interesting that some of these reductions appear to be promoted by the presence of triethylamine (see Part 111, Chapter 5 ) .

Introductory Review

ix

Although no really striking developments in polymer photochemistry as such have appeared during the year, Schaaf and his co-workers have developed several polymer-bound dye and porphyrin sensitizers for the photochemical generation of singlet oxygen: these appear to have the advantage of functioning heterogeneously, and they show greater resistance to bleaching than do sensitizers not bound in this way. Boden has used ‘crowii ethers’ to render certain dyestuff salts for singlet oxygen generation soluble in organic solvents. Potassium perchromate (K,Cr08) is no longer recommended as a ‘clean’ source of singlet oxygen: according to Pitts et al., <60/, of singlet oxygen seems actually to be formed. The question whether hydroperoxidation of alkenes by singlet oxygen occurs by an essentially concerted ene-reaction or via an intermediate perepoxide continues to lack a definitive answer. Opinion in recent years has been tending to favour the former alternative, but two theoretical studies have now given increased support to the latter (Inagaki and Fukui; Dewar and Thiel). It begins to look as though either mechanism may operate, depending on the nature of the alkene. Naphthalene 1,4-endo-peroxide cannot be formed directly from naphthalene and singlet oxygen, but has now been prepared indirectly by a twostage route involving dye-sensitized oxidation of a [lOIannulene. It proves to be fairly stable, but on warming gives naphthalene and singlet oxygen (Vogel and co-workers). The common use of 1,3-diphenylisobenzofuran (-> o-dibenzoylbenzene) as a diagnostic trap for singlet oxygen may not after all be wholly specific for this reagent, according to Howard and Mendenhall. The reaction may occur to some extent also by free radical-initiated oxidation. It is noteworthy that peracids give the product quantitatively without involvement of singlet oxygen (Boyer et a!.). In the field of solar energy conversion, there has been increasing interest in the photoproduction of hydrogen from water. Unfortunately, as already noted, a question mark now hangs over the striking report of the photodissociation of water in the presence of a monolayer-bound ruthenium complex. Further developments here will be awaited with the greatest interest. There are, however, a number of systems from which the photoproduction of molecular hydrogen has been observed, and attention is drawn to the important review by Stein. The three main approaches are (a) the use of transition metal compounds to provide photo-redox systems, sometimes in conjunction with an n-type semiconductor (see Creutz and Sutin), (b) photoelectrolysis of water using Ti02 or strontium- or potassium-titanate electrodes, and ( c ) the use of photosynthetic organisms containing the enzymes nitrogenase and hydrogenase. Among a number of interesting developments during the year, one may particularly note Lin and Sutin’s photogalvanic cell based on a R ~ ( b p y ) ~ + / F e ~ + system, and the ingenious 3-electrode electrical storage battery described by Hodes and co-workers which is chargeable by sunlight with a reported efficiency of up to 90%. Some important improvements have been made in the technology of manufacturing silicon suitable for photocells : these may substantially reduce the present high manufacturing costs (Chalmers and co-workers). Soukup and Shah’s ‘high voltage vertical multijunction solar cells’ may also provide practical photovoltaic devices of markedly improved efficiency. Some very efficient solar cells

X

Introductory Review

based on cadmium sulphide heterojunctions with other semiconductors have been further described, notably those employing CdS/InP junctions (Shay et al., Boer). James and Moon, among others, have described some promising solar cells incorporating gallium arsenide. The field solar energy conversion is undoubtedly attracting increased academic and industrial attention as the urgency of the need for renewable energy sources becomes more widely appreciated. Important practical developments in the years ahead are very much to be hoped for and, I think, expected. February 1977 D. BRYCE-SMITH.

Contents

lnfroduction and Review of the Year By D. Bryce-Smith

iii

Part I Physical Aspects of Photochemistry Chapter 1 Developments in Instrumentation and Techniques By M. A. West

3

1 Introduction

3

2 Plasma Sources

4

3 Laser Sources CW Lasers Pulsed Gas Lasers Dye Lasers Laser Dyes Solid-state Lasers Frequency Conversion Laser Measurements

5 5

6 9 10 12

4 Monochromators and Light Filters

15

5 Absorption Spectrometry U .v.-Visi ble Spectrometry 1.r. Spectrometry Two-phot on Absorption Techniques Photoacoustic Spectroscopy C.D. and M.C.D.

17 17 20 21 22 22

6 Preparative Techniques

23

7 Light Detection and Measurement Photodiodes Photomultipliers Other Photodetectors Radiometry and Photometry Miscellaneous

25 25 27 29 29 32

13

14

xii

Contents 8 Fluorescence and Phosphorescence Spectrometry U.v.-Visible Fluorescence Spectrometry Signal Processing Fluorescence Techniques Other Luminescence Equipment and Techniques Applications of Fluorescence Phosphorescence Spectrometry Raman Spectroscopy

33 33 34 35 37 38 41 43

9 Transient Absorption Spectroscopy Conventional Flash Photolysis Nanosecond Flash Photolysis Subnanosecond Photophysical Techniques Probe Technique Opt ical-gate Technique Streak Camera Technique Miscellaneous Applications

44 44 46 49 49 50 52 53

10 Transient Emission Spectroscopy Instruments and Methods Applications

53 53 57

11 Signal Processing

58

Chapter 2 Photophysical Processes in Condensed Phases By K. Salisbury 1 Introduction

60

60

2 Excited Singlet State Processes Radiative and Non-radiative Processes Ionic and Radical Phenomena Excimers Singlet Quenching by Energy Transfer Exciplexes and Electron Donor-Acceptor Complexes Heavy Atom Quenching

60 60 80 83 84 85 91

3 The Triplet State Radiative and Non-radiative Processes Triplet Quenching and Triplet Energy Transfer

92 92 96

4 Two-photon Processes

99

5 Photo-oxidation

100

6 Chemiluminescence

101

...

Contents

XI11

7 Photochromism

102

8 Some Low-temperature and Crystal Studies

103

Chapter 3 Gas-phase Photoprocesses By D. Phillips

105

1 Introduction

105

2 Alkanes, Alkenes, and Alkynes

105

3 Aromatic Molecules

108

4 Carbonyl and Oxygen-containing Compounds Free Radical Reactions

115 123

5 Nitrogen-containing Compounds

126

6 Sulphur-containing Molecules

131

7 Halogen Atoms and Halogenated Compounds

132

8 Metal Atom Reactions Mercury Cadmium, Zinc, and Magnesium Alkali Metals and Alkaline Earths Miscellaneous

139 139 141 142 144

9 Miscellaneous

144

10 Laser Isotope Separation

146

11 Atmospheric Photochemistry Extraterrestrial Phenomena Thermospheric and Stratospheric Reactions Tropospheric Reactions and Pollutants Detection and Estimation of Atmospheric Pollutants and Constituents Rare Gases Atomic and Molecular Hydrogen Atomic and Molecular Oxygen and Ozone HO, Reactions Atomic and Molecular Nitrogen, NO, Reactions CO, Reactions SO2 and H,S Reactions

148 148 148 150

151 153 154 155 159 161 163 164

Contents

xiv

Part /I Photochemistry of Inorganic and Organometallic Compounds By J , M. Kelly 1 Photochemistry of Transition-metal Complexes

Titanium Vanadium Chromium Molybdenum Manganese Rhenium Iron Ruthenium and Osmium Cobalt Rhodium and Iridium Nickel Platinum Copper, Silver, and Gold Mercury Lanthanides Actinides

167 171 171 172 177 177 177 177 180 184 189 191 191 191 192 192 194

2 Transition-metal Organometallics and Low-oxidation-state Compounds Titanium, Zirconium, and Hafnium Vanadium, Niobium, and Tantalum Chromium, Molybdenum, and Tungsten Manganese and Rhenium Iron and Ruthenium Cobalt, Rhodium, and Iridium Platinum Copper Gold Mercury

196 196 198 198 203 206 213 218 218 218 219

3 Metalloporphyrins and Related Compounds Haems and Cytochromes

22 1 225

4 Water, Hydrogen Peroxide, and Anions

226

5 Main-group Elements Magnesium Boron Aluminium and Thallium Silicon, Germanium, and Tin Lead Phosphorus, Arsenic, Antimony, and Bismuth Sulphur Selenium and Tellurium Halogens

228 228 228 229 229 23 1 231 232 232 233

xv

Contents

Part /I/ Organic Aspects of Photochemistry Chapter 1 Photolysis of Carbonyl Compounds

237

By W . M.Horspool 1 Introduction

237

2 Norrish Type I Reactions

238

3 Norrish Type I1 Reactions

244

4 Rearrangement Reactions

253

5 Oxetan Formation

255

6 Fragmentation Reactions

256

Chapter 2 Enone Cycloadditions and Rearrangements: Photoreacti o ns of Cyclo hexad ienones and Quinones By W. M. Horspool

262

1 Cycloaddition Reactions Intramolecular Intermolecular Dimerization

262 262 266 27 1

2 Enone Rearrangements

273

3 Photoreactions of Thymines etc.

288

4 Photochemistry of Dienones Linearly Conjugated Dienones Cross-conjugated Dienones Miscellaneous Dienones

292 292 294 299

5 1,2-, 1,3-, and 1,4-Dienones

300

6 Quinones

310

Chapter 3 Photochemistry of Olefins, Acetylenes, and Related Compounds By W. M. Horspool 1 Reactions of Alkenes Addition Reactions Hydrogen Abstraction Reactions Halogeno-olefins Group Migration Reactions cis-trans-Isomerizat ion

314 314 314 315 317 318 32 1

xvi

Contents 2 Reactions involving Cyclopropane Rings

322

3 Isomerization of Dienes

335

4 Reactions of Trienes and Higher Polyenes

340

+

5 [2 21 Intramolecular Reactions

347

6 Dimerization and Intermolecular Cycloaddition Reactions

350

7 Reactions of Acetylenic Compounds

353

8 Miscellaneous Reactions

354

Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert

362

1 Introduction

362

2 Isomerization Reactions

362

3 Addition Reactions

367

4 Substitution Reactions

382

5 Intramolecular Cyclization Reactions

391

6 Dimerization Reactions

406

7 Lateral-nuclear Rearrangements

41 1

Chapter 5 Photo-reduction and -oxidation By H. A. J. Carless

413

1 Conversion of C=O into C-OH

413

2 Reduction of Nitrogen-containing Compounds

426

3 Miscellaneous Reductions

430

4 Singlet Oxygen

434

5 Oxidation of Aliphatic and Alicyclic Unsaturated Systems

436

6 Oxidation of Aromatic Compounds

443

7 Oxidation of Nitrogen-containing Compounds

447

8 Miscellaneous Oxidations

453

Contents

xvii

Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S. T. Reid

455

1 Nitrogen-containing Compounds Rearrangement Addition Miscellaneous Reactions

455 480 488

2 Sulphur-containing Compounds

492

3 Compounds containing other Heteroatoms

499

Chapter 7 Photoelimination By S. T. Reid

455

503

1 Photodecomposition of Azo-compounds

503

2 Elimination of Nitrogen from Diazo-compounds

509

3 Elimination of Nitrogen from Azides

515

4 Photodecomposition of other Compounds having N-N Bonds

521

5 Photoelimination of Carbon Dioxide

524

6 Fragmentation of Organosulphur Compounds

526

7 Miscellaneous Decomposition and Elimination Reactions

532

Part /I/ Polymer Photochemistry By D. Phillips 1 Introduction

541

2 Photopolymerization Photoinitiation of Addition Polymerization Photocondensation Polymerization and Photochemical Cross-linking Photograft ing

54 1 54 1

3 Optical Properties and Luminescence of Polymers

545

4 Photochemical Reactions in Polymers Photochemical Reactions in the Absence of 0, Photo-oxidation and Weathering PoIy(o1efins) (PE, PP) Poly(styrene) (PS)

549 549 55 1 55 1

544 545

551

xviii

Contents Poly(amides) and Poly(urethanes) Poly(viny1 chloride) (PVC) Elastomers Cellulose Wool Photodegradable Polymers U.V. Stabilization 5 Appendix: Review of Patent Literature Photopolymerizable Systems Table A1 : Prodegradants and U.V. Sensitizers Table A2: U.V. Absorbers and Stabilizers Table A 3 : Optical Brightening Agents

552 552 552 553 553 553 553 554 554 558 562 566

Part V Photochemical Aspects of Solar Energy Conversion By M. D.Archer 1 General Reviews

571

2 Photochemistry Valence Isomerizations Photochemical Decomposition of Water Electron Transfer Reactions

572 572 573 574

3 Photoelectrochemistry Photogalvanic Effects and Cells Semiconductor Electrodes Titanium Dioxide Electrodes Other Semiconducting Oxide Electrodes Cadmium Sulphide Electrodes Miscellaneous

575 575 577 578 579 580 582

4 Photochemistry in Vesicles, Micelles, and Artificial Membranes

582

5 Photosynthesis The Structure and Function of Photosynthetic Membranes Primary Photochemical Events in Photosynthesis Photosynthetic Hydrogen and Oxygen Evolution

583 583 585 585

6 Photovoltaic Cells Silicon Cadmium Sulphide Heterojunction Cells Gallium Arsenide Other Semiconductor Heterojunctions Shottky Barrier Solar Cells Theory Inorganic Materials Organic Materials

586 586 587 588 589 589 589 590 590

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Contents

Part V / Chemical Aspects of Photobiology By G.Beddard 1 Introduction

593

2 Photosynthesis Chlorophylls in viva and in vitro Photosystem I (PS I) Photosystem I1 (PS 11) Photosynthetic Bacteria Fast Fluorescence from the Light-harvesting Pigments

593 593 599 60 1 602

3 Vision

607 607 608

Retinals and Retinols Visual Pigments

Author I ndex

605

612

Part I PHYSICAL ASPECTS OF PHOTOCHEMISTRY

1 Developments in Instrumentation and Techniques BY M. A. WEST

1 Introduction Although the progressive trend is for more and more physics to enter into chemical applications, a state of affairs which has attracted comment by analytical chemists (Aiialyt. Chem., 1975,47, 2073), photochemists must surely welcome the application of lasers and electro-optic developments to aid their research. Fields such as absorption and emission spectroscopy, chemical kinetics, and more recently, preparative chemistry, have all benefited through higher spectral resolution, selectivity, sensitivity, etc. This two-year review (July 1974 to June 1976) discusses most of the obvious advances in instrumentation and techniques in photochemistry, photophysics, and related spectroscopy as well as referring to fringe and other developments which have potential for, or have yet to be applied to, studies on the interaction of light with matter. With such a wide subject content, it is not possible to be very critical of publications or to include all publications within the confined space of this chapter. Furthermore, although subjects have been arbitrarily separated into 10 sections, some areas could be equally well placed in several sections, for example, two-photon absorption in sections dealing with pulsed lasers, absorption, or even emission spectroscopy. Several key developments have taken place recently in a number of relatively new techniques. Photoacoustic spectroscopy, though discovered 95 years ago, has benefited considerably by recent research which shows its considerable potential for absorption spectrometry of solids and semi-solids. Preparative photochemistry using i.r. lasers is already proving itself as a powerful technique for isotopic separations and for producing specific products. The time resolution in transient absorption measurements has now been pushed back to femtoseconds, beyond which, chemistry, as we know it, does not exist because of the uncertainty principle. A list of recommended terms for spectroscopy was tabulated in a previous volume (Vol. 6, p. 62) and was reputably based on the S.I. system of units. Unfortunately, inconsistencies in these terms have been indicated by Mielenz,l who recommends use of more logical adjectives and nouns to describe quantities and terms which are based on the transport of energy according to the laws of geometrical optics. For example, by defining absorbance as the negative logarithm to base ten of internal transmittance, it should be clear that this refers to the transmittance of an absorbing material exclusive of losses at boundary surfaces K. D.Mielenz, Analyt. Chem., 1976,48, 1093.

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Photochemistry

and effects of interreflection between them. Any instrument used for the measurement of spectra should simply be called a spectrometer. The word spectrophotometer, though commonly used, is a misnomer since a photometer is an instrument that measures luminous flux. Since the adjective ‘luminous’ implies the integral effect of visual radiation as perceived by the human eye, the spectral analysis of luminous flux has no physical meaning. It is certainly more accurate and logical to use the term absorption spectrometer and in the same way the confusion over spectrofluorimeters and spectrofluorometers would be eliminated by the term fluorescence spectrometer. One suggestion unlikely to find acceptance by photochemists, however, is replacement of the firmly established quantum yield by radiant yield or photon yield.

2 Plasma Sources The low-pressure mercury lamp so commonly used for photochemistry has been and the intensity of the 253 nm line examined as a function studied recently of Hg pressure, tube radius, and operating current. The intensity rises to a peak at about 7 mTorr pressure and falls at higher Hg pressures and, at constant pressure, increases linearly with ~ u r r e n t . A ~ useful review emphasizing the chemical developments of inorganic phosphors discusses their applications in changing the output wavelength of an Hg lamp.* Instabilities in the output of an HPK mercury lamp have been overcome by operation from an optically stabilized supply resulting in a drift of 0.1%h-l over a 30 h period.6 The amount of obnoxious and hazardous ozone generated by xenon short arc lamps is reduced considerably by passing the normal cooling air through a baffled aluminium chamber containing iron oxide.* This ‘filter’ decomposes the ozone to oxygen with high efficiency, but only after a warm-up time of 3040 min. A comparison of Xe-Hg, D2arc, and H2 hollow-cathode lamps has been made in an evaluation of a suitable source for background correction in atomic absorption spectrometry.’ At shorter wavelengths, a new type of source generating the line radiation of the rare gas ions achieves an enhanced ion flux by incorporating a charged particle arrangement. Intense line spectra are obtained from the He, Ne, and Ar ions, affording a convenient windowless source of He(@ (30.4 and 25.6 nm) and Ne(I1) (46 nm) suitable for photoelectron spectroscopy.8 A microwave-discharge U.V. light source has been reported to yield significant photon fluxes at 26.9 and 40.81 eV.O Mention will be made in other sections of the use of light from a synchrotron, but it is worth noting here a collection of papers dealing with this intense plasma source and its applications.l* 2p

* lo

T. J. Hammond and C. F. Gallo, Appl. Optics, 1976, 15, 64. T. J. Hammond and C . F. Gallo, Appl. Optics, 1976, 15, 308. A. L. N. Stevens, J. Luminescence, 1976, 12/13, 97. R. E. Pulfrey, Appl. Optics, 1976, 15, 308. W. C . Neely, A. D. West, and T. D. Hall, J. Phys. ( E ) , 1975, 8, 543. M. S. Epstein and T. C . Rains, Analyt. Chem., 1976,43, 528. F. Burger and J. P. Maier, J . Phys. ( E ) , 1975, 8,420. T. V. Vorburger, B. J. Waclawski, and D. R. Sandstrom, Rev. Sci. Instr., 1976, 47, 501. ‘Collection of Papers on Synchrotron Radiation and Applications in Vacuum U.V. Physics’, ed. E.-E. Koch, R. Haensel, and C. Kunz, Pergamon, 1974.

Developments in Instrumentation and Techniques

5

3 Laser Sources Before reporting developments in laser sources, it is appropriate to comment on safety codes regarding eye protection. Although there is little doubt that nearly every laser system radiates a beam which is hazardous to the eye, current safety codes in this country and elsewhere need to be revised regularly in view of developments in laser sources. Minimum permissible exposures depend on laser wavelength, exposure time, and peak power and, for many lasers, are estimated and certainly not based on ophthalmic measurements of thresholds for retinal or corneal lesions. Although some current safety codes have been criticized for being confusing, too conservative, and unrealistic (Laser Report, 1976, 12, 6, 7), there has been a report that standards for the near-u.v. may be inadequate (Laser Focus, 1976, 12(1), 41) since the corneal-damage threshold for the Nz laser for 10 ns pulses is only 10 pJ cm-2. Even more disturbing is recent evidence showing that the eye is 800 times more susceptible to damage from blue light than from radiation in the near-i.r.ll Both laser users and developers must be aware of realistic safety requirements, particularly in view of present and planned legislation on safety. The following sections outline some of the numerous publications on lasers with a reporting bias towards high-energy U.V. and tunable sources of all wavelengths which are being, or can be, used in photochemistry and spectroscopy.

CW Lasers.-There are few U.V. lasers known with adequate CW output power, and frequency doubling of visible lasers is not normally very efficient. Intracavity SHG, with temperature-tuned KDP or ADP crystals in a folded argon ion laser cavity, produces an output power of 300 mW at 257.25 nm.12 The important design criteria for this 32% power conversion efficiency are: (i) temperature tuning of the SHG crystal to better than kO.02 "C; (ii) cutting the crystals at the Brewster angle; and (iii) producing a 50 pm beam waist in the crystal. A lower cost and potentially useful laser for photochemistry is a CW CuII laser obtained by exciting a neon discharge in a copper hollow ~ a t h 0 d e . l ~ Lines at 248.6, 250.6, 259.1, and 259.9 nm at a power output of between 7 and 210 mW have been reported. The He-Cd laser, which usually emits at 325 and 441 nm,14 can produce simultaneous emission on five wavelengths in the red, green, and blue which can be mixed to give a 'white light' 1aser.16 CW laser oscillations on 23 transitions of CU(II)between 450 and 799 nm were obtained by exciting He-Ar, He-Ne, or He-Xe discharge in a hollow copper cathode.16 A high output power (0.5 W) and a bandwidth of 0.004 nm have been reported for a rhodamine 6G (Rh6G) laser pumped by an argon ion 1aser.l' Removal of unwanted background fluorescence from this type of laser within 0.5 nm of the exciting Ar+ l1 l2 l3

W. T. Ham, H. A. Mueller, A. I. Goldman, B. E. Newman, L. M. Holland, and T. Kuwabara, Science, 1974, 185, 362. P. Huber, ODtics Comm.. 1975. 15, 196. J. R. McNei, G. J. Collins, K:B. Persson, and D. L. Franzen, Appl. Phys. Letters, 1976,28, 207.

l8

D. C. Fromm, G . M. Neumann, and E. M. Schmidt, Optics andLaser Technology, 1976,8, 68. J. Meckley, Laser Focus, 1975, 11(11), 44. J. R. McNeil, G. J. Collins, K. B. Persson, and D. L. Franzen, Appl. Phys. Letters, 1975, 27,

l7

E. E. Marinero, A. M. Angus, and M. J. Colles, Optics Comm., 1975, 14, 226.

l4 l6

595.

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Photochemistry

line at 488 nm line has been achieved using an external diffraction grating.l* Two astigmatic and coma-free prism ring dye lasers have been described for the jet-stream CW system.lg CW laser action at 546 nm from an Hg laser 20* 21 has been obtained in one case 31 in a sealcd-off system, suggesting use as a low-power (3 mW) green laser. DOTC and hexacyanine-3 cyanine dyes pumped by a 1.5 W krypton laser produce laser emission covering the range 754-888 nm.22 A compact external cavity for use with Group 111 and IV compound semiconductor injection lasers incorporates a grating which allows tuning from 860 to 910 nm.23 Among i.r. lasers reported are those obtained by non-linear mixing of emission from Nd-YAG and Rh6G lasers in LiI03 (range 1.28-1.62 a spin-flip Raman laser for the range 1905-1850 cm-l which was calibrated by absorption spectroscopy of COS, NO, DBr, and H,O using acousto-optic detection,25and chemical lasers of HI; 27 and DF.20In one case, F atoms produced in a mixture of SF, and He by microwave-discharge apparatus produced a laser with a CW output power of 4 W between 2.5 and 2.9 pm.26 Laser gain profiles (at 10.8 pm) were measured in a low-pressure Na-catalysed N20-CO transverse flow chemical laser under a variety of flow conditions.28 261

Pulsed Gas Lasers.-The search for new U.V. lasers that are highly efficient has been particularly stimulated by requirements of isotope separation and laserinduced thermonuclear fusion. Electron-beam pumping of high-pressure noble gases is well known to be 30 and recent studies with xenon 31-33 and xenon-He-Ar mixtures 34 revealed a continuously tunable source over 5 nm at 172 34 Investigations of laser systems using collisional energy transfer to create population inversions between electronic states of acceptor molecules have concentrated on electron-beam pumping of gas mixtures, e.g. Xe-0,, Ar-N2.35 An intense band emission at 3 4 G 3 4 4 n m from Ar-I, mixtures has been attributed to emission from molecular iodine with an overall fluorescence yield of 13 k 4%. Since Velazco and Setser36 suggested that the diatomic 1 1 1 ~ 1 . ~ ~ 9

la

l8 2o

21 2a 23 24 26

28

27

28 29

30 31

s8 34 86

86

K. Jain, W. T. Wozniak, and M. V. Klein, Appl. Optics, 1975, 14, 811. G. Marowsky, Appl. Phys. Letters, 1975, 26, 647. N. Djeu and R. Burnham, Appl. Phys. Letters, 1974, 25, 350. M. Artusy, N. Holmes, and A. E. Siegman, Appl. Phys. Letters, 1976, 28, 133. A. Donzel and C. Weisbuch, Optics Comm., 1976, 17, 153. H. Heckscher and J. A. Rossi, Appl. Optics, 1975, 14, 94. W. Lahmann, K. Tibulski, and H. Welling, Optics Comm., 1976, 17, 18. R. J. Butcher, R. B. Daniels, and S. D. Smith, Proc. Roy. Soc., 1975, A344, 541. D. Proch, H. Pummer, K. L. Kompa, and J. Wanner, Rev. Sci. Instr., 1975,46, 1101. J. M. Gagne, L. Bertrand, Y . Counturie, S. Q. Mah, and J. P. Monchalin, J . Opt. SOC.Amer., 1975, 65, 876. R. C. Benson, C. B. Bargeron, and R. E. Walker, Chem. Phys. Letters, 1975, 35, 161. D. J. Bradley, in ‘Lasers in Physical Chemistry and Biophysics’, ed. J. Joussot-Dubien, Elsevier, Amsterdam, 1975, pp. 7-23. J. P. Girardeau-Montaut, Onde Electr., 1974, 54, 456, 463. J. B. Gerard0 and A. W. Johnson, Appl. Phys. Letters, 1975,26, 582. D. J. Bradley, D. R. Hull, M. H. R. Hutchinson, and M. W. McGeoch, Optics Comm., 1975, 14, 1. S. C. Wallace and R. W. Dreyfus, Appl. Phys. Letters, 1974, 25, 498. J. K. Rice and A. W. Johnson, J. Chem. Phys., 1975, 63, 5235. M. V. McCusker, R. M. Hill, D . L. Huestis, D. C. Lorents, R. A. Gutcheck, and H. H. Nakano, Appl. Phys. Letters, 1975, 27, 363. J. E. Velazco and D. W. Setser, J. Chem. Phys., 1975,62, 1990.

Developments in Instrumentation and Techniques

7

noble-gas halides were possible laser systems, the following have been observed to lase following electron-beam excitation: XeBr at 282 nm;38-40 KrF at 249 nm;37940-44 XeCl at 308 nm;40,43 XeF at 351 and 353 nm;40945-47 and ArF at 193 nm.37 These systems are of great interest as a new class of powerful tunable U.V. lasers. For example, using an axial electron-beam excitation scheme to excite a mixture of Ar, Kr, and F2, 108 J of laser energy corresponding to a peak power of 1.9 GW was obtained from KrF and 1.6 G W from ArF.37 Electron-beam pumping is not essential since transverse electrical excitation (similar to that used in the N, laser) of mixtures of He or Ne, Xe, and NF, at pressures between 300 and 1000 Torr produced strong laser emission at 351 and 353 nm (attributed to XeF) with an energy of 7 mJ (compared with 2 mJ from nitrogen under the same condition^).^^ Laser action on the U.V. bands of I2 at 342 nm49-51 and bromine at 292 nm 5 2 following electron-beam irradiation has been reported with an experimental arrangement similar to that used for the rare gas halide lasers.44 The nitrogen laser (at 337 nm) must be the most common laser used in photochemical laboratories. The literature on these devices up to 1974 has been reviewed,53 and a detailed analysis of their dynamic behaviour 54 and circuit theory and design55presented. In the last paper, the usual flat-plate design is modified to spiralled striplines rolled around the cavity. In this way, a reproducible power of 1.2 MW was obtained at a charging voltage of only 12 kV.55 Other models constructed include a double parallel-plate design similar to that of Basting and Steyer (Vol. 4, p. 88) with a third electrode in the cavity for preionization, giving an output power of > 3 MW and pulse energy of >20 mJ,56 a low divergence (0.2 x 0.3 mR) laser of maximum intensity 5 MW m~ad-,,~'a MW system from a simple 25 cm device,58and a low-threshold coaxial arrangement using a Nanolite pulser of maximum power 140 kW.5B A stabilization technique employing a corona-type discharge prior to pulsing a Blumlein circuit has been employed,60and calculations made on laser intensity 49s

J. M. Hoffman, A. K. Hays, and G . C. Tisone, Appl. Phys. Letters, 1976,28, 538. S. K. Searles and G. A. Hart, Appl. Phys. Letters, 1975, 27, 243. s9 S. K. Searles, Appl. Phys. Letters, 1976,28,602. 40 C . A. Brau and J. J. Ewing, J. Chem. Phys., 1975,63,4640. I1M. L. Bhaumik, R. S. Bradford, and E. R. Ault, Appl. Phys. Letters, 1976, 28, 23. Ia G. C. Tisone, A. K. Hays, and J. M. Hoffman, Optics Comm., 1975,15, 188. I 3 J. J. Ewing and C. A. Brau, Appl. Phys. Letters, 1975,27, 350. 44 J. A. Margano and J. H. Jacob, Appl. Phys. Letters, 1975, 27,495. 46 E. R. Ault, R. S. Bradford, and M. L. Bhaumik, Appl. Phys. Letters, 1975, 27, 413. 40 C. A. Brau and J. J. Ewing, Appl. Phys. Letters, 1975, 27, 435. I7 C. P. Wang, H. Mirels, D. G. Sutton, and S. N. Suchard, Appl. Phys. Letters, 1976,28, 326. 48 R. Burnham, N. W. Harris, and N. Djeu, Appl. Phys. Letters, 1976, 28, 86. p B J. J. Ewing, J. H. Jacob, J. A. Mangano, and H. A. Brown, Appl. Phys. Letters, 1976,28,656. J. J. Ewing and C. A. Brau, Appl. Phys. Letters, 1975, 27, 557. 61 R. S. Bradford, E. R. Ault, and M. L. Bhaumik, Appl. Phys. Letters, 1975, 27, 546. 6a J. R. Murray, J. C. Swingle, and C. E. Turner, Appl. Phys. Letters, 1976, 28, 530. 53 J. P. Girardeau-Montaut, Nouv. Rev. Opt., 1974, 5, 367. P. Richter, J. D. Kimel, and G. C. Moulton, Appl. Optics, 1976, 15, 756. 65 A. J. Schwab and F. W. Hollinger, I.E.E.E. J . Quantum Electron., 1976, QE-12, 183. 6 8 J. I. Levatter and S.-C. Lin, Appl. Phys. Letters, 1974, 25, 703. 67 B. Godard and M. Vannier, Optics Conzm., 1976, 16, 37. 6 8 H. M. von Bergmann, V. Hasson, and D. Preussler, Appl. Phys. Letters, 1975, 27, 553. H. Fischer, R. Girnus, and F. Ruhl, Appl. Optics, 1974, 13, 1759. O 0 V. Hasson, H. M. von Bergmann, and D. Preussler, Appl. Phys. Letters, 1976, 28, 17. s7

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Photochemistry

and linewidth.61 Construction of an N, laser operating at 1 atm pressure and producing 335 pJ in a 1 ns pulse6, and a segmented flat-plate Blumlein circuit generating 400 ps pulses at a peak power of 1 MW 63 have also been reported. Addition of SF6 to a nitrogen laser has been found to produce a considerable increase in output power 66 up to Electron-beam pumping of Ar-N, mixtures results in laser emission at 357.7 nm 67-69 with a much higher efficiency (0.08-0.4%) than nitrogen A compact Ar-N, excitation transfer laser emits 40 ns pulses at a repetition rate of 1 kHz with a peak power up to 300 kW.69 In this case, a 12-stage Marx bank generator drives the cathode directly with an input voltage of 540 kV. Travelling-wave excitation of high-pressure nitrogen can produce single pulses from the second positive band of N, with the duration decreasing from 300 ps at 1 atm to 50 ps at 6 Mixtures of argon and iodine-donor compounds (HI, CF31,or CH31) can be electron-beam pumped to produce lasing from iodine at 301 nm,71at average output powers up to 25 MW. Further investigations of the copper laser (reported in Vol. 6, p. 69) have shown that this could have potential as a high-energy visible Quasicontinuous pulsed laser output at 510.6 and 578.2 nm has been reported from 600 "C copper iodide discharges at repetition rates near 8 kHz 73 and up to 30 kHz with copper At slightly longer wavelengths, laser oscillations have been observed on the green bands of XeO and KrO excimers pumped by an electron beam at around 550 nm with peak powers up to 100 kW.75 A multiple wavelength laser could be obtained in a single laser tube by using metals known to lase individually. Copper and gold as laser materials, for example, produce a total power of 17 mW at repetition rates up to 1.7 kHz with simultaneous emission at 510.6, 578.2, and 627.8 nm.76 A discharge-heated lead vapour laser with emission at 406.2 and 405.7 nm has been r e p ~ r t e d . ~ ' High-power photochemical iodine lasers (emission at 1.31 5 pm) have the potential of providing the short and powerful pulses which are necessary for laser fusion. In order to reach maximum inversion quickly, it is necessary to pump CF31 or C3F,I molecules with a flash lamp or light from a laser-produced 649

62 63

a4 Bs 66

67

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e9 70

71 7a

73 74 76

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P. Richter, J. D. Kimel, and G. C. Moulton, Appl. Optics., 1976, 15, 1117. E. E. Bergmann, Appl. Phys. Letters, 1976, 28, 84. H. Salzmann and H. Strohwald, Optics Comm., 1974, 12, 370. C. S. Willett and D. M. Litynski, Appl. Phys. Letters, 1975, 26, 118. J. Itani, K. Kagawa, and Y . Kimura, Appl. Phys. Letters, 1975, 27, 503. 0. Judd, I.E.E.E. J. Quantum Electron., 1976, QE-12,78. S. K . Searles, Appl. Phys. Letters, 1974, 25, 735. E. R. Ault, M. L. Bhaumik, and N. T. Olson, Z.E.E.E. J . Quantum Electron., 1974, QE/10, 624; N. G . Basov, V. A. Danilychev, V. A. Dolgikh, 0. M. Kerimov, A. N. Labonov, and A. F. Suchlov, Pis'rna Zhur. Eksp. i Teor. Fiz., 1974,20, 124 (Chem. Abs., 1975, 81, 129 065). E. R. Auk, Appl. Phys. Letters, 1975, 26, 619. H. Strohwald and H. Salzmann, Appl. Phys. Letters, 1976, 28, 272. A. K. Hays, J. M. Hoffman, and G. C. Tisone, Chem. Phys. Letters, 1976, 39, 353. J. A. Piper, Optics Comm., 1975, 14, 296. I. Liberman, R. V. Babcock, C. S. Liu, T. V. George, and L. A. Weaver, Appl. Phys. Letters, 1974, 25, 334. C. J. Chen and G . R. Russell, Appl. Phys. Letters, 1975, 26, 504. H. T. Powell, J. R. Murray, and C. K. Rhodes, Appl. Phys. Letters, 1974, 25, 730. T. S. Fahlen, I.E.E.E. J. Quantum Electron., 1976, QE/12, 200. R. S. Anderson, B. G. Bricks, T. W. Karras, and L. W. Springer, I.E.E.E. J. Quantum Electron, 1976, QE-12, 313.

Developments in Instrumentation and Techniques

9

plasma,78,79 or in one case, to increase the number density of the iodide by shock c o m p r e s ~ i o n . ~The ~ high-gain characteristics of these lasers may result in premature super-radiant emission along an amplifier chain unless the various amplifier stages are optically isolated from each other. This was accomplished by a single saturable absorber consisting of an electric discharge passed through a cell containing CF31 gas or iodine vapour.80 Q-switching and modelockinga2have been achieved with this laser, resulting in the latter case in 160 ps pulses. There have been numerous reports of carbon dioxide lasers, lasing at 10.6 pm, with details of high-power TEA lasers,83a chemical waveguide laser with energy from the exothermic chain reaction between D, and F2 initiated by flash p h o t o l y ~ i s ,and ~ ~ laser amplification by stimulated emission of CO, by transfer from products of the oxidation of alkaline-earth metal vapours in N20.85 Q-switching with aromatic halogenated hydrocarbons and rapid modulation by operating a thin film Pb,-,Sn,Te optical shutter have also been d e s ~ r i b e d . ~ ~ Conversion of a Coherent Radiation CO, laser to create a CO laser results in laser emission at 5.4-5.6 pm with a power of 1 W.88 Pumping CH3F gas with a 200 MW TEA CO, laser produced far4.r. laser pulses (at 496 pm) with powers > 1 MW.89 Laser action at 11.5 and 12.2 urn was observed in electron-beam stabilized electric discharges with He-Co-CS, and He-Ne2-CS, mixtures.g0 Dye Lasers.-The welcome development of dye lasers offering higher output powers, shorter pulse durations, and higher repetition rates have been accompanied by numerous studies of fluorescent dyes. There has been a growing interest in studies of both the photophysical and photochemical properties of these dyes in attempts to achieve conditions of high output power and minimum photochemical degradation. Average dye laser output powers of > 100 W have been demonstrated at repetition rates of 350 Hz in an arrangement in which the dye flow was transverse to the laser axis and pumped by a vortex-stabilized flashlamp in an elliptical r e f l e ~ t o r .A ~ ~similar transverse flow system has been used at repetition rates up ~ ,slab dye laser designed originally for photocoagulation produced to 1 ~ H Z . A 20 m J output (from Rh6G) with a 50 J flash.s3 A sound suggestion for increasing 78 79

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84

87

88

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O3

S. Ishii, B. Ahlborn, and F. L. Curzon, Appl. Phys. Letters, 1975, 27, 118. L. D. Pleasance and L. A. Weaver, Appl. Phys. Letters, 1975, 27, 407. S. Ishii, K. Fong, and B. Ahlborn, Rev. Sci. Instr., 1976, 47, 600. S. Ishii and B. Ahlborn, Rev. Sci. Instr., 1975, 46, 1287. E. D. Jones, M. A. Palmer, and F. R. Franklin, Opt. Quantum Electron, 1976, 8, 231. A. C . Walker and K. R. Rickwood, J. Phys. (E), 1976,7,432; P. F. Browne and P. M. Webber, Appl. Phys. Letters, 1976, 28, 662; F. Rheault, J.-L. Lachambre, P. Lavigne, H. Pepin, and H. A. Baldis, Reo. Sci. Instr., 1975, 46, 1244; J. Domey, ibid., p. 811; H.-S. Kwok and E. Yablonovitch, ibid., p. 814. T. 0. Poehler, R. E. Walker, and J. W. Leight, Appl. Phys. Letters, 1975, 26, 560. D. J. Benard, Chern. Phys. Letters, 1975, 35, 167. J. R. Izatt, G. F. Caudle, and B. L. Bean, Appl. Phys. Letters, 1974, 25, 446. A. V. Nurmikko and G. W. Pratt, Appl. Phys. Letters, 1975, 27, 83. J. A. Davis, Rev. Sci. Instr., 1975, 46, 323. D. E. Evans, L. E. Sharp, B. W. James, and W. A. Peebles, Appl. Phys. Letters, 1975,26, 630. L. Y . Nelson, C. H. Fisher, and S. R. Byron, Appl. Phys. Letters, 1974, 25, 517. W. W. Morey and W. H. Glenn, I.E.E.E. J. Quantum Electron., 1976, QE/12, 31 1. H. W. Friedman and R. G. Morton, Appl. Optics, 1976,15, 1494. P. Burlamacchi, R. Pratesi, and U. Vanni, Rev. Sci. Instr., 1975,46, 281.

10

Photochemistry

the output power of a conventional flashlamp-pumped laser is to ignite a lowenergy prepulse discharge through the flashlanip just prior to firing the main flashlamp This prepulse or simmer mode of operation of flash lamps is well known to enhance the U.V. spectral output. Spectral characteristics of lamps have been given in a description of a coaxial dye cell pumped by a linear lamp, and considerable variations in peak power and energy were found at different wavelengths of a xenon d i s ~ h a r g e . ~ ~ High intensities in the 200-300 nm range needed to pump U.V. and blue dyes were obtained in a novel arrangement using a C0,-laser-produced Output powers of > 10 kW from p-terphenyl (lasing at 340 nm) were produced by a 10.6 pm input pulse energy of 5.6 J and more than 30 kW was obtained under the same conditions from Rh6G. By mixing two dyes in a single cell (e.g. Rh6G with coumarin 120) it is possible to obtain two or more lasing wavelengths on pumping with an N, laser.Q7A sixchannel flat-plate quartz dye cell has been used in a similar pumping arrangement to select specific dyes.ss This cell offers advantages over the rotating carousel because of a larger pumping area, more effective stirring, and a large dye reservoir (which reduces photodecomposition). Dye lasers of narrow bandwidth O Q s looare required for high-resolution spectral studies and selective photochemical excitation. In one example, peak powers of 50 kW in the visible at linewidths down to 6 x nm have been reported for a pressure-tuned, dye laser oscillator with an external interferometer and twostage amplifier pumped by a 1 MW N2laser.lo0 Theoretical analysis of dye-laser pumping has shown that only the singlet manifold needs to be considered in kinetic studieslO1 and that a rate equation, agreeing with experimental measurements, can be based on a study of the effects of pump radiation on singlet state absorption and fluorescence quantum yield.lo2 Laser Dyes. Many hundreds of fluorescent dyes have been examined for laser action, and only a few reports of these can be mentioned here. U.V. lasers, which are much needed for photochemical work, are still few and far between although a review, with useful references to Russian work, lists 30 flashlamppumped dyes lasing between 330 and 400nm.lo3 POPOP remains the most efficient dye in the vapour phase, producing radiation tunable from 380 to 410 nm.104-10s Unfortunately, the gain reaches a maximum well before the M. H. Ornstein and V. E. Derr, Appl. Optics., 1974, 13, 2100. M. Drake and R. I. Morse, Optics Comm., 1974, 12, 132. Q6 W. T. Silvfast and 0. R. Wood, Appl. Phys. Letters, 1974, 25, 275. 97 W. T. Silvfast and 0. R. Wood, Appl. Phys. Letters 1975, 26,447; R. K. Jain and A. Dienes, Spectroscopy Letters, 1974, 7, 491. P. E. Oettinger, Appl. Spectroscopy, 1976, 30, 362. J. E. Lawler, W. A. Fitzsimmons, and L. W. Anderson, Appl. Optics, 1976, 15, 1083; A. Yamagishi and H. Inaba, Optics Comm., 1976, 16,223. loo R. Wallenstein and T. W. Hansch, Optics Contm., 1976, 14, 353. lo1 S. Speiser, Chem. Phys., 1974, 6, 479. loa E. Saher, D. Treves, and I. Wieder, Optics Comnt., 1976, 16, 124. lo3 G. A. Abakumov, V. V. Fadeev, and R. V. Khokhlov, Spectroscopy Letters, 1975, 8, 651. lo' P. F. Liao, P. W. Smith, and P. J. Maloney, Optics Comm., 1976,17,219. lo6 G . Marowsky, F. P. Schaefer, J. Keto, and F. K. Tittel, Appl. Phys., 1976,9,143;P. W. Smith, P. F. Liao, C. V. Shank, T. K. Gustafson, C. Lin, and P. J. Maloney, Appl. Phys. Letters, 1974, 25, 144. lo6 M. Maeda and Y. Miyazoe, Jap. J. Appl. Phys., 1974,13,827. O4

!as J.

Developnzents in Instrumentation and Techniques

11

maximum of the pumping pulse and falls rapidly. This premature termination, attributed to photodeconiposition, is a severe limitation to high overall efficiency.lo4 Tables of lasing dyes with output wavelengths < 440 nm lo6and between 420 and 750 nni lo7have been published. Several new coumarin derivatives have been reported,lo8,logsome of which have superior thermo-optical properties with flashlamp pumping and lasing with high efficiency between 450 and 520 nm.lo8 A useful study of photodegradation of 7-diethylamino-4-methylcoumarinin ethanol showed that two chemically different reaction routes could be identified.l1° One of these routes leads to compounds which lase and the other to a substance which absorbs at the laser wavelength. Although oxygen is usually employed to quench the triplet state, in this case addition causes formation of the laser-inhibiting compound. Improved performance could be obtained by adding a non-oxidizable tripletstate quencher, replacing the CH, group on the coumarin by a less reactive CF3 group, and removing the inhibitor by appropriate chemical filtering. A comprehensive review on the lasing properties of 4-methylun~belliferone(lasing range 390-560 nm) includes other related coumarins.lll Laser action in trans-l,l,4,4-tetraphenylbutadiene in cumene shows that at low temperatures (- 80 "C) emission occurs at 435 nm, between -70 and - 80 "Cat both 435 and 498 nm, and at room temperature at 498 nm This behaviour has been attributed to a temperature-dependent photoisomer. Fluorol 7GA dye, if sufficiently purified, is a stable lasing material for the region 530-600 nrn.ll3 Chemical impurities in Rh6G [Cl-] decrease the quantum efficiency of fluorescence and shorten the usable lifetime of any lasing Other studies with this popular lasing dye include relaxation kinetics of dimerization 115and lasing properties in polyacrylonitrile polymers.l16 Studies of the lasing properties of Kiton red S and rhodamine B dyes with both long- and short-pulse excitation have shown how the performance is strongly influenced by substituents bonded to the C-9 of the main c h r o m ~ p h o r e . ~ ~ ~ Population inversion is difficult to achieve with cresyl violet, and optical filtering procedures designed to lower the laser threshold have been described.l18 Mixing the dye with RhGG produces efficient lasing between 646 and 700 nni because of transfer of excitation attributed to depletion by stimulated emission of population in an upper laser At longer wavelengths, efficient (211%) emission between 838 and 923 nm has been reported for various polymethine dyes pumped by a frequency-doubled Nd:YAG laser,119and a range of lo' J. B. Marling, J. G. Hawley, E. M. Liston, and W. B. Grant, Appl. Optics, 1974, 13, 2317. lo8 K.H.Drexhage, G. R. Erikson, G . H. Hawks, and G. A. Reynolds, Optics, Comm., 1975,15, 399.

E. J. Schimitschek, J. A. Trias, P. R. Hammond, R. A. Henry, and R. L. Atkins, Optics Comm., 1976,16,313. B. H. Winters, H. I. Mandelberg, and W. B. Mohr, Appl. Phys. Letters, 1974, 25, 723. ll1 S. C. Naydon, Spectroscopy Letters, 1975, 8, 815. lla C.Rulliere, J. P. Morand, and J. Joussot-Dubien, Optics Comm., 1975, 15, 263. 113 M.Lambropoulos, Optics Comm., 1975, 15, 35. 11* J. M. Drake and R. I. Morse, Optics Comm., 1975, 13, 109. 116 M. M. Wong and Z. A. Schelly, J . Phys. Chem., 1974, 78, 1891. ll6 S. Reich and G . Neumann, Appl. Phys. Letters, 1974,25, 119. 11' J. M. Drake, R. I. Morse, R. N . Steppel, and D. Young, Chem. Phys. Letters, 1975,35, 181. 11* D. E. Evans, J. Puric, and M. L. Yeoman, Appl. Phys. Letters, 1974, 25, 151. 119 C. D. Decker, Appl. Phys. Letters, 1975, 27, 607. loB

12

Photochemistry

dyes useful between 71&1080 nm was discovered with a Q-switched ruby laser as the pump source.120 A dye laser pumped by a pulse train from a mode-locked solid-state laser can emit short pulses coincidental in time with pumping pulses when the relative cavity lengths are mafched.l2l Recent work using Fabry-Perot tuning elements showed that transform-limited (12 ps) pulses covering a broad spectral tuning range (549-727 nm) could be generated with high efficiency in several laser dyes.121 By choosing a cavity of correct photon cavity decay time (e.g. 60 ps) and by controlling the level of pumping, it is possible to obtain high repetition rate, tunable, dye-laser pulses of subnanosecond duration from N,-laser-pumped dyes in the near U.V.and visible.122 Narrow-band ps pulses have been generated in an ultrashort (50 pm) cavity of rhodamine B pumped by a 530 nm modelocked Nd 1 a ~ e r . lDouble ~~ mode-locked operation occurs in mixtures of Rh6G and cresyl violet resulting in an ultrashort pulse at 574 nm followed by one at 644nm.124 A novel system for generating a single tunable high-power ps pulse involves passing a mode-locked dye-laser train through a dye amplifier which is pumped by a N2 1 a ~ e r . lAs ~ ~the amplifier gain is only available for a few ns, only one pulse in the train is amplified. Other reports of mode-locked pulses include the injection locking of a mode-locked high-power train to a (low-power) mode-locked CW laser to improve laser optical properties,126and the use of a mode-locked Arf laser to pump a dye and obtain < 500 ps (detector-limited) ~u1ses.l~~ Solid-state Lasers.-Ruby and neodymium lasers remain the most powerful and useful solid-state lasers and major developments have been directed towards more efficient harmonic conversion (see next section). CaLaS0AP:Nd [Ca2La7.,,Nd,.,,(Si0,),0,1 has been evaluated as a replacement for Nd:YAG in high repetition rate 1060 nm Q-switched laser systems; it was shown to operate at an average power of >1 J at 30Hz.12* Developments in electro-optic and other switches, particularly for single-pulse extraction from a mode-locked train, include descriptions of a multichannel laser-triggered spark gap which will switch up to 10 kV in five channels with a risetime of <300 ps;129a lithium niobate Pockels ce11;130 a Kerr cell with a risetime of 100 ps;131and a method for pulse selection by use of a laser-initiated Krytron-switched Blumlein structure which replaces the usual spark gap.132 In the latter example, the result is an r.f. noise-free pulse selector with a risetime of a few ns. A spark gap triggered by a P. E. Oettinger and C. F. Dewey, I.E.E.E. J . Quantum Electron., 1976, QE-12, 95. L. S. Goldberg and C. A. Moore, Appl. Phys. Letters, 1975, 27, 217. 122 C. Lin and C . V. Shank, Appl. Phys. Letters, 1975, 26, 389. lZ3 B. Fan and T. K. Gustafson, Appl. Phys. Letters, 1976,28, 202. l Z 4 Z . A. Yasa and 0. Teschke, Appl. Phys. Letters, 1975,27,446. la5 A. J. Schmidt, Optics Comm., 1975, 14, 287. E. I. Moses, J. J. Turner, and C . L. Tange, Appl. Phys. Letters, 1976, 28, 258; J. J. Turner, E. I. Moses, and C. L. Tang, Appl. Phys. Letters, 1975, 27, 441. 12' J. M. Harris, R. W. Chrisman, and F. E. Lytle, Appl. Phys. Letters, 1975, 26, 16. 128 K. B. Steinbruegge and G. D. Baldwin, Appl. Phys. Letters, 1974,25220. n8 C. L. M.Ireland, J . Phys. ( E ) , 1975, 8, 1007. 130 V. J. Corcoran, R. W. McMillan, and P. M. Rushworth, Appl. Optics, 1975,14, 643. 131 J.-C. Diels, Rev. Sci. Instr., 1975, 46, 1704. lS2 R. C. Hyer, H. D. Sutphin, and K. R. Winn, Rev. Sci. Instr., 1975,46, 1333.

I3O

lol

Developments in Instrumentation and Techniques

13 dye-laser beam for switching a single pulse from a mode-locked train is distinguished by a low threshold peak power (2 kW) and 5 pJ pulse energy.133 Passive Q-switching of a neodymium laser has been achieved with india ink 134 and nickel diethiene.135 In the latter case, recovery times were found to be solventdependent and as short as 25 ps in ethyl sulphide. Picosecond pulse generation from a frequency-tripled Nd laser (at 353 nm) has been A novel application of photoconductivity produced by ps pulses in silicon has been to switch electrical signals as large as 1.5 kV which have been used in turn to drive a travelling-wave Pockels cell for efficient optical switching with a measured risetime of about 25 ps.13' Frequency Conversion.-Conversion efficiencies of third harmonic generation with Nd lasers in alkali-metal vapours have been improved from 0.1 to 10% with a phase-matched mixture of rubidium and xenon.138 The conversion efficiency is limited by experimental difficulties in attaining high metal-vapour pressures 138 or buffer gas pressures.139 Coherent radiation around 189.5 and 179.8 nm was obtained by frequency-mixing two tunable dye lasers,14oand a review by the same researchers discusses how resonant enhancement of cubic nonlinearities in atomic vapours can be used to provide tunable radiation in both the vacuum-u.v. and near4.r. by sum and difference mixing.141 A number of non-linear crystals for frequency conversion have been investigated and some offer better performance than either ADP or KDP. Lithium perchlorate trihydrate, for example, is transparent to 210nm light and will efficiently convert 530nm into 265 nm at power densities less than 100 MW cm-2.142 Unlike ADP and KDP this material does not need an oven since its refractive indices are not temperature-dependent. At low power levels, lithium formate monohydrate crystal has been reported to produce third harmonic radiation from an Nd laser at about three times the efficiency of ADP.143 Rubidium dihydrogen phosphate (RDP) is one of the more attractive crystals for frequency-tripling of Nd lasers because of its high efficiency (21%) and high damage thresh01d.l~~ Potassium pentaborate crystals show excellent transmission to 200 nm and would appear to be an ideal candidate for SHG with dye lasers. No optical damage for focused fundamental light at 450 nm was found at 1 GW cm-2,146 and a conversion efficiency of 9.2% has been reported at 217nm for a peak power of 15 kW at 434.2 n111.l~~ K. Schildbach and D. Basting, Rev. Sci. Instr., 1974, 45, 1015. G. Dube, Appl. Optics, 1975, 14, 553. 136 B. Fan and T. K. Gustafson, Optics Comm., 1975, 15, 32. 136 D. T. Attwood, E. L. Pierce, and L. W. Coleman, Optics Comm., 1975, 15, 10. 13' P. LeFur and D. H. Auston, Appl. Phys. Letters, 1976, 28, 21. 13* D. M. Bloom, G. W. Bekkers, J. F. Young, and S. E. Harris, Appl. Phys. Letters, 1975, 26, 687. 130 D. M. Bloom, J. F. Young, and S. E. Harris, Appl. Phys. Letters, 1975, 21, 390. lrlo R. T. Hodgson, P. P. Sorokin, and J. J. Wynne, Phys. Rev. Letters, 1974,32, 343. 141 J. J. Wynne and P. P. Sorokin, Laser Focus, 1975,11(4), 62. 14a J. G. Bergman, D. Williams, G. R. Crane, and R. N. Storey, Appl. Phys. Letters, 1975, 26, 571. 143 K. Kato, Optics Quantum Electron., 1976, 8, 261. 144 K. Kato, Appl. Phys. Letters, 1974,25,342. C. F. Dewey, W. R. Cook, R. T. Hodgson, and J. J. Wynne, Appl. Phys. Letters, 1975,26,714. lr16 H. J. Dewey, I.E.E.E. J. Quantum Electron., 1976, QE-12, 303. ls3

131

2

14

Photochemistry

The most efficient SHG crystal for the Nd fundamental is claimed to be CDA, which at 50 MW pump power converts 57% into 530 nni radiation.lq7 CD*A lq7 and RDAlq8are the next best contenders with conversion efficiencies of 45 and 34%, respectively. An overall conversion efficiency of 15% for frequency-quadrupling an Nd:YAG laser has been achieved with a CDA-ADP cascade.lq9 The most efficient crystal for ruby lasers has been reported to be RDA, which has a maximum loss-free power conversion efficiency of 58%.160 A number of devices have been described for tuning dye lasers, including birefringent Fabry-Perot etalons 151 and pressure tuning.lo0S162s 153 The latter method is claimed to have many advantages 153 for high-resolution outputs. For example, a frequency-doubled oscillator-amplifier dye-laser system gave a fundamental output power of about 6 MW with a spectral width of ~ 0 . 0 0 3 nm at 564 nm and a second harmonic at 282 nm of 0.6 MW peak power and bandwidth <0.0015 nm with pressure tuning.163 Very rapid tuning of a dye laser over a limited bandwidth (15 nm) has been achieved by applying an r.f. signal at 100 MHz to a KD*P tuning c r y ~ t a 1 . l ~ ~ The short-pulse output from a synchronously mode-locked tunable dye laser was sum- and difference-mixed in KDP, ADP, and LiIO, with the 1064 and 532nm pulses from its mode-locked Nd:YAG pump. This method produced efficient generation of narrow bandwidth tunabIe short pulses in the U.V. from 270 to 432 nm and in the i.r. from 1.13 to 5.6 pm.lsS Tunable i.r. radiation from 500 urn to 1 mm 156 and 1.4 to 4 pm ls7was obtained by frequency-mixing in a LiNb03 crystal, and from 854 to 1410 nm 168 using CDA pumped from a doubled Nd:YAG laser. At much longer wavelengths, tunable stimulated electronic Raman scattering and 11.7-15 pm;15@upwas observed in the ranges 2.5-4.75, 5.67-8.65, conversion in Cd-Se using an HF laser can cover the range 8-25 pm,lS0and far4.r. radiation (95 cm-l) was generated by different frequency-mixing with a spin-flip laser.lal 1521

Laser Measurements.-A laser beam monitor which uses a beam splitter set at 20" (instead of the usual 45") is claimed to be less sensitive to small changes in the direction of incidence of polarized laser beams, laser polarization, and beam divergence.ls2 Accurate intensity determinations of ps pulses usually require both temporal and spatial pulse shapes to be known. However, a method has K. Kato, I.E.E.E. J. Quantum Electron., 1974, QE10, 616. K. Kato, Optics Comm., 1975,13, 93. 149 K. Kato, Optics Comm., 1975, 13, 361. lSo K. Kato, Z.E.E.E. J . Quantum Electron., 1974, QE-l0,622. lS1 M. Okada, K.Takizawa, and S. Ieiri, Appl. Optics, 1976,15,472; M. Okade, S. Shimizu, and S. Ieiri, ibid., 1975, 14,917. lS* R. Flach, I. S. Shahin, and W. M. Yen, Appl. Optics, 1974, 13, 2095. A. Moriarty, W. Heaps, and D. D. Davis, Optics Comm., 1976, 16, 324. lS4 J. M. Telle and C. L. Tang, Appl. Phys. Letters, 1975,26, 10, 572. l8li C. A. Moore and L. S. Goldberg, Optics Comm., 1976,16, 21. lSe A. Koster and A. Vossough, J. Phys. (E), 1974, 7, 340. lS7 R. L. Herbst, R. N. Fleming, and R. L. Byer, Appl. Phys. Letters, 1974, 25, 520. lti8 G. A. Massey and R. A. Elliot, I.E.E.E. J. Quantum Electron, 1974, QE-10, 899. 16g D. Cotter, D. C. Hanna, and R. Wyatt, Optics Comm., 1976, 16,256. le0 A. Ferrario and M. Garbi, Optics Comm., 1976, 17, 158. V. T. Nguyen and T. J. Bridges, Appl. Phys. Letters, 1975,26, 452. lea I. S. Falconer, R. A. Niland, and M. I. Turk, J. Phys. (E), 1975,8,216. 14'

140

Developments in Instrumentation and Techniques

15

been described which uses the intensity dependence of the energy transmission through a two-photon absorbing sample instead of the (non-linear) transmission characteristics through a saturable absorber.la3 Temporal and spectral characteristics of ps pulses have been determined using a streak camera 16*and a variation of the optical Kerr effect.1as In the latter case, a frequency sweep can be obtained during a single 10 ps 530 nm pulse. An optical mixing arrangement which measures the duration of subnanosecond pulses from mode-locked tunable dye lasers has been described.lss An energy detector for a COa laser uses the pressure rise generated in a vacuum cell by gaseous i.r. absorption to provide linear energy detection over several decades 167 with good reproducibility, a wide aperture, and in-line beam monitoring. 4 Monochromators and Light Filters Despite the obvious theoretical advantages of holographic gratings (low stray light and wider spectral coverage in comparison to conventional ruled gratings), few manufacturers to date are incorporating these in commercial monochromators. The theory and performance of holographic gratings and their specific use in Seya-Namioka monochromators have been discussed.168 The design of stigmatic monochromators for the U.V. region using concave holographic gratings has been r e ~ 0 r t e d . l ~ ~ The overall quantum efficiency and instrument polarization of a McPherson model 218 grating monochromator from the U.V. to i.r. have been determined and related to known theoretical e q ~ a t i 0 n s . lA ~ ~clever slit servo-mechanism maintained the output of a monochromator at a constant level as the wavelength was scanned by directing part of the optical radiation onto a thermocouple detector which provided a signal to a stepping motor coupled to the ~ 1 i t s . l ~ ~ The introduction of a rotating mirror near the exit slit of a Rowland circle mounting with a rapid scanning spectrometer capable of being synchronized with short pulses of radiation resulted in a wavelength range between 115 and 200 nm in 34.5 nm increments at a scan speed of 30 nm 111s-l.l~~A rotating mirror spectrograph has been used to examine the time-resolved emission of exploding silver wire at a time resolution of 0.3 p s at 250 nm and 0.6 ps at 400 nm with the spectra recorded on 2000 ASA film.173 A compact Littrow mounted grating spectrometer uses a cooled linear array of silicon diodes as a detector for the range 400-1 100 nm.174 Spectrographs for the v a c u u r n - u . ~ . ,covering ~~~ the 16s

le4

le6

167

16* 168 170 171 178 173 174

A. Penzkofer and W. Falkenstein, Optics Comm., 1976, 17, 1. T. R. Royt, W. L. Faust, L. S. Goldberg, and C. H. Lee, Appl. Phys. Letters, 1974,25, 514. A. N. Rubinov, M. C. Richardson, K. Sala, and A. J. Alcock, Appl. Phys. Letters, 1975, 27, 358; Optics Comm., 1974, 12, 188. H. Mahr and M. D. Hirsch, Optics Comm., 1975, 13, 96. A. A. Offenberger, P. R. Smy, and N. H. Burnett, Rev. Sci.Instr., 1974, 12, 188. H. Noda, T. Namioka, and M. Seya, J. Opt. SOC.Amer., 1974, 64, 1031, 1037, 1043. M. Povey, Appl. Optics, 1974, 13, 2739. N. Andersen, K. Jensen, J. Jepsen, J. Melskens, and E. Veje, Appl. Optics, 1974,13, 1965. A. T. Collins, J. Phys. (El, 1975, 8, 1021. C. Weiser, Rev. Sci. Instr., 1975, 46, 830. R. D. Sacks, and J. A. Holcombe, Appl. Spectroscopy, 1974, 28, 518. G. A. H. Walker, V. L. Buchholz, D. Camp, B. Isherwood, J. Glaspey, R. Coutts, A. Condel, and J. Gower, Rev. Sci. Instr., 1974, 45, 1349. G. M. Lawrence and E. J. Stone, Rev. Sci. Instr., 1975, 46, 432.

16

Photochemistry

range 58-180 nm, and f a r - i . ~ . covering , ~ ~ ~ the range 5-70 pm, have also been reported. Further developments have been reported on the use of acousto-optic filters as a restricted range monochromator (see Vol. 6, p. 77). In one case, a commercially available CaMoO, crystal was incorporated in a rapid scanning spectrometer covering the wavelength range 450-750 nm at a bandwidth between 0.2 and 0.6 nm.177 Although the transmission of this device was only comparable to that of an j74 monochromator, the stray light was naturally high but could be eliminated electronically by transmission modulation or phasesensitive detection of the output signal. The transmission of the filter can be switched in 20 ps and modulation frequencies up to 25 kHz are attainable without loss of transmission. Similar performance specifications on transmission can be obtained with a TeO, c r y ~ t a 1 . lA~ ~more conventional narrow-gap interferometer of high transmission has been used as a tunable filter for the wavelength range 440-560 n111.l~~ Until recently, interference filters for i.r. wavelengths were generally limited to bandwidths of the order of 50 nm. Even though filters have large inherent throughput advantages over grating and prism spectrometers, they cannot compete either with the frequency-scanning facility of these instruments or with the fact that high resolution (< 2 cm-l) has only been achieved with non-filter devices. Recently, however, filters have been produced lSowith extremely narrow bandwidths (0.2 cm-l), high transmission (50%), and, most important, the facility to scan over a broad range (k 10%of the central wavelength) by simply tilting the filter several degrees with respect to the incident radiation axis. Glass filters and plastic filters lg2 for laser attenuation, limitations to the manufacture and performance of U.V. interference filfers,ls2 and techniques to produce narrow bandwidth nm) reflection grating filters ls3 have been described in other papers. Isolation of the 185 nm light from a low-pressure Hg lamp without interference from the dominant 254 nm light was accomplished by filtering with a 6oCo gamma-irradiated lithium fluoride Considerable care has to be taken with these discs because the radiation-induced centres which filter out the 254nm light are bleached on exposure to u.v., and regular monitoring of absorbance is necessary.lg4 Simulation of solar U.V. radiation has been accomplished with a 6 kW xenon source and filter systern.ls6 Among other optical devices to be described are a simple modification to the multiple-reflection White cell to give an additional path length without altering the basic beam configuration,ls6 an in-line achromatic focusing assembly to l 7 6 D.P. McNutt, K. Shivanandan, M. Daehler, and P. D. Feldman, Appl. Optics, 1975,14,117. W. S. Shipp, J. Biggins, and C. W. Wade, Rev. Sci.Instr., 1976, 47, 565. I. C. Chang, Appl. Phys. Letters, 1974, 25, 370. 178 N. K. Reay, J. Ring, and R. J. Scaddan, J . Phys. ( E ) , 1974, 7, 673. 180 A. E. Roche and A. M. Title, Appl. Optics, 1975, 14, 765. lsl W. B. Alexander, Electro-optical Sytems Design, 1975, 7(4), 23A. laa S. L. Bryn, Laser Focus, 1974, 10 (7), 45. 183 R. V. Schmidt, D. C. Flanders, C. V. Shank, and R. D. Standley, Appl. Phys. Letters, 1974, 25, 651. 184 D. Kamra and J. M. White, J. Photochem., 1975, 4, 361. 1.86 W. B. Sisson and M. M. Caldwell, Photochem. and Photobiol., 1975,21, 453. 188 D. E. Jennings, W. E. Blass, and N. M. Gailer, Appl. Optics, 1976, 15, 864. 177 178

Deuelopments in Instrumentation and Techniques

17

focus a sharp image onto a ~ p e c t r o m e t e rand , ~ ~ a~ three-mirror multi-transversal absorption cell of minimum volume and astigmatism.lss Quartz plates have been used to provide two output wavelengths from a monochromator separated by up to 2 nm lagand to enable point-by-point absorption spectra to be obtained Fibre optics have been used to transmit a from a single-beam coherent image from a helium-filled Dewar flask at 0.3 Klgl and, with an integrating sphere, to measure scattered light.lg2 Graded refractive index antireflection films applied to glasses sensitized by a phase-separating heat treatment have reduced reflectance losses from -8 to <0.5% in the wavelength region 350-2500 nm,lg3 and plasma-polymerized coatings of perfluorobut-2-ene were found to be effective single-layer AR coatings for PMMA.lg4 5 Absorption Spectrometry

U.v.-Visible Spectrometry.-Nomenclature in spectrometry has already been discussed in the introduction,l and several of Mielenz’s recommendations have been adopted here. Since so many quantitive data in photochemistry are related to measurements taken by absorption spectrometry, this section will be liberally based and is planned to complement the comprehensive biennial review of U.V. and visible spectrometry covering the literature published in 1973-l975.lg5 Precision and accuracy of u.v.-visible spectrometric measurements are of universal concern, and a useful article reviews instrumental errors (slit widths, scanning speeds, stray light), sampling handling errors (cell positioning, cell cleaning), and limitations of liquid and solid reference mate1-ia1s.l~~ A detailed theoretical and experimental investigation of factors affecting precision in absorption spectrometry includes errors due to amplifier noise, dark current, and cell positioning.lg7 Practical secondary absorption (and fluorescence) standards with suitable long-term stability enables checks to be made of wavelength and photometric accuracy with a range of solutions contained in c u v e t t e ~ .Wavelength ~~~ calibration methods have been reviewed in an article dealing with visible-range ~ p e c t r o m e t e r ~ Polarization .~~~ errors in spectrometers may cause unpredictable variation in light transmitted, reflected, or absorbed, and since most spectrometers have some significant degree of polarization in their sample illumination (up to 30%) the spectra of all solid samples must be suspect and samples checked for dichroism.200 An optical system used in a versatile precision spectrometer incorporates periodic switching through three optical equal paths with rotating mirrors to F. M. Phelps and K. B. Newbound, J. Opt. SOC.Amer., 1975, 65, 1283. G. J. Rayl, Appl. Optics, 1976, 15, 921. D. W. Brinkman and R. D. Sacks, Analyt. Chem., 1975,47, 1723. lgo S. P. Varma, Rev. Sci. Instr., 1975, 46, 1424. lV1 G. A. Williams and R. E. Packard, Rev. Sci. Instr., 1974, 45, 1029. lg2 F. W. Ostermayer and W. W. Benson, Appl. Optics, 1974, 13, 1900. ln3 M. J. Minot, J. Opt. SOC.Amer., 1976, 66, 515. lg4 T. Wydeven and R. Kubacki, Appl. Optics, 1976,15, 132. lV6 R. Hummel and D. Kaufman, Analyt. Chem., l976,48,268R. lg6 J. 0. Erickson and T. Surles, Amer. Laboratory, 1976, 8(6), 41. lg7 L. D. Rothman, S. R. Crouch, and J. D. Ingle, Analyt. Chem., 1975,47, 1226. lg8 M. A. West and D. R. Kemp, Internat. Laboratory, 1976, ( 5 ) , 27. lBQ D. H. Alman and F. W. Billmeyer, J. Chem. Educ., 1975,52, A281, A315. F. Grum and L. F. Costa, Appl. Optics, 1974, 13, 2228. la’

18

Photochemistry

give consecutive readings of IoT, IoR, Io, and a dark signal.201 Direct measurement of absorbance values as high as 4 is possible on a double-beam spectrometer designed for solid-state samples at low temperatures; the usual rotating mirror is replaced by a chopper wheel to arrange for reference and sample beams to pass through the same set of Dewar flask windows.202Digital techniques are suitable for measuring absorption spectra at low flux densities, and use of a reference channel before the sample in one arrangement ensured a constant flux per wavelength interval independent of the spectral distribution of the light source.2o3 The absorption spectrum of [2H,]naphthalene vapour at 70 mTorr pressure and an absorption pathlength of 4.2 m were given to illustrate this arrangement. High-resolution molecular spectroscopy (of the tetrahedral fine structure in the v3 of methane) with a precision and reproducibility of 5 x 10-4cm-1 has been reported; the analysing light is produced by mixing A narrow-band Ar+ laser with a tunable dye laser in a lithium niobate pulsed dye-laser system has been used to record the rotational absorption spectrum of molecular iodine using fluorescence detection.205 Absorption coefficients of the order of 0.05cm-l in thin semiconductor layers have been measured with a double-beam single-detector spectrometer using a laser as a monitoring source.2o6 Triplet-triplet absorption spectra of aromatic hydrocarbons in low-temperature solutions have been recorded with a computercontrolled spectrometer which uses intermittent U.V. irradiation to excite a sample.2o7Modulation excitation spectrometry has been used to record singlet and triplet absorption spectra of polycyclic hydrocarbons 208 and spectra of solvated electrons at low concentration^,^^^ and to study transient species produced in the reaction of Hg(h3Po)with H,O and NH,.210 The vidicon scanning spectrometer reported earlier (Vol. 6, p. 77) has been shown to be useful at wavelengths as low as 200 nm for scan rates up to 250 Hz and repetition rates between 0.2 and 250 H z . ~ ~Other ' rapid scanning spectrometers have been used for stopped-flow 212 and for microsecond acquisition and processing of spectrometric data.213 Signal-processing equipment for absorption spectrometers 214 has improved signal to noise ratios and accuracy. A programmable calculator provided a linear wavelength scale on a prism spectrometer.216 Other related electronic accessories included a simple analogue divider for plotting transmittance or aoi a oa

A. P. DeFonzo, Rev. Sci. Znstr., 1975, 46, 1329. L. F. Mollenauer and D. H. Olson, Rev. Sci. Znstr., 1975, 46, 677. 2 03 W. E. Howard, H. L. Selzle, and E. W. Schlag, J. Phys. ( E ) , 1975, 8, 783. 204 A. S. Pine, J. Opt. SOC.Amer., 1976, 66, 97. aos R. Wallenstein and T. W. Hansch, Appl. Optics, 1974, 13, 1625. a06 M. Gal, K. Nemeth, and G. Eppeldauer, J . Phys. ( E ) , 1974, 7, 484. 2 07 U. B. Ranalder, H. Kanzig, and U. P. Wild, J. Photochem., 1975, 4, 95. a 08 M. A. Slifkin and A. 0.Al-Chalabi, Spectrochim. Acta, 1976,32A, 661 ; Chem. Phys. Letters, 1974, 29, 405. aos P. Krebs, J. Phys. Chem., 1976, 79, 2941. 210 A. B. Harker and C. S. Burton, J. Chem. Phys., 1975, 63, 885. a i l M. J. Milano and H. L. Pardue, Analyt. Chem., 1975,47,25. a i a R. M. Wightman, R. L. Scott, C . N. Reilley, R. W. Murray, and J. N. Burnett, Analyt. Chew., 1974,46, 1492. a13 J. A. Miller, P. Levoir, J. C . Fontaine, F. Garnier, and J. E. Dubois, Analyt. Chem., 1975, 47, 29. 214 C. W. Wade, Rev. Sci. Znstr., 1975,46, 987; S . P. Lee, W. Tscharnuter, and B. Chu, Rev. Sci. Instr., 1975, 46, 1278. a16 S. Loughin, C. Y. Yang, and J. E. Fischer, Appl. Optics, 1975, 14, 1373.

Developments in Instrumentation and Techniques

19 reflectance characteristics as a function of wavelength,216and a differential amplifier for scale expansion on a u.v.-visible ~ p e c t r o m e t e r . ~ ~ ~ Other publications related to u.v.-visible spectrometry include a description of a modification to a Cary 15 spectrometer to allow simultaneous observations and exposure of biological materials to microwave radiation;21s a probe colorimeter using a green LED and photodiode for measurements at 560 nm;219 two portable spectrometers (400-1000 nm) to measure scattering and absorption in polar ice and snow;220and a description of the Perkin-Elmer model 356 spectrometer.221A new method for determining atomic oscillator strengths used resonance fluorescence emitted measurements.222A flow tube technique developed to study the kinetics of reaction of CS and O2 used a trombone-like movable injector to alter the time at which injected gas was in contact with the transient CS. In this case, CS concentrations were monitored by absorption at 257.6 nm with a D2larnp-~pectrometer.~~~ Absorption studies of materials under non-ambient conditions fall into categories of measurement at variable temperature,224constant low temperatures,226high temperatures and pressures,227low temperatures and high pressures,228and high A variable pathlength cell for studies of gaseous absorption at ambient temperature has also been described.230 Resonance absorption and related techniques have been used with great effectiveness to measure low concentrations of atoms, for example, fluorine (> loll atoms C M - ~ ) at 95 nm 231 by resonance absorption, sodium 232 using a tunable CW laser, caesium ( > 9 x lo7atoms) by intracavity laser quenching,233 and sodium-20 by the same technique.234A boxcar detection method (see p. 34) has been used in a measurement of Br, pressures prior to laser pulse dissociat i ~ n and , ~ ~an~ intracavity dye laser technique employed to study photodetachment of electrons from gaseous OH- in an ion cyclotron resonance ~pectrometer.~~~ 217 218 218 220

221

322 z2s

224

226

z26

227 228 229

230 2s1 z32 233 234 236

236

A. M. Ferendeu, Rev. Sci. Instr., 1974, 45, 1166. C. Burgess, F. R. Hartley, and V. Hughes, Lab. Practice, 1975, 24, 669. J. W. Ellis, C. M. Weil, and D. C. Janes, Rev. Sci. Instr., 1975, 46, 1344. T. Anfalt, A. Graneli, and M. Strandberg, Analyt. Chem., 1976, 48, 357. R. R. Roulet, G. A. Maykut, and T. C. Grenfell, Appl. Optics, 1974, 13, 1652. D. S. Botten, UV Spectrometry Group Bull., 1974 (2), 11. M. A. A. Clyne and L. W. Townsend, J.C.S. Faraday 11, 1974, 70, 1863. W. H. Breckenridge, W. S. Kolln, and D. S. Moore, Chem. Phys. Letters, 1975,32,290. R. Hocken, M. R. Moldover, E. Muth, and S. Gerner, Rev. Sci. Znstr., 1975, 46, 1699; E. Grimley and P. Gordon, J . Phys. (E), 1975,8, 1063. I. G. Dance and J. E. Cline, Chem. Instrumentation, 1975, 6, 319; R. D. Alexander, A. W. L. Dudeney, and R. J. Irving, J. Phys. (E), 1974, 7 , 522. L. Pross, J. Hemmerich, and K. Rossler, Rev. Sci. Znstr., 1976, 47, 353. E. Gallei and E. Schadow, Rev. Sci. Znstr., 1974, 45, 1504. J. S. Schilling, U. F. Klein, and W. B. Holzapfel, Rev. Sci. Znstr., 1974, 45, 1353. P. Laporte, Rev. Sci. Znstr., 1974, 45, 1386; E. K. Fleischmann, E. G. Conze, H. Kelm, and D. R. Stranks, ibid., p. 1427; W. J. Le Noble and R. Schlott, ibid., 1975,46, 770; B. Welber, ibid., 1976, 47, 183; R. K. Williams, ibid., 1975, 46, 250; 0. Kajimoto and R. J. Cvetanovic, J . Chem. Phys., 1976, 64, 1005. S. Sandroni and E. Brambilla, Appl. Spectroscopy, 1976, 30. 238. P. P. Bemand and M. A. A. Clyne, J.C.S. Faraday ZZ, 1976, 72, 191. G. M. Carter, D. E. Pritchard, and T. W. Ducas, Appl. Phys. Letters, 1975, 27, 498. W. J. Childs, M. S . Fred, and L. S. Goodman, Appl. Optics, 1974, 13, 2297. F. C. M. Coolen and H. L. Hagedoorn, J . Opt. SOC.Amer., 1975, 65, 952. F. J. Wodarczyk and P. B. Sackett, Chem. Phys., 1976,12, 65. J. R. Eyler, Rev. Sci. Znstr., 1974, 45, 1155.

20

Photochemistry

1.r. Spectrometry.-Errors in i.r. spectrometry are dealt with in part in a series of papers which deal with transmission measurements using thin cells and films and dispersion distortion.237 One of the most advanced commercial i.r. spectrometers (Perkin-Elmer model 580) which features ratio-recording electronics, integrated scan modes, an automatic wavenumber converter, and remote computer control illustrates the state-of-the-art in i.r. optical design and electronic processing.238 Experimental problems of measuring i.r. spectra of single surface monolayers have been examined, and some spectra of sterate films obtained with a specially-designed s p e c t r ~ r n e t e r . ~ ~ ~ 1.r. reflectance spectrometry is dealt with in several publications,240and an ellipsoidal mirror reflectometer for use between 2 and 34 pm has been described.241 Measurements of optical properties of thin adsorbed films have been made on an automated i.r. ellipsometer which makes use of two stationary polarizers that bracket a rotating polarizer and the reflecting surface.242 A comprehensive review on instrumentation and cells for optical spectroscopy at high pressures lays specific emphasis on i.r. and Raman applications,243Lowtemperature cells,244a variable path-length gas cell,245a cell with a sinall dead volume for studying adsorbed species,246and a temperature stabilizer for N,-flushed spectrometers 247 are examples of accessories for use in the i.r. Other i.r. applications include use of a tunable Pb-Sn-Se diode laser for highresolution studies ca. 667 cm-1;24san i.r. vapour analyser using a He-Ne laser as a source of 3.39 pm radiation to analyse ethanol vapour-air mixtures (from human subjects);249a double-resonance arrangement to study V- Y energy transfer to CO;2Kostopped-flow kinetic measurements between 1800 and 4000 cm-1;261 determination of the equilibrium constant for the reaction N204+ 2N02 using Fourier transform spectroscopy to measure the v 2 band of NO2 at 13.3 pm;252rapid scan analysis for the far4.r. (100 pm-2 mm) in a few ms by mounting the reflector of a Fabry-Perot interferometer on the cone of a loudspeaker;253and observation of new i.r. absorptions following proton-beam irradiation of Ar-CCI, mixtures.254 An i.r. analyser to measure CO, NO, SOz, HCI, and H F concentrations in chimney stacks relies on a gas filter-correIation technique to attain sensitivities 237

2sB

241 24a

243 244 a46 248

247

z4*

24D 261 a62

2ss

J. P. Hawranek, P. Neelakantan, R. P. Young, and R. N. Jones, Spectrochim. Acta, 1976,32A 75, 8 5 ; J. P. Hawranek and R. N. Jones, ibid., p. 99. R. Murton, Lab. Equip. Digest, 1975, 13(11), 53. J. F. Blanke, S. E. Vincent, and J. Overend, Spectrochim. Acta, 1976,32A, 163. H. G . Tompkins and D. L. Allana, Rev. Sci. Inst., 1974, 45, 1221; M. Ito and W. Suetaka, J . Phys. Chem., 1975, 79, 1190; R. J. Obremski, Industrial Research, 1976, 18(5), 59. B. E. Wood, J. G . Pipes, A. M. Smith, and J. A. ROUX,Appl. Optics, 1976, 15, 940. R. W. Stobie, B. Rao, and M. J. Dignam, Appl. Optics, 1975, 14, 999. J. R. Ferraro and L. J. Basile, Appl. Spectroscopy, 1974, 28, 505. J. Stokr, Z. Ruzicka, and S. R. Ekwal, Appl. Spectroscopy, 1974,28,479; B. 0 . Fowler anp E. C. Lambert, ibid., p. 591. R. L. Musselman and C. P. Nash, Appl. Spectroscopy, 1975,29, 527. J. Erklelens and W. J. Wosten, J. Phys. Q, 1974,7, 607. A. T. Collins, J. Phys. ( E ) , 1974, 7, 254. J. R. Aronson, P. C. von Thuna, and J. F. Butler, Appl. Optics, 1975, 14, 1120. T. A. A. Alobaidi and D. W. Hill, J. Phys. (E), 1975,8, 30. P. Brechignac, G . Taieb, and F. Legay, Chem. Phys. Letters, 1975,36,242. S . E. Brady, J. P. Maher, J. Bromfield, K. Stewart, and M. Ford, J. Phys. (El, 1976,9, 19. R. J. Nordstrom and W. H. Chan, J. Phys. Chem., 1976,80,847. D. S. Komm, R. A. Blanken, and P. Broissier, Appl. Optics, 1975, 14, 460. R. 0. Allen, J. M. Grzybowski, and L. Andrews, J. Phys. Chem., 1975,79, 898.

Developments in Instrumentation and Techniques

21

of 5-10 ~ . p . m Laser . ~ ~ ~resonant absorption methods have been used to determine C0,256C02,267 and 03.258 Absorption coefficients of NH3, C2Ha,and O3 have been determined with a C 0 2 laser,259of CHI, N20, C 0 2 , and HDO with a DF laser,26oand of NO in NO-N, mixtures with a CO laser.261 In the last case, optimum detection sensitivity occurred when the mixture was situated in a magnetic field which shifted the absorption wavelength into coincidence with a fixed laser wavelength. Stratospheric i.r. absorption spectra taken on airborne experiments with Concorde were used to measure NO, NO2, and HN03 concentrations.26a Two-photon Absorption Techniques.-Two-photon absorption experiments have increased considerably in number during the past few years, and high sensitivities have been reported when fluorescence was used for detection. Possible sources of error, due in part to the effect of spatial and temporal variations of the laser beam, have been 264 A new spectrometer relying on direct two-photon absorption (as opposed to two-photon excitation) uses two TEMoo laser beams propagating in opposite directions through a sample.263 A tunable pulsed dye laser generates a powerful beam which is crossed by a continuous krypton ion laser, providing a constant-frequency probe beam. The two-photon absorptivity of diphenylbutadiene at 29 259 cm-l reported with this spectrometer was as high as cm4s photon-l molecule-1, indicating the presence of a previously undetected -A, excited state as the lowest excited singlet state for this molecule. The two-photon absorption spectrum of biphenyl and bridged biphenyls in CCl, has been reported; direct absorption and excitation techniques were A highly polarized pulsed dye-laser pulse (covering the range 425-700 nm) was passed through a Glan polarizer and into a fluorescence cell. Scattered fluorescence was detected with a sodium salicylate-coated56AVP photomultiplier tube. The laser beam subsequently passed through Fresnel rhombs which caused the polarization of the light entering the second cell to remain linear or to become right-or-left circular. The second photomultiplier monitoring this cell was identical with the first. The beam was finally monitored with a flat response photodiode. These polarization measurements were used to allow the strong two-photon absorptions to be assigned without resort to any subsidiary vibrational analysis.264 Two-photon absorption measurements by excitation have also been made on crystalline [2,2]-para~yclophane,~~~ benzene and [2H6]benzene,266 and solids at 265 256 257

269

W. F. Herget, J. A. Jahnke, D. E. Burch, and D. A. Gryvnak, Appl. Optics, 1976,15,1222. R. G. Shortridge and M. C. Lin, Chem. Phys. Letters, 1975, 35, 146. R. T. Ku, E. D. Hinkley, J. 0. Sample, Appl. Optics, 1975, 14, 854. J. Shewchun, B. K. Garside, E. A. Bullik, C. C. Y . Kwan, M. M. Elsherbiny, G. Hogenkamp, and A. Kazandjian, Appl. Optics, 1976, 15, 340. R. R. Patty, G. M. Russwurm, W. A. McClenny, and D. R. Morgan, Appl. Optics, 1974,13, 2850.

260

2R1 263 26s

268

a6a

D. J. Spencer, G. C. Denault, and H. H. Takimoto, Appl. Optics, 1974, 13, 2855. P. A. Bonczyk, Rev. Sci. Instr., 1975, 46, 456. J.-C. Fontanella, A. Girard, L. Gramont, and N. Louisnard, Appl. Optics, 1975, 14, 825. R. L. Swofford and W. M. McClain, Rev. Sci. Znstr., 1975, 46, 246. R. L. Swofford and W. M. McClain, Chem. Phys. Letters, 1975,34,455. K. Fuke, S. Nagakura, and T. Kobayashi, Chem. Phys. Letters, 1975, 31, 205. L. Wunsch, H. J. Neusser, and E. W. Schaag, Chem. Phys. Letters, 1975,31,433; ibid., 1975, 32, 210.

22 Photochemistry low A technique for simple measurements of two-photon absorption coefficients involves comparison with a reference sample and use of a reference channel to eliminate any dependency on laser intensity or spatial and temporal profile.268 A technique called ‘optical-optical double resonance’ (OODR), in which barium oxide is sequentially excited by two lasers, is a special case of two-photon spectrometry involving excitation of a molecule via a real intermediate level.2sg Photoacoustic Spectroscopy.-Optoacoustic (or photoacoustic) spectroscopy has been mentioned previously (Vol. 5 , p. 226; Vol. 7, p. 82) in applications dealing with concentration measurements or the study of radiationless transitions in gases. Although this technique was discovered in 1881, its recent applicability to absorption spectra of solids, semi-solids, and living tissues has prompted one recent review to state that it ‘may prove to be one of the most important spectroThe reader is referred to recent reviews scopic innovations of the 1970’~’.~~O for the theoretical aspects of the technique 272 and illustrations of its 273 Since photoacoustic spectroscopy is unaffected by light scattering, it is possible to obtain absorption spectra from t.1.c. plates274and biological materials, including whole blood smears.275 Applications of this technique to the steady-state and time-resolved photochemistry of solids and light-scattering solutions are obvious. CW tunable laser radiation has replaced the usual xenon source in an arrangement to detect NO2 at concentrations of 10 ~ . p . b .High-sensitivity ~~~ measurements, reported for NH3,277 involved the photoacoustic detection cell incorporated in a dye-laser cavity. Carbon dioxide lasers are commonly used for photoacoustic spectroscopy of atmospheric pollutants in the i.r,, and an adaptation of this method allowed detection of low concentrations of explosive vapo~rs.~~* 2719

C.D. and M.C.D.-Circular dichroism (c.d.) spectrometers may not be reliable if they are calibrated at only one wavelength; a procedure has been described which determines the influence of instrumental parameters on c.d. Instrumental papers include a useful review on 0.r.d. and c.d. spectrometers,280 an arrangement to measure o.r.d., c.d., and absorbance,281a versatile spectrometer to measure both fluorescence and (natural and magnetic) c.d.,282and an In the last paper, polarization is i.r. (5000-350 cm-I) c.d. aa7

271

a78

276 s76 a77

278 279 280

281 282

28s

U. Fritzler, Ph. Keller, and G . Schaack, J. Phys. (E),8, 530. H. Lotem, J. H. Bechtel, and W. L. Smith, Appl. Phys. Letters, 1976, 28, 389. R. W. Field, G. A. Capelle, and M. A. Revelli, J. Chem. Phys., 1975,63, 3228. T. H. Maugh, Science, 1975, 188, 38. A. Rosencwaig, Physics Today, 1975,28(9), 23. J. G. Parker, Analyt. Chem., 1975, 47, 1189A. A. A. King and G. F. Kirkbright, Lab. Practice, 1976, 25, 377. A. Rosencwaig and S. S. Hall, Analyt. Chem., 1975, 47, 548. A. Rosencwaig, Science, 1973, 181, 657. A. M. Angus, E. E. Marinero, and M. J. Colles, Optics Comm., 1975, 14, 223. G . Stella, J. Getfand, and W. H. Smith, Chem. Phys. Letters, 1976, 39, 146. P. C. Claspy, Y . - H . Pao, S. Kwong, and E. Nodov, Appl. Optics, 1976,15,1506. A. Davidsson and B. Norden, Spectrochim. Acta, 1976, 32A, 717. K.-P. Wong, J . Chem. Educ., 1975, 52, A9, A83; ibid., 1974, 51, A573. D. V. Amato and G. W. Ewing, Analyt. Letters, 1974, 7 , 763. J. C. Sutherland, G . D. Cimino, and J. T. Lowe, Rev. Sci. Znstr., 1976,47, 358. I. Chabay and G. Holzwarth, Appl. Optics, 1975, 14, 454.

Developments in Instrumentation and Techniques

23

varied at 8 kHz by means of a germanium photoelastic modulator, and phasesensitive detection is used to obtain a precision of 4 x absorbance units at 3000cm-l. A kinetic method for following changes in c.d. with time has been used to follow structural changes which are known to occur in haemoglobin on A rapid the reversible photodissociation of CO bound to the modulator (50 kHz) altered the polarization of the analysing beam and a pulsed dye laser was used for photolysis. The system was reported to be sensitive to occurring in 1 ms. Rapid measurements of changes in polarization of 2 x optical rotation changes in the presence of large absorbance changes, described with a stopped-flow technique,285are obviously applicable to flash photolysis. In this case, the time resolution is related to sensitivity and is 10 ps at 0.018" sensitivity and 4 ms at 0.0016". A high-resolution c.d. spectrometer was constructed principally to observe the low-temperature m.c.d. spectra of crystals.2ss Use of a photoelastic modulator to provide alternatively right- and left-circularly polarized light in an m.c.d. spectrometer allowed data to be recorded continuously with wavelength.287 In this arrangement, a cavity-dumped tunable dye laser was synchronized to the modulator and gated signal amplification enabled weak dichroic signals to be recorded at a higher S/N ratio. Other papers include details of a magnetic circular polarization spectrometer which uses an Ar+ laser,288and modifications to a Jouan c.d. spectrometer for low-temperature m.c.d. measurements.289 The determination of linear dichroism (between 325 and 695 nm) on a modified single-beam spectrometer following flash excitation allowed orientation and spectral characteristics of chromophores in isolated cells to be recorded over a A photoelectric polarimeter has been described which time-scale of relies on synchronous detection of light signals modulated by a rotating p01arizer.~~~ Experiments with light-scattering materials showed that very weak fluxes of polarized light and light with a small degree of polarization (up to could be measured. 6 Preparative Techniques There is little doubt that laser-induced photochemistry is a potentially valuable method for enhancing isotopic abundances for many of the light elements and, in some cases, for selective decompositions or 293 An overview of laser processes with emphasis on the use of visible lasers to enrich chlorine isotopes 294 is a useful introduction to this technique. The three pre-requisites for separation of isotopic species by photochemical means are (i) selective 284

28b 286

287 288

28B 290

2e1 2ga

293 294

F. A. Ferrone, J. J. Hopfield, and S. E. Schnatterly, Rev. Sci. Instr., 1974, 45, 1392. M. Tsuda, Rev. Sci. Znstr., 1975, 46, 1419. J. C. Collingwood, P. Day, R. G. Denning, P. N. Quested, and T. R. Snellgrove, J. Phys. ( E ) , 1974, 7, 99 1. W. C. Egbert, P. M. Selzer, and W. M. Yen, Appl. Optics, 1976,15, 1158. B. C. Cavenett and G. Sowersby, J . Phys. ( E ) , 1975,8,365. R. E. Koning, R. M. E. Vliek, and P. J. Zandstra, J. Phys. ( E ) . 1975, 8, 710. F. I. Harris and E. F. MacNichol, J . Opt. SOC.Amer., 1974, 64, 903. C. Cortese, F. Aramu, and V. Maxia, Optics Cornrn., 1975, 15, 296. C. B. Moore, Accounts Chem. Res., 1973, 6, 323. E. Grunwald, and K. J. Olszyna, Laser Focus, 1976, 12(6), 41. M. Lamotte, H. J. Dewey, J. J. Ritter, and R. A. Keller, in ref. 29, pp. 153-164.

24

Photochemistry

excitation of the desired isotopic species, (ii) preferential reaction of this species with a scavenger, and (iii) separation of the photoproducts from the reaction mixture without the occurrence of appreciable isotopic scrambling. Irradiation of a reaction mixture of ICl and BrHC=CHBr causes a dramatic change in the 35Cl to 37Cl ratio corresponding to a 37Cl content of 85% in the transCIHC=CHCl p h o t o p r ~ d u c t .In ~ ~this ~ work, an intracavity absorption technique used an absorption cell containing 135Clinside the cavity of a broad-band dye laser, thereby removing those unwanted output frequencies that can excite 1 3 T l in an external reaction mixture, and requiring no prior knowledge of the high-resolution spectrum of the molecule being excited. Other examples include separation of 12C/13Cisotopes by selective predissociation of HCHO using a frequency-doubled dye laser;296separation of 14N/15Nand 12C/13Cisotopes in sym-tetrazine vapour with a CW dye laser,297and alteration of chlorine isotopic abundances of t h i o p h o ~ g e n e .Concentrations ~~~ of loB in mixtures of BCl, and H2S were increased from 19.5 to 29.2% following irradiation with a CO, laser.299 Highly selective isotopic separation of deuterium has been demonstrated in laser-irradiated formaldehyde mixtures of H 2 C 0 and HDCO,,OO using a CW He-Cd laser at 325.03 nm. In this case, the laser was locked to a frequency suitable for isotope separation by virtue of coincidence between the laser emission and a molecular absorption wavelength, avoiding possible frequency instability or drift problems. Enrichment of 104-foldfor 15N and 13Chas been achieved by irradiating sym-tetrazine in benzene at 1.8 K with an N,-laser-pumped dye laser.3o1 Although the technique is still in its infancy, preparative chemistry using lasers rather than thermal excitation could well prove to be an ideal method for synthesizing specific p h o t o p r o d u ~ t s .For ~ ~ ~example, a mixture of BC13 and H2 irradiated with a pulsed CO, laser yielded BHCI, and HCl as the only The reaction was characterized by a large quantum yield of about 122 photons at 10.6 pm per BHCI, molecule. Other boron compounds examined include H,B,PF, ,03 and B2H6.304Monochromatic CW radiation (1.5 W) at 10.6 pm initiates a chain reaction in B2H6, resulting in B20H16as the main Deuteriated methylene chloride (CD2C12)is selectively decomposed by 10 MW 1.r.-induced i.r. pulses of a CO, laser in the presence of methylene changes in CH,OH-Br, mixtures have been monitored in an i.r. spectrometer following irradiation with a 50 mJ pulsed HF laser and a 650 W tungsten lamp.3o6 Quantum yields of only a few solid-state photochemical reactions have been determined because of experimental difficulties in measuring product yields, absorbed dose, etc. Quantum yields may be determined from reflectance measure296 286

297

300

301 302 303 304 305

306

s. Datta, R. W. Anderson, and R. N. Zare, J. Chem. Phys., 1975, 63, 5503. J. H. Clark, Y. Haas, P. L. Houston, and C. B. Moore, Chem. Phys. Letters, 1975, 35, 82. R. R. Karl and K. K. Innes, Chem. Phys. Letters, 1975, 36, 275. M. Lamotte, H. J. Dewey, R. A. Keller, and J. J. Ritter, Chem. Phys. Letters, 1975, 30, 165. S. M. Freund and J. J. Ritter, Chem. Phys. Letters, 1975, 32, 255. J. B. Marling, Chem. Phys. Letters, 1975, 34, 84. R. M. Hochstrasser and D. S. King, in ref. 29, pp. 177-182. S. D. Rockwood and J. W. Hudson, Chem. Phys. Letters, 1975, 34, 542. E. R. Lory, S. H. Bauer, and T. Manuccia, J. Phys. Chem., 1975,79, 545. H. R. Bachman, H. Noth, R. Rinck, and K. L. Kompa, Chem. Phys. Letters, 1974, 29, 627. A. Yogev and R. M. J. Benmair, J. Amer. Chem. SOC.,1975, 97,4430. C. Willis, R. A. Beck, R. Corkum, R. F. McAlpine, and F. K. McClusky, Chem. Phys. Letters, 1976, 38, 336.

Developments in Instrumentation and Techniques

25

ments, and equations have been derived to describe the rate of a photochemical reaction of an infinitely thick sample.3o7A theoretical paper has outlined photochromic conversion in optically thick samples.3o8A more practical use of solidstate reactions is preparative photochemistry at low temperatures to synthesize highly reactive molecules 309 or reduce secondary reactions. Optically active 1,2-dithiane is produced in a hydrocarbon glass at 77 K by photolysis with circularly polarized light.310 Irradiation in a solid matrix at low temperatures minimized spontaneous racemization. A static irradiation system has been used for studies on the photo-oxidation of SO, as a function of time in which the extent of reaction was monitored Both static and flow systems were using the Raman bands of SO, and so3.311 employed to investigate product formation following irradiation of SO, and SO, with NO, COz, and 0, at 313 nm and between 370 and 400 nm.312 Other flow systems were used in a direct NO, photolysis rate meter 313 and for photolysis Decomposition of organic mercury compounds by of CH4-H2 irradiation with low-pressure Zn, Cd, or Hg lamps was carried out in a toroidal irradiation cell placed around the lamp.315 An apparatus for vacuum-u.v. photolysis of gases using Ar, Kr, and Xe resonance lamps has been described.316 High densities of O(lS0) produced by the vacuum-u.v. photolysis of NzO immersed in high-pressure argon were used to produce Ar0.317 Excitation into various portions of the T*-T absorption (at 166.5 nm) of fluoroethylene in low temperature doped matrices was effected with N and Br resonance lamps.318 Effective stirring in the ‘merry-go-round’ type of photochemical reactor is necessary because the incident light is often absorbed with a thin layer. One proposed solution is a magnetic device which causes Teflon-coated magnetic bars in the irradiation tubes to perform up-and-down motion with intermediate trembling phases.319 Glass filters for this photoreactor have also been An all-glass system with two reservoirs containing reactants which may be mixed in accurate amounts is a useful reaction vessel for fluorescence and other measurements at various concentration^.^^^

7 Light Detection and Measurement Photodiodes.-The instabilities of silicon photodetectors when they are exposed to near-u.v. radiation is attributed in part to a decrease in uniformity of response 307

3u8

nio 311

312 313

314

315

s16 317 318

330

321

E. L. Simmons and W. W. Wendlandt, J. Phys. Chem., 1975,79, 1158; E. L. Simmons, Appl. Optics, 1976, 15, 95 1 . W. J. Tomlinson, Appl. Optics, 1976, 15, 821. 0. L. Chapman, Pure Appl. Chem., 1974, 40, 511. B. Nelander and B. Norden, Chem. Phys. Letters, 1974,28, 384. P. A. Skotnicki, A. G . Hopkins, and C. W. Brown, J . Phys. Chem., 1975, 79, 2450. K. Chung, J. G. Calvert, and J. W. Bottenheim, International J. Chem. Kinetics, 1975,7, 161. J. 0. Jackson, D. H. Stedman, R. G. Smith, L. H. Hecker, and P. 0. Warner, Rev. Sci. Instr., 1975,46, 376. J. P. Merris and C. T. Chen, J . Amer. Chem. SOC.,1975, 97, 2962. A. M. Kiemeneij and J. G . Kloosterboer, Analyt. Chem., 1976,48, 575. J. Gawlowski and J. Niedzielski, Rocz. Chem., 1974, 48, 1085. W. M. Hughes, N . T. Olson, and R. Hunter, Appl. Phys. Letters, 1976. 28. 81. W. A. Guillory and G. H. Andrews, J . Chem. Phys., 1975, 62,4667. R. Straatmann and H. J. Kuhn, Mol. Photochem., 1976, 7 , 203. B. F. Plummer, Mol. Photochem., 1974, 6, 241. A. R. Watkins, Mol. Photochem., 1974, 6, 325; J. Phys. Chem., 1974, 78. 2555.

26

Photochemistry

across the cell surface and enhancement of the detector response after prolonged irradiation.322 Properties of four commercial U.V. photodiodes have been outlined,323and graphs giving unilluminated noise current of photodiodes under various bias conditions have been presented with details of a low noise (< A Hz-4) preamplifier developed for these Two diodes originally used for pulse radiolysis (E.G. and G. type SHS-100 silicon diode and Barnes A-100 InAs diode) have been assessed for high-output current The former has a linear output up to at least 6.5 mA, a &98% response time of 15 ns and is distinguishable from many Si diodes in not having a slow component to its response. The A-100 diode can be used for absorption measurements between 450 and 3200 nm, has an output linear to at least 2 mA and a &98% response time of about 60 ns.325 The multiplication of a photocurrent in reverse-biased p-n junctions can be regarded as the solid-state analogue of photomultipliers. In both cases, photons can trigger avalanches leading to electrical pulses in an external circuit. Alloyed Ge diodes, slightly biased above breakdown voltages at 77 K, have been shown to produce self-sustaining avalanche pulses for individual photons in the range 1.2-1.8 pm with a risetime of less than 1 ns.326 Photomultipliers are usually insensitive above about 1 pm, although there is a recent report of an Ag-O-Cs photocathode with an enhanced i.r. sensitivity (threshold 1.25 pm) and average luminous sensitivity of about 30 PA lm.327In the near-i.r., high-sensitivity InSb detectors 328 and PbSnTe have been discussed and a new class of detectors based on LaF, has been An ultralow noise voltage preamplifier and bias current supply for the CdHgTe photodetector has been described.330 One interesting application of the quasimetallic photoconductivity produced in silicon transmission lines by mode-locked laser pulses are electronic gates and switches which can be turned on and off in a few picoseconds. Electrical signals as large as 100 V can be switched by a few pJ of optical energy.331 Mode-locked pulses have also been used to investigate the performance of a fast vacuum ph~todiode.~~~ Photodiode arrays (discussed earlier in Vol. 6 , p. 91) have been reassessed in an authoritative article which points out their limitations in terms of restricted sensitivity (compared to photomultiplier tubes) and wavelength coverage (with reasonable spectral resolution) and comments that effective and inexpensive solutions are not close at hand.333 Nevertheless, a self-scanning semiconductor array in the electron bombardment induced mode has been shown to be capable a23

M. A. Lind and E. F. Zalewski, Appl. Optics, 1976, 15, 1377. F. W. Duncan, Laser Focus, 1974,10(8), 41 ;E. Danahy, Electro-optical Systems Design, 1976,

826

W. H. Havens, Appl. Optics, 1974, 13, 2209. G. G. Teather, N. V. Klassen, and H. A. Gillis, International J. Radiation Phys. Chem., 1976,

a22

8(4), 12. 8,477. ax6

a23

328 330 831

W. Fichtner and W. Hacker, Rev. Sci. Znstr., 1976, 47, 374. M. Srinivasan, B. M. Blat, and N. Govindarajan, J . Phys. ( E ) , 1974, 7, 859. D. N. B. Hall, R. S. Aikens, R. Joyce, and T. W. McCurnin, Appl. Optics, 1975, 14. 450: R. F. Leftwich, Industrial Research, 1976, 18(1), 91. A. Sher, C. L. Fales, and J. F. Stubberfield, Appl. Phys. Letters, 1976, 28, 676. W. G. Gore and G. W. Smith, J. Phys. ( E ) , 1974,7, 644. D. H. Auston, Appl. Phys. Letters, 1975, 26, 101. B. Sipp, J. A. Miehe, and G. Clement, J. Phys. (E), 1975, 8, 298. G. Horlick, Appl. Spectroscopy, 1976, 30, 113.

Developments in Instrumentation aizd Techniques

27

of detecting single electrons and predicted to have a sensitivity close to the theoretical limit if incorporated in a photoelectronic Reviews on arrays and their applications 335 discuss the mechanisms of charge storage. Photodiode arrays have been used in a rapid scanning spectrometer 336 and to measure the beam cross-section, two-photon fluorescence, and Kerr cell photographs of single picosecond Photomultipliers.-A useful review of high-sensitivity photomultipliers has appeared which includes a discussion of pulse versus d.c. detection in a publication aimed at astrophysical applications 338 but which is relevant to fluorescence spectrometry and fluorometry. Transit time fluctuations of photoelectrons in the cathode-first dynode space due to their initial kinetic energy spread form one of the fundamental limitations of time resolution of photomultipliers used in single photoelectron time measurements, A synchrotron radiation source (which provides a continuum with a duration independent of photon energy) has revealed a considerable wavelength Irradiation of the central 2 mm part of this effect with the RCA 8850 photomultiplier with this light produces an increase in risetime from 0.4 to 0.55 ns as the wavelength is reduced from 550 to 350 nni.339 The main factor contributing to this time spread with incident wavelength is the difference in path lengths for secondary electrons emitted at the first dynode. Dark noise is the principal limitation to sensitivity to photomultipliers and is especially significant with red-sensitive tubes where the dark noise is greatly increased by the high level of thermionic emission from the cathode. Noise reduction may be accomplished by cooling the photomultiplier 340 or by magnetic d e f o c u ~ i n g .In ~ ~the ~ latter case, an external magnetic field will deflect electrons from the edge of the cathode away from the electron multiplier (in an end-on tube such as the RCA 8850 and 8852). Although significant improvements in dark count can be found for these tubes, the usefulness of the magnetic defocusing technique is limited by the production of a ‘fringe’ area on the photocathode in which the overall counting efficiency varies rapidly with position and in which the pulse height distribution is degraded. Typical noise levels of 5 C.P.S. with a cathode area of 1 cm2 were obtained with the 8852 tube at -25 “C at a counting efficiency of 0.91.341A number of reports have been made of enhanced dark currents induced by short bursts of light. Incident irradiances as low as 0.14pWcm-2 (at 605 nm) and lasting only a few microseconds produce a significant enhancement in dark noise of a 56TUVP tube in the few hundreds of microseconds following removal of the signal from the cathode.342 Quantitive S. B. Mende and E. G. Shelley, Appl. Optics, 1975, 14, 691. M. A. Venn, Optics andLaser Technology, 1974,6,209; P. W. Fry, J. Phys. (E), 1975,8,337. 3313 D. A. Yates and T. Kuwana, Analyt. Chem., 1976,48, 510. 337 W. Seka and J. Zimmermann, Rev. Sci. Instr., 1974, 45, 1175. s3* A. T. Young, in ‘Methods of Experimental Physics’, ed. N. Carleton, Vol. 12, part A, Academic, New York, 1974. 33s B. Sipp, J. A. Miehe, and R. Lopez-Delgado, Optics Comm., 1976, 16, 202. 9 4 0 S. Benci, P. A. Benedetti, and M. Manfredi, Appl. Spectroscopy, 1974, 13, 1555; G. C. King, R. E. Imhof, and A. Adams, J. Phys. (E), 1974,7, 882; S. D. Hoyt and J. D. Ingle, Analyt. Chem., 1976, 48, 232. 841 P. B. Coates, J . Phys. (E), 1975, 8, 614. sra R. E. W. Pettifer and P. G. Healey, J. Phys. (E), 1974, 7 , 617. 834

3aC

28

Photochemistry

measurements have also been made with the EM1 9558 Fluorescence and phosphorescence in photoinultiplier window materials induced by charged particle irradiation (e.g. the radiation environment of space) can be significant and alter dark count rates. The phosphorescence decay can last from several minutes to several Afterpulse spectra of a photomultiplier tube used for photon counting have been determined with a simple experiment technique.345Fatigue characteristics of RCA 8850, 8852 and EM1 9558, 9658 and 9659 tubes have been investigated at low anode currents, and stabilization of photomultiplier gain was suggested as one solution to combat fatigue Measurements of the variation in sensitivity and reflection coefficients of the photocathode of a 9558 tube have been made with changes in the angle of the incident light beam.347 Operation of a 1P28 tube with only four dynodes and an interdynode voltage of 250 yielded a risetime of only 360 ps with a linear range in anode current extending to 8 mA.348 This performance should be compared to that of fast photodiodes (risetimes typically 100 ps) and a recently reported gated crossed-field photomultiplier (risetime 1 ns).349 A further investigation of the quantum efficiency of sodium salicylate showed it to be constant (to within +8%) from 10 to 60 nm, enabling the material to be used with a photomultiplier as a wavelength converter in the vacuum-u.v.350 Pulsed lasers and light sources are often used in luminescence and absorption studies. Since the duty cycle of these sources is often low, the detection electronics should only respond for a short time interval around this pulse in order to improve the S/N ratio. For example, the output of an N,-laser-pumped dye laser might consist of 5 ns pulses at a repetition rate of 10 Hz. With photomultiplier risetimes of a few ns, photon counting could not be used except at very low light levels (less than about 1 photoelectron per pulse) without avoiding pulse pile-up. It is, therefore, necessary to integrate the charge from the photomultiplier and use a gating technique. Even a slow gate (e.g. 10 ps operating at 10 Hz) will reduce the background noise from a typical low-noise, high-gain tube to 1 photoelectrons-l. A simple analogue gate has been described and used in combination with an electrometer/ratemeter to measure small pulsed signals in the presence of a large d.c. photoelectron background.351 A similar principle (using a 100 ps gate at 50 Hz) is employed in a boxcar integrator and filter fluorimeter (Section 8). Photomultipliers in liquid scintillation counters are a particular source of instability due to variation of gain with temperature, tube current, and aging. An interesting paper has described the use of a small light-emitting diode placed directly in front of the photomultiplier and in the line of the normal collection 343

D. P. Jones and G. C. Kent, J. Phys. ( E ) , 1974,7,744. W. Viehmann, A. G . Eubanks, G . F. Piper, and J. H. Bredekamp, Appl. Optics, 1975, 14, 2 104.

348 347 348 349 360

361

J. S. Gethner and G . W. Flynn, Rev. Sci. Znstr., 1975,46, 586. P. B. Coates, J . Phys. ( E ) , 1975, 8, 189. D. P. Jones, Appl. Optics, 1976, 15, 910. G. Beck, Rev. Sci. Instr., 1976, 47, 537. A. Kono and S. Hattori, Appl. Optics, 1974, 13, 2002. J. A. R. Samson and G. N. Haddad, J. Opt. SOC.Amer., 1974,64,1346. H. Rosen, P. Robrish, and G. Jan de Vries, Rev. Sci. Instr., 1975, 46, 1115.

Developments in Instrumentation and Techniques

29

optics to provide a reference beam and feedback to the gain There are obvious applications of the principle in both absorption and fluorescence spectrometry. Direct measurement of In(I,/&) in kinetic spectrophotometric determinations of atomic species without imposing limitations on time resolution has been achieved by use of a rapid response logarithmic circuit connected to the output of a solar blind photomultiplier Devices to measure photomultiplier current gain of side-window tubes 354 and simultaneous determination of both anode current and pulse counting 355 have been described. Among other circuits for photomultipliers are those for gating an ll-stage tube in a time of a and for providing an analogue divider from few ps,356for digitizing the two photometric signals.358

Other Photodetectors.-Subpicosecond time resolution from the vacuum-u.v. to near4.r. has been realized with a new design of streak camera which has a photocathode extraction field of 20 kV cm-l and provides a spatial resolution of 10-18 line pairs 111rn-l.~~~ Streak cameras have been used to record a single (30 ps) pulse from an Nd:YAG laser using a mosaic tube to record the resident luminosity of the phosphor 360 and the width of the fine structure pulse from a linac by observing associated cerenkov radiation.361 Applications in picosecond flash photolysis are given in Section 9. Sensitization and desensitization of silver halide films have enabled i.r. photography at 5 and 10 pm to be achieved with a linear resolution of 6-8 line pairs 11lm-l.~~~ Polaroid type 57 positive film sensitized indirectly with a fluorescent overlay has been shown to be a useful detector from 650 nm to the v a c u u m - u . ~ .Loss ~ ~ ~ of reciprocity has been reported with Kodak i.r. film type 2481 exposed to ruby laser pulses compared to conventional red light.364 Radiometry and Actinometry.-A hydrogen arc operated at 20 000 K has been used as a primary standard source of spectral radiance between 124 and 300 nm to an accuracy of k 5%,365and a low-pressure variation used to provide absolute radiometric calibrations between about 120 and 170 nm to an absolute accuracy of k Black-body radiation of 12 000 K f 15% is provided by a 1 ms discharge lamp which provides a standard continuum from 260 nm to the visible E. Soini, Rev. Sci. Znstr., 1975, 46, 980. D. Husain and A. N. Young, J.C.S.Faraday ZZ, 1975,71, 525; I. S. Fletcher and D. Husain, Chem. Phys. Letters, 1976, 39, 163; D. Husain, S. K. Mitra, and A. N. Young, J.C.S. Faraday IZ, 1974, 70, 1721. 364 N. W. Bower and J. D. Ingle, Analyt. Chem., 1975, 47, 2069. 366 R. L. Klobucher, J. J. Ahumada, J. V. Michael, and P. J. Karol, Rev. Sci. Znstr., 1974, 45, 1071. M. Yamashita, Rev. Sci. Instr., 1974, 45, 956. 867 F. M. Hamm and P. B. Zeeman, Appl. Spectroscopy, 1976, 30, 70. A. M. Ferendeci, Rev. Sci. Instr., 1974,45, 1166. 369 D. J. Bradley and W. Sibbett, Appl. Phys. Letters, 1975, 27, 382. s60 L. A. Lompre, G. Mainfray, and J. Thebault, Appl. Phys. Letters, 1975, 26, 501. 361 G. S. Mavrogenes, C. Jonah, K. H. Schmidt, S. Gordon, G. R. Tripp, and L. W. Coleman, Reo. Sci. Instr., 1976, 47, 187. 86a G. F. Frazier, T. D. Wilkerson, and J. M. Lindsay, Appl. Optics, 1976, 15, 1350. R. Engleman and L. J. Radziemski, Appl. Optics, 1975, 14, 2821. w4 R. 0. Rice and J. D. Macomber, J . Opt. SOC.Amer., 1975, 65, 1489. 366 W. R. Ott, K. Behringer, and G. Gieves, Appl. Optics, 1975, 14, 2121. 368 W. G. Fastie and D. E. Ken, Appl. Optics, 1975, 14, 2133. 36a 868

30

Photochemistry

without superimposed absorption or emission lines from 2ny windows.367 A simple Pen-Ray low-pressure Hg lamp has been shown to be a suitably stable source for calibration at 253.7 nm of a standard p h o t o i n ~ l t i p l i e r . ~Other ~~ calibrated detectors cover the range 20-60 nm 369 and 135-300 nm.370 In the latter case, a comparison was made against a thermopile of the U.V. flux from a monochromator and the total flux from a black-body simulator with an overall uncertainty of 20%. Calibration of U.V. standard lamps which emit a line spectrum (e.g. Hg) was based on comparison with the output of a synchrotron whose relative spectral intensity is A new approach to radiant power measurenients used a continuously tunable CW dye laser to measure the absolute spectral response of a silicon photodiode and narrow bandpass filter by comparison with an electrically calibrated pyroelectric detector. The filtered photodetector was subsequently used to measure the spectral power density of a standard lamp previously calibrated by a classical technique.372 Pyroelectric detectors are finding more and more use for radiometric measurements, including 3 ns pulses in the U.V. to visible with powers from a few pW to mW 373 and as a reference in a commercial fluorescence Accurate measurements of radiative flux require that measurements are independent of the way in which the flux enters the detector (because of nonuniform sensitivity). Integrating spheres are commonly used to present polychromic light to a detector, and a new design has been described covering the range 200-3000 nm which uses a powdered fluorocarbon (G-80 Halon manufactured by Allied Chemical, U.S.P. 3 764 363) as a reflecting wall coating. The sphere transmittance is 0.32 at 200 nm, rising rapidly with wavelength to a nearmaximum theoretical value of 0.56 over the remainder of the range.374 The reflectance of the fluorocarbon varies from 0.93 at 200 nm to 0.99 at 400 nm, and it exhibits some fluorescence 375 which must be considered when using u.v.-rich sources. Other reflective materials proposed include potassium ~ ~ the ~ latter case, U.V. sulphate-barium sulphate 376 and barium ~ u l p h a t e . In irradiation has been shown to have a deleterious effect on impure material. A new system for absolute total diffuse transmission and reflection measurements used two integrating spheres, one coated with BaSO, for use between 185 and 2000 nm and the other coated with sulphur flowers for use between 1.5 and 12 pm.378 There is a continuing need for reliable actinometers in the vacuum-u.v., despite the common use of N 2 0 and CO,. Physical radiometry at 147 nm relies on the photoionization quantum yields of aliphatic amines and measurement of 3749

367 368 3eB 370

371 372 373

374 376 376

377 378

K. Guenther and R. Radtke, J. Phys. (E), 1975, 8, 371. G. Marette, G. Jegoudez, H. Poncet, and J. P. Lepeltier, Optics Comm.,1976, 16, 149. E. B. Saloman and D. L. Ederer, Appl. Optics, 1975,14, 1029. G. Marette, Appl. Optics, 1976, 15, 440. H. Kaase, J . Phys. (E), 1975, 8, 590. J. Geist, B. Steiner, R. Schaefer, E. Zalewski, and A. Corrons, Appl. Phys. Letters, 1975,26, 309. J. Geist, H. J. Dewey, and M. A. Lind, Appl. Pftys. Letters, 1976, 28, 171. K. L. Eckerle, W. H. Venable, and V. R. Weidner, Appl. Optics, 1976, 15, 703. R. D. Saunders and W. R. Ott, Appl. Optics, 1976, 15, 827. J. B. Schutt, J. F. Arenas, C. M. Shai, and E. Stromberg, Appl. Optics, 1974, 13, 2219. W. Erb, Appl. Optics, 1975, 14, 493. W. G. Egan and T. Hilgeman, Appl. Optics, 1975,14, 1137.

Developments in Instrumentation and Techniques

31

saturation Although this is no more accurate than the chemical method ( + 5 % ) , it is more convenient and precise. Photoreaction should be suitable for a convenient and exact chemical actinometer if its rate law is easy to calculate and if the time-dependent change of product concentration can be measured in a simple way with high accuracy. The former can be determined for the photoisomerization of azobenzene in methanol by application of the ‘linear interpolation method’ and the latter by U.V. Advantages of this method which have been proposed include insensitivity to oxygen and preirradiation with stray light, and lack of extensive sample preparation. Further work is clearly needed to check these claims. The now-classical ferrioxalate actinometer has been shown to be suitable for measuring N2laser pulses 381 but to be unsuitable for use above 570 nm, even with intense laser pulses, because of two-photon-induced photo~hernistry.~~~ Decafluorobenzophenone in propan-2-01 solution (proposed as an actinometer in Chern. Cornm., 1970, 1413) has been found to exhibit a decomposition which depends on both wavelength and light and is not a suitable a ~ t i n o m e t e r33* .~~~~ Good evidence has been presented to justify use of a chemical actinometer instead of the more usual photoelectronic methods to measure both CW and pulsed laser intensities.385 The chemical reaction involved in this actinometer is the tris-(2,2’-bipyridine)ruthenium(11) sensitized photo-oxidation of tetramethylethylene (TME) in methanol. Radiation is absorbed by the intensely orange-coloured ruthenium compound which is quenched by dissolved oxygen to form lo2which, in turn, is consumed by the TME. Oxygen uptake is read directly from a burette at an efficiency of 0.76 moles O2 per Einstein. The observed quantum yield is constant to within 5% over the range 280-560 nm. Although the upper wavelength limit is relatively low for laser work, the authors suggest use of other metal complexes or organic dyes which they predict will extend the range to > 1000 nm.386 An electronically integrating actinometer uses a beam splitter to direct part of a U.V. beam to be measured to a quantum counter cell and part through a sample cell onto a second quantum counter cell.386 Silicon photodiodes are used to record the light output of the two fluorescent cells. A comparison with ferrioxalate actinometry showed good agreement at wavelengths of 254, 280, 313, and 366nm. Possible sources of error not discussed by the authors include corrections for reflected light in the sample cell (Vol. 5, p. 96), changes of penetration with wavelength (and hence solid angle observed by the silicon detector) for the quantum counter (see ref. 405), and sample fluorescence. In a report on the sensitivity of bacteria to the U.V. component of sunlight, mention was made of a sun-burning U.V. meter which uses a magnesium tungstate a80 a81

a8s a84

a8a

D. Salomon and A. A. Scala, J. Chem. Phys., 1975, 62, 1469. G. Gauglitz, J. Photochem., 1976, 5, 41. E. I. Aleksandrov and T. A. Lopasova, Kuantouaya Electron (Moscow), 1974,1, 1464. H. Zipin and S. Speiser, Chem. Phys. Letters, 1975, 31, 102. G. Gauglitz and U. Kolle, J. Photochem., 1975, 4, 309. P. Margaretha, J. Gloor, and K. Schaffner, J.C.S. Chem. Comm., 1974, 565. J. N. Demas, E. W. Harris, and R. P. McBride, in ref. 29, pp. 477-484. W. Amrein, J. Gloor, and K. Schaffner, Chimia, 1974, 28, 185.

32

Photochemistry

phosphor with a response approximating that of the erythema action The light emitted by the phosphor was detected by a phototube which produced a proportional electrical signal. A more conventional instrument was used to record the ratio of diffuse to direct solar irradiances in the middle U . V . ~ ~ ~ Miscellaneous.-Information gleaned from numerous other references to photodetectors and their applications include work on the deposition of an s-1 (Ag-0-Cs) type photocathode onto a diffraction grating substrate to provide a significant enhancement in quantum efficiency at pre-selected frequencies for specified light angles of incidence ;389 quantum efficiency measurements of electron multipliers between 120 and 240 nm;390and a report that the human eye can respond to radiation at wavelengths at least as far as 1064 nm. In the latter case, continuous 1064nm laser light appeared red but a 1060nm pulsed laser source appeared green, which suggested the occurrence of second harmonic generation in the retina.391 An electro-optic technique has been used to study the flash discharge products of N2-C mixtures. CN radicals were examined in emission, and a 20 nm portion of the spectrum was dispersed by a Czerny-Turner spectrograph. This spectrum was focused onto the photocathode of a three-stage image intensifier prior to a recording of the intensified image with film. This arrangement preserved the nm) and photometric accuracy with greatly increased resolution limit (5 x speed over classical photographic techniques.392 Critical parameters of image intensifier tubes are included in a review of image converters and intensifiers.393 Other topics reviewed include TV-type multichannel detectors and their application in spectroscopy 394 and the SSR optical multichannel a n a l y ~ e r . ~ ~ ~ A theoretical study has indicated that optoelectronic sampling of photoelectrons would be capable of measuring time-resolved periodic low-level incoherent optical signals with risetimes of a few p i c o ~ e c o n d ~ The .~~ sensitivity ~ of PbSe epitaxial films to atomic hydrogen was used to detect the photodissociation of H,S by U.V. radiation.397 A room-temperature photodichroic material (NaF crystal) allows information to be recorded at 300-365 nm and read at 514 nm.398 Although the sensitivity (100 mJ cm-2) of the crystals is not comparable with film, this is a reversible process and high resolution is attainable. Amplifiers, displays, and transmission lines used for measuring fast optical signals (> 100 ps, < 100 ns) have been reviewed.39B 387 388 s8g

381

3D3 394

395 Sg6 397

303

3DB

D. Billen and A. E. S. Green, Photochem. and Photobiol., 1975,21, 449. A. T. Chai and A. E. S. Green, Appl. Optics, 1976, 15, 1182. J. G. Endriz, Appl. Phys. Letters, 1975, 25, 261. F. Paresce, Appl. Optics, 1975, 14, 2823. D. H. Sliney, R. T. Wangemann, J. K. Franks, and M. L. Wolbarst, J. Opt. SOC.Amer., 1976, 66, 339. M. Bridoux, B. Dessaux, J. M. Selle, and H. Tourbez, Canad. J. Spectroscopy, 1975, 20, 42. P. Schagen, J . Phys. (E), 1975, 8, 153. Y. Talmi, Analyt. Chem., 1975, 47, 658A. D. E. Olten, Industrial Research, 1975, 17(10), 82. J. J. Wiczer and H. Merkelo, Appl. Phys. Letters, 1975, 27, 397. J. T. Young and J. N. Zemel, Appl. Phys. Letters, 1975, 27, 455. D. Casasent and F. Caimi, Appl. Optics, 1975, 15, 815. F. E. Lytle, Analyt. Chern., 1974, 46, 817A.

Developments in Instrumentation and Techniques

33

8 Fluorescence and Phosphorescence Spectrometry The popularity of fluorescence and phosphorescence analysis is illustrated by the 1200 references contained in a biennial review covering the period 19731975.400 Although many of the references covered applications in organic and inorganic chemistry and biology, instrumentation, methods, and techniques illustrated the increasing use of lasers as excitation sources.

U.v.-Visible Fluorescence Spectrometry.-It is worrying that nearly all commercial and many home-made fluorescence spectrometers have such poorly designed excitation optics that correct excitation spectra (which at low concentrations should mimic singlet absorption spectra) would not be obtainable over any moderate wavelength range. The principal cause of this defect is use of nonachromatic lens systems, which can produce appreciable changes in focal length with wavelength, especially in the U.V. Even certain types of mirror system are not fully corrected for optical aberrations. Good image quality (within limits) may be obtained with an in-line Cassegrainen arrangement which uses a concave mirror (with a central hole) and a convex Two fluorescence spectrometers using this arrangement have been described. In one designed for highly concentrated or light-scattering solutions, fluorescence is collected from the dark cone within the extension of the converging excitation beam.40aIn this way, no excitation light can reach the analysing system although scattering requires the use of low-light-scattering monochromators (double-prism). Extensive details of the design of an absolute fluorescence spectrometer employing Cassegrainentype reflecting objectives 403 have been given in a description of an optically fast fluorescence spectrometer. Distribution of fluorescence around the cuvette could be obtained by mounting the emission monochromator on a goniometer arm. Light losses with a Cassegrainen system due to the central obstruction of the beam, however, can amount to 28%,exclusive of reflective losses, which could not be tolerated in any fast spectrometer. The other principal defect in spectrometers provided with a corrected excitation arrangement is the use of a quartz plate beam-splitter, because of changes in reflectivity and polarization with wavelength. One method of eliminating this is to use a rotating mirror which alternatively irradiates a quantum counter and allows excitation light to be reflected from a stationary mirror before exciting a sample.4o4 A more practicable static arrangement is to deflect a small and representative part of the excitation beam to the quantum counter cell. Changes in penetration depth (and hence light collection) and polarization with wavelength for rhodamine B quantum counters have led to the development of a mushroomshaped cell.405Passage of the excitation light through a long cylindrical entrance tube of dye ensured that only a central cone of rays passed straight through. The hemispherical rear portion avoids losses due to any diverging effect and bends some of the rays towards the detector. A comparison of the mushroom cell with a 400

(01 40a

*OS

404 406

C. M. O’Donnell and T. N. Solie, Analyt. Chem., 1976, 48, 175R. K. D. Mielenz, Appl. Optics, 1974, 13, 2931. A. Bierzynski and J. Jasny, J. Photochem., 197415, 3, 431. K. D. Mielenz, Appl. Optics, 1974, 13, 2581. W. H. Melhuish, Appl. Optics, 1975, 14, 26. E. D. Cehelnik and K. D. Mielenz, Appl. Optics, 1976,15,2259.

34

Photochemistry

1 cm cell showed that strong deviations as a function of wavelength of the ratio of normalized spectra between 260 and 600 nm to 545 nm (maximum of strongest absorption band) were considerably reduced with the former celLPo5 Two recently introduced commercial fluorescencespectrometers use differential techniques to remove any signal from a blank solution or solvent. The PerkinElmer model 512 alternately presents fluorescence signals from two samples to a single photodetector, obtaining any differencee l e c t r ~ n i c a l l y .A~ commendable ~~ reference system in the Aminco model SP500 used a pyroelectric detector which is reported to have a wider range than a quantum counter (220-800 nm with a response flat to 3%) and to be faster and less sensitive to ambient temperature changes than a Surprisingly, neither instrument incorporates photon counting or digital electronics for processing. A double-beam, singlephotomultiplier fluorescence spectrometer has been suggested as a means of improving the stability of a normal two-detector instrument to better than k 1% over a 10 min period.408In this case, a rotating mirror was used to deflect excitation light alternately on to a scatterer or quantum counter. Signal Processing.-Computer processing of the output of the high-resolution luminescence spectrometer described earlier (Vol. 5, p. 103) enables wavelength calibration, dark count correction data smoothing, and curve presentation to be made as well as corrected emission spectra by reference to a standard lamp.4o9 A luminescence spectrometer interfaced to a digital computer will record excitation, emission, and detailed three-dimensional Prompt and delayed fluorescence of 1 ,Zbenzanthracene has been recorded at low temperatures using a chopped xenon lamp for excitation.4f1 Although a CW laser would usually be considered for spectral measurements because of the simplicity of a d.c. detection system, repetitive pulsed lasers (especially flashlamp-pumped or N,-laser-pumped dye lasers) are often cheaper and have a wider tuning range. Repetitive signals produced by such lasers may be processed by means of a gated boxcar integrator, and an excellent introductory article describes the principles and applications of this device.412 Examples of the application of this pulsed technique to fluorescence may be found in papers dealing with the detection of aflatoxins (0.2 ng) adsorbed on t.1.c. the spectrum of CrO,,Cl, ;414 high-resolution emission spectra of sym-tetrazine at 1.8 K;415 Rh6G (at concentrations as low as M) 416 and quinine sulphate and A somewhat similar gated technique is used in a novel filter fluorimeter which uses a low-power (7.5 W) repetitive flashlamp for excitation.418 406 407 408 409

410 411

4lP 413 414 415. 416

4l7 4l8

R. E. Anacreon and Y. Ohnishi, Appl. Optics, 1975,14, 2921. I. Landa and J. C. Kremen, An&. Chem., 1974,46, 1694. W. H. Melhuish, J . Phys. ( E ) , 1975,8, 815. Vo-Dinh Tuan and U. P. Wild, J. Phys. ( E ) , 1974,13,2899. G. R. Haugen, B. A. Raby, and L. P. Rigdon, Chem. Instrumentation, 1975,6 , 205. H.Staerk, J . Luminescence, 1976, 11, 413. G. K. Klauminzer, Laser Focus, 1975, 11(11), 35. M. R. Berman and R. N. Zare, Analyt. Chem., 1975,47, 1200. J. R. McDonald, Chem. Phys., 1975,9,423. J. H. Meyling, R. P. van der Werf, and D. A. Wiersma, Chem. Phys. Letters, 1974,28,364. A. B. Bradley and R. N. Zare, J . Amer. Chem. Soc., 1976,98,620. B. W.Smith, F. W. Plankey, N. Omenetto, L. P. Hart, and J. D. Winefordner, Spectrochim. Acta, 1974,30A, 1459. M. A. West, Internat. Laboratory, 1975 (6/7),41.

Developments in Instrumentation and Techniques

35 Fluctuations in lamp intensity from flash to flash are compensated by means of ratio-recording producing a long-term stability of less than 1% drift in 12 h. The sensitivity is < 0.5 p.p.b. quinine sulphate. Fluorescence Techniques.-Either component of a fluorescent mixture may be determined with a useful new instrumental technique which involves wavelength modulation of either the excitation or emission monochromator and scanning the modulated ax. signal with the second m ~ n ~ ~ h r ~ m aExamples t o r . ~ ~were ~ given of p.p.m. spectra of aromatic hydrocarbons separated spectrally with this method, which is claimed to be most useful when the spectra of interfering compounds is too small for simple wavelength selection. Derivative techniques can be used in cases where a sacrifice in S/N ratio is justified to increase A combination of synchronous and detectability of minor spectral derivative fluorescence spectrometry has been suggested as a method for characterizing materials with featureless spectra (e.g. crude Synchronous measurements involve a continuous scan of excitation and emission wavelengths separated by a constant increment, and the determination of spectral distributions as a function of this wavelength increment.421 Accurate quantum yield measurements are fundamental to a photophysical characterization of a fluorescent material. However, absolute values are subject to a number of potential errors including (i) polarization effects, (ii) fluorescence reabsorption, (ii) internal reflections in the cell, (iv) detector calibration, (v) refractive index effects, and (vi) variation of absorbance with instrument source bandwidth. Most of these problems may be circumvented through use of a so-called integrating sphere fluorimeter where errors due to (i) and (v) are largely eliminated, and errors from (ii) and (iii) are also eliminated by extrapolation to zero absorbance.422The detection system now becomes the sphere, monochromator, and photodetector, which can be calibrated against a standard lamp. Findings with this instrument support a value of 1.OO ( k 5%) for the fluorescence quantum yield of diphenylanthracene with reference to a value of 0.546 for quinine s ~ l p h a t e .Other ~ ~ ~ studies on diphenylanthracene at different temperatures and exciting wavelengths 423 and low concentrations 424 have shown that aDf depends , ~ ~ in ~ benzene4z4)and only reaches 1.00 in on the solvent (0.6 in n - h e ~ t a n e0.82 carefully degassed solutions of cyclohexane (where there is no triplet fo~rnation).*~3 Application of the n2 correction factor for changes in refractive index has been questioned,424and it was shown that proper correction is a string function of geometry of the sample Another effect of refractive index of more fundamental importance is its influence on the intrinsic radiative parameters of the excited Studies with diphenylanthracene have shown that a reduction in temperature from ambient to - 160 "C (n increases from 1.351 to 1.439 in isopentane) causes a decrease in experimental lifetime from 8.8 to 7.5 ns, consistent with an r2 to r3 dependence of the radiative lifetime. Quinine T. C. O'Haver and W. M. Parks, Analyt. Chem., 1974,46, 1886. G. L. Green and T. C. O'Haver, Analyt. Chem., 1974,46, 2191. 421 P. John and I. Soutar, Analyt. Chem., 1976, 48, 520. l Z 2W. R. Ware and W. Rothman, Chem. Phys. Letters, 1976,39,449. l S sG . Heinrich, S. Schoof, and H. Gusten, J. Photochem., 1974/5,3, 315. 434 J. V. Morris, M. A. Mahaney, and J. R. Huber, J . Phys. Chem., 1976, 80, 969. l T 5 J. Olmsted, Chem. Phys. Letters, 1976, 38, 287. ll9 *20

36

Photochemistry

bisulphate has long been regarded as a suitable quantum yield standard despite its dramatic photodegradation,lB8and an absolute method relying on measurement of a temperature rise due to light energy absorbed but not emitted as fluorescence gave a value of 0.561 3. 0.039 (and 0.84 for diphenylanthracene in degassed cyclohexane at 25 0C).426 A technique developed to determine absolute emission quantum yields of powdered samples makes a comparison of reflected plus emitted light from powders and the intensity of reflected light from a reference standard employing a constant response thermopile Quantum yield determinations have also been made for turbid ~ ~ I u t i o nmonomolecular s,~~~ systems of fluorescent and of L-tryptophan and indole relative to rhodamine B at two different excitation Correcting fluorescence emission spectra is usually regarded as a tedious exercise on uncorrected fluorescence spectrometers. A simple and cheap solution of applying a correction function (obtained from a standard lamp or quantum counter) to a spectrum as the wavelength is changed employed the use of PROMS (programmable read only memories).431 Excitation spectra of cyclic ketones (as transparent films at 20 K) have been measured using a method designed for low-temperature Polarization measurements of fluorescence which are important for the analysis of electronic transition bands of molecules are usually made with an analyser set in one direction (horizontal or vertical) at different times. Use of a rotating polarizer in an arrangement to record the polarization spectra of coronene and 1,12-benzperylene has enabled measurement of Ivy IhyIv + &, and Iv - Ih/& Ih.433 Gratings used in fluorescence spectroIv - Ih, meters invariably polarize both excitation and emission light into horizontal and vertical components whose magnitudes depend on wavelength. A correct fluorescence spectrum from a polarized sample may be obtained by viewing in the horizontal plane at an angle of 45" (or 135") to the direction of propagation of the exciting radiation with a polarizer set at 54.75" (or 125.25") from the vertical direction.434 Under these conditions, the reading is proportional to the total flux emitted by the sample and is dependent on the state of polarization of the exciting radiation and the emission anisotropy of the sample. A conventional apparatus to determine luminescence polarization at 77 K has been employed to study benzenethiol and t h i o a n i s ~ l e , and * ~ ~a fluorescence polarimeter designed to measure both birefringence and scattering has been used for aniostropic Fluorescence correlation spectroscopy, an extension of fluorescence depolarization, enables determinations to be made of reorientational correlation

+

426

427 428 428

431 432

433 434 436

436

B. Gelernt, A. Findeisen, A. Stein, D. Moore, and J. A. Poole, J. Photochern., 1976, 5, 197. M. S. Wrighton, D. S. Ginley, and D. L. Morse, J . Phys. Chem., 1974, 78, 2229. D. Spitzer and J. J. Ten Bosch, Appl. Optics, 1976, 15, 934. D. Mobius and G. Debuch, Chent. Phys. Letters, 1974, 28, 17. H. B. Steen, J. Chem. Phys., 1974, 61, 3997. F. Gruneis, S. Schneider, and F. Dorr, J. Phys. (E), 1975, 8, 402. L. T. Molina and E. K. C. Lee, J . Phys. Chem., 1976,80,244. K. Ohno and H. Inokuchi, Chem. Phys. Letters, 1975, 33, 585. K. D. Mielenz, E. F. Cehelnik, and R. L. McKenzie, J. Chem. Phys., 1976, 64, 370. P. G. Russell, J. Phys. Chem., 1975, 79, 1347. J. Fuhrmann and M. Hennecke, Colloid Polymer Sci., 1976, 254, 6.

Developments in Instrumentation and Techniques

37

times as well as translational diffusion rates and chemical reaction rates. Theories 43A together with discussions of S/N of this technique have been ratio and prospects for application.437

Other Luminescence Equipment and Techniques.-A fluorescence spectrometer designed for measurement of weakly fluorescent biological materials has been reported. It uses a conventional xenon source and double monochromator for excitation, with the sample mounted on an X- Y table; photon counting was also A tunable dye laser producing 358-641 nm light was employed as an excitation source in a new type of fluorescence spectrometer in which a single modified monochromator was used as both the laser source and scanning monochromator for emission.44o In addition to the biennial review cited earlier,4ooa report of the 1975 Tokyo conference on luminescence 441 covers all basic theoretical and experimental aspects of luminescence phenomena in both inorganic and organic materials, with an emphasis on the former. Thermoluminescence flow spectra are widely used to investigate carrier traps both in dielectrics and semiconductors. Two vacuum-operating thermoluminescence spectrometers (of high light collection efficiency) have been described; they possess sufficient thermal insulation to enable samples to be placed 15 mm from the light detector even if samples are heated to 500 "C or cooled to - 157 0C.442A real-time display of luminescence from tin oxide single crystals over the range 480-670 nm was obtained with a low monochromator equipped with a PAR optical multichannel a n a l y ~ e r . ~ ~ ~ Laser-induced fluorescence has been used in an interesting application of laser Doppler v e l ~ c i m e t r y .Fluorescence ~~~ detection has the advantage of eliminating all scattered laser light (by optical filtering), allowing a more accurate determination of particle Measurements of fluorescein in a flow line (at sensitivities greater than 5 p.p.b. in water) have been made on a simple filter fluorimeter using a cadmium sulphide photoresistor as a detector.446 Rapid changes in chlorophyll a fluorescence intensity, which occur within the first few moments of illumination in all photosynthetic plants (the Kautsky effect), have been measured using a red LED (emitting at 670 nm) and a phototransistor as a Front-surface illumination has been applied to measurements of oat The collection cultivars447and the contents of a 1 mm diameter micr0ce11.~~* efficiency of a front-surface arrangement has been improved using an ellipsoid mirror with a photomultiplier installed directly at the second focal point.449 4R7 @dB

@39 440 441

442 443 444 445 046

44c 448

44B

J. T. Yardley and L. T. Specht, Chem. Phys. Letters, 1976,37, 543. S. R. Aragon and R. Perora, J . Chem. Phys., 1976, 64, 1791. P. Vigny and M. Duquesne, Photochem. and Photobiol., 1974, 20, 15. D. C. Harrington and H. V. Malmstadt, Analyt. Chem., 1975, 47, 271. 'Tokyo Conference on Luminescence,' 1975, J. Luminescence, 1976, Vol. 12/13. F. Aramu, V. Maxia, and C. Muntoni, Rev. Sci. Instr., 1974, 45, 1414. J. P. Fillard, M. de Murcia, J. Gasiot, and S. Chor, J. Phys. (E), 1975, 8, 993. W. H. Stevenson, R. dos Santos, and S. C. Mettler, Appl. Phys. Letters, 1975,27, 395. U. Schreiber, L. Groberman, and W. Vidaver, Rev. Sci. Instr., 1975, 46, 538, P. E. Engler, P. R. Calabrese, and F. A. Giori, Rev. Sci. Instr., 1974, 45, 1469. E. J. Brach and B. Baum, Appl. Spectroscopy, 1975, 29, 326. M. F. Bryant, K. O'Keefe, and H. V. Malmstadt, Analyt. Chem., 1975,47, 2325. M. De Mets, J. Phys. (E), 1975, 8, 971.

Photochemistry

38

Instabilities in the output of an Aminco fluorimeter have been attributed to large random changes in the lamp intensity and solved (5% variation in 12 h) with a constant voltage power A more significant paper reveals that a 150 W Eimac illuminator (a miniature high-pressure xenon arc lamp with an integral collimating mirror) used in the Aminco instrument will give three times the photon flux of a standard xenon lamp in an ellipsoidal condensing system when used above 350 nm.451 The spectral range of a Farrand Mk I fluorimeter has been extended to 850 nm and modifications suggested for this instrument by use of an R446 enable single-beam absorption spectra to be determined at - 180 0C.463A continuous flow He cryostat has been described for use in luminescence Applications of Fluorescence.-Novelty in technique or method has been used as a criteria for inclusion in this section of examples of fluorescence applications in photochemistry. Space limitations do not permit a comprehensive review. The resonance-fluorescence method is not only very sensitive because many photons per second can be resonantly scattered from each individual atom, but also very selective in that only specific transitions are observed. This enables detection of rare atoms even in the presence of another material. Use of a CW dye laser for excitation enabled absolute density measurements to be made of sodium vapour from lo2atoms cm-s at -28 "C to loll at 144 0C.454At present, ground-state transitions of 87 elements whose energy levels are known can be detected, in theory at least, using either existing CW or pulsed lasers at sensitivities from lo2 to los atoms ~ m -It ~has been predicted that caesium -~ be detectable with this method atoms at concentrations of 0.1 atom ~ r n should using a 20 mW blue CW laser. This represents many orders of magnitude higher sensitivity than present atomic absorption or emission techniques. Atomic detection by fluorescence has also been used for Biz* in a flow arrangement,455 z4Mgatoms in an atomic beam at a fluorescence width of 40 M H z , sodium ~ ~ ~ at concentrations of > 7 x g ~ m using - ~ a flashlamp-pumped dye and rubidium atoms.45s At shorter wavelengths in the vacuum-u.v., resonance fluorescence, used to detect C1 atoms, combined with i.r. fluorescence enabled rate constants to be determined for the reaction of HCl (v = 1) with oxygen atoms,45swith atomic hydrogen produced by the flash photolysis of h y d r a ~ i n eand , ~ ~with ~ oxygen atoms at 130.6 nm.461 Reactions of hydroxyl radicals have been followed by resonance uio

451

452 46s

464 464 456

466

457 468 45s

460 461

P. Froehlich and E. Kenny, Analyt. Letters, 1976,9,349.

R. J. Perchalski, J. D. Winefordner, and B. J. Wilder, Analyt. Chem., 1975,47, 1993. W. C.Neely, T. D. Hall, S. Cravitt, and J. H. DeLap, Appl. Spectroscopy, 1975, 29, 205. W. C. Neely and T. D. Hall, Appl. Spectroscopy, 1974,28, 578. W. M. Fairbank, T. W. Hansch, and A. L. Schawlow, J . Opt. SOC.Amer., 1975,65,199. aF. Aurich, J. Marquard, and K. D. Schumacher, J. Phys. ( E ) , 1975,8,447. G.Gerber, H. Sakurai, and H. P. Broida, J . Chem. Phys., 1976,64, 3410. H.M.Gibbs and T. N. C. Venkatesan, Optics Comm., 1976, 17, 87. H. L.Brod and E. S. Yeung, Analyt. Chem., 1976,48,344. P. W. Pace and J. B. Atkinson, J. Phys. ( E ) , 1974,7 , 556. Z.Karng, B. Katz, and A. Szoke, Chem. Phys. Letters, 1975,35, 100. L.J. Stief and W. A. Payne, J. Chem. Phys., 1976,64,4892. R. N . Dubinsky and D. J. McKenney, Canad. J. Chem., 1975,53, 3531.

Developments in Instrumentation arid Techniques

39

fluorescence with a detection limit of 1O1O molecules cm-3,462and by polarized Fluorescence of the H2 Lyman bands fluorescence excitation excited by the 106.6 nm argon resonance line has been resolved with 2 vacuumU.V. monochromator and detected by an EMR 542-9-08-18 phototube with photon counting.464Monochromatic light from a synchrotron has been used to excite solid xenon at 20 K for emission studies at 176 nm.465Luminescence from benzene in an inert matrix at 5 K has been observed following excitation with 123.6, 147, or 206.2nm light from resonance A study of Xe(3PP1) -+ Xe(,P2) quenching by Ar in Xe-Ar matrices involved excitation by the 147 nm Xe line and observation of the Xe2* second emission Light emission of photofragments produced by the photolysis of NH, and ND3468 and with rare gas resonance lamps has been measured in a cell with two Wood's horns at right angles to reduce scattered light. Tunable lasers have already made a considerable impact as excitation sources in molecular fluorescence, and a review of their uses in analytical chemistry lists applications in Raman and absorption spectroscopy as well as Frequency-doubled tunable dye lasers, for example, have found use as excitation sources for hydroxyl radicals,471formaldehyde concentrations at sub p.p.m levels in air 4 7 2 and SO,, OH, and I2473 at high resolution (0.001-0.015 nm). Tunable CW laser excitation has been applied to measurements of the hyperfine structure 474 and polarized fluorescence spectra475of NH2, the fluorescence spectra of the vibronic side-bands of Cr3+ in LaAlO, from 735 to 760n111,~~~ and iodine v a p o ~ r . ~Experiments '~ on pulsed laser-excited fluorescence spectra of perylene in n-octane and ethanol hosts at 4 K revealed that monochromatic excitation leads to a dramatic sharpening of the emission bands.478Pulse sampling following excitation with a pulsed dye laser has enabled the fluorescence of UF, (Of= 5 x to be Interest in quantitative measurements of small gas molecules (such as NO, and SO,) has been prompted in part by interest in atmospheric pollutants. Rapid and continuous monitoring of SO, in air, for example, over a concentration range 8.6 p.p.b. to 1.8 p.p.m., was based on fluorescence induced by a Zn lamp (at 213.8 nm).480 A similar arrangement enabled NO concentrations in air at G. W. Harris and R. P. Wayne, J.C.S. Faraday Z, 1975,71, 610. G. A. Chamberlain and J. P. Simons, Chem. Phys. Letters, 1975, 32, 355. 464 E. C. Y . Inn and W. L. Starr, J . Opt. SOC.Amer., 1975, 65, 320. 465 R. Brodman, R. Haensel, U. Hahn, U. Nielsen, and G. Zimmerer, Chem. Phys. Letters, 1974, 29, 250. 460 L. Hellner and C. Vermeil, J. Mol. Spectroscopy, 1976, 60, 71. 467 R. Atzmon, 0. Cheshnovsky, B. Raz, and J. Jortner, Chem. Phys. Letters, 1974, 29, 310. 468 J. Masenet, A. Gilles, and C. Vermeil, J. Photochem., 197415, 3, 417. 469 C. Lalo and C. Vermeil, J. Photochem., 1974/5, 3, 441. 470 J. R. Atkins. Analyt. Chem., 1975,47, 753A. 471 R. K. Lengel and D. R. Crosley, Chem. Phys. Letters, 1975, 32, 261. 472 K. H. Becker, U. Schurath, and T. Tatarczyk, Appl. Optics, 1975, 14, 310. 47s M. A. A. Clyne, I. S. McDermid, and A. H. Curran, J. Photochem., 1976, 5, 201. 474 G. W. Hills, D. L. Philen, R. F. Curl, and F. T. Tittel, Chem. Phys., 1976, 12, 107. 475 M. Kroll, J . Chem. Phys., 1975, 63, 1803. 476 S. Aoki and I. D . Abella, Appl. Phys. Letters, 1975, 26, 653. 477 M. H. Ornstein and V. E. Derr, J. Opt. SOC.Amer., 1976, 66, 233. l i 8 I. I. Abram, R. A. Auerbach, R. R. Birge, B. E. Kohler, and J. M. Stevenson, J. Chem. Phys., 1975, 63,2473. 479 A. Andreoni and H. Buchler, Chem. Phys. Letters, 1976, 40, 237. 480 F. P. Schwarz, H. Okabe, and J. K. Whittaker, Analyt. Chem., 1974, 46, 1024. 4aa

46s

40

Photochemistry

0.015-7 p.p.m. to be High sensitivity measurements of NO, (> 0.6 p.p.b.v.) employed a He-Cd laser (442 nm) for excitation with photoncounting techniques.482 Liquid filter cells (containing solutions of Na,Cr,O, or CoS0,-NiSO,) effectively attenuated the laser light without fluorescing (unlike glass interference filters). Fluorescence of NO, in the first predissociation region (398-420nm) has been studied with a modulated xenon source and phasesensitive detection.483Other studies with this compound included low-temperature (3 K) measurements of excitation spectra 484 and determining spatial concentrations in a cylindrical flow reactor by Ar+-laser-excited Decrease in NO concentrations due to reaction with Fain a flow tube arrangement has been followed by resonance fluorescence scattering at 226 nm,486and highresolution studies of this compound between 190 and 240 nm were made following excitation with an Hg 184.9 nm line isolated from the 191.4 nm line with a cell containing a low pressure of NO.487 Concentrations of O(,P) atoms generated by the sinusoidally-modulated r.f.-powered Hg sensitization of NzO were determined by following the emission of NO, using a Iock-in Likely analytical applications of chemiluminescence at ambient and subambient temperatures, including oxidative degradation and other degradation Eechanisms of materials such as hydrocarbons and plastics, have been reviewed.489 Chemiluminescence spectra have been reported for reactions of BCl, and H2S following excitation with a CO, laser,49oand 0 and H with NO.491 A 340 1 cell was employed in chemiluminescence studies of OH* reactions with inert gases.492 Luminescence accompanying free metal-atom oxidation is currently receiving interest because of possible use as an electronic transition visible chemical laser, and is illustrated in a high-temperature fast-flow study of Sn-H,0.493 The efficiency of electrogenerated chemiluminescence of several systems has been measured using ferrioxalate actinometry and a calibrated p h o t ~ d i o d e . ~ ~ ~ Absolute measurements of radiation-induced luminescence (and optical absorption to 25 pm) have been made in an optical system entirely built in a standard He c r y ~ s t a t . Related ~~~ papers include luminescence spectra from He+-0, interactions,496observation of CN emission bands following electron impact and dissociative excitation of simple cyanides,497and corona discharge emission spectra of N,, He, and air.498 F. P. Schwarz and H. Okabe, Analyt. Chem., 1975, 47, 703. A. W. Tucker, M. Birnbaum, and C. L. Fincher, AppE. Optics., 1975, 14, 1418. 483 W. M. Uselman and E. K. C. Lee, J . Chem. Phys., 1976, 64, 3457. 484 R. E. Smalley, L. Wharton, and D. H. Levy, J . Chem. Phys., 1975,63,4977. IR6 T. B. Stewart and H. S. Judeikis, Rev. Sci. Znstr., 1974, 45, 1542. C. E. Kolb, J. Chem. Phys., 1976, 64, 3087. d87 T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. Ichimura, and Y. Mori, J. Chem. Phys., 1975, 63, 5470. CR8 R. Atkinson and J. N. Pitts, J . Phys. Chem., 1974, 78, 1780; 1975, 79, 295. 489 R. A . Nathan, G . D. Mendelhall, J. A. Hassell, and J. D. Wallace, Industrial Research, 1975, 17(12), 62. 490 S. D. Rockwood, Chem. Phys., 1974, 10, 453. 491 T. Ibaraki, K. Kodera, and I. Kusunoki, J. Phys. Chem., 1975, 79, 95. G . E. Streit and H. S. Johnson, J . Chem. Phys., 1976, 64, 95. 493 W. Felder and A. Fontjin, Chem. Phys. Letters, 1975, 34, 398. 494 C. P. Keszthelyi, N. E. Tokel-Takvoryan, and A. J. Bard, Analyt. Chem., 1975,47, 249. '95 B. C.Passenheim, B. D. Kitterer,T. M. Flanagan, and R. Denson, Rev. Sci. Znstr., 1974,45,1365. 4sf3 H. H. Harris, M. G. Crowley, and J. J. Leventhal, Chem. Phys. Letters, 1974, 29, 540. 497 I. Tokue, T. Urisu, and K. Kuchitsu, J. Photochem., 197415, 3, 273. 498 F. Gruma and L. F. Costa, Appl. Optics, 1976, 15, 76.

4R1

4s2

Developments in Instrumentation and Techniques

41

Applications of i.r. fluorescence are contained in a review which gives details of experimental techniques including laser sources and detector^.^^^ Carbon dioxide lasers are featured as excitation sources in fluorescence measurements of ethylene, propylene, ethanol, and CO and isotopic species at 78 K,601and ethylene in air.602 The i.r. fluorescence of both bromine603 and active nitrogen 604 has been recorded in flow systems. Included in other applications of luminescence are publications on the velocity distribution of an Na, supersonic beam using a chopped Ar+ laser beam for excitation and a downstream CW laser to determine changes in laser-induced fluorescence;606flow velocity distributions in solution using a pulsed light to excite luminescent particles ;60a fluorescencedetectors for liquid chromatography ;607 triboluminescence accompanying phase changes in methanol at 157.8 K;508 and use of a molecular beam to obtain high concentrations of collision-free molecules (2 x 1017~ m - ~ such ) as pyrene, thereby enabling the S , fluorescence spectra to be measured at high r e ~ o l u t i o n .Admission ~~~ of 0, to phosphorescing samples of aromatic hydrocarbons has been shown to induce a fluorescence which can be explained as a singlet oxygen-triplet molecule annihilation fluorescence.51oAn optical double-resonance method for determining vibrational lifetimes of matrix-isolated C,- using CW lasers to pump and probe has been claimed to have considerable advantages over i.r. fluorescence techniques.611 Phosphorescence Spectrometry.-Quantitive phosphorescence analysis of organic materials is usually carried out at low temperatures in order to minimize collisional quenching and maximize triplet-state lifetime, As a result, the need for cryogenic equipment makes this technique more cumbersome and expensive than conventional fluorescence spectrometry. Phosphorimetry at room temperature has obvious advantages, and one method applicable to a wide variety of ionic aromatic and heteroaromatic compounds of biological and pharmaceutical importance employed filter paper as the support matrix.612 Solutions in ethanol were deposited on 6 mm discs of Eaton Dikeman 613 filter paper which were dried under controlled conditions. Reported detection limits - eosin ~ Y to a few mg for other materials could be improved of 20 ng ~ m for by use of a more sensitive fluorescence spectrometer, especially with photoncounting. Though not mentioned in the publication, non-ionic materials (such as aromatic hydrocarbons) may be readily dissolved in thin polymer films (e.g. polymethylmethacrylateand epoxy resin) and will produce strong phosphorescence R. T. Bailey and F. R. Cruickshank, Appl. Spectroscopy Rev., 1976. 10, 1. J. W. Robinson and N. Katayama, Spectroscopy Letters, 1974,7, 581. H. Dubost and R. Charneau, Chem. Phys., 1976,12,407. ma N. Katayama and J. W. Robinson, Spectroscopy Letters, 1975, 8, 61. Z . Karny and B. Katz, Chem. Phys., 1976,14, 295. E. M. Gartner and B. A. Thrush, Proc. Roy. SOC.,1975, A346, 103. T. D. Gaily, S. D. Rosner, and R. A. Holt, Rev. Sci. Instr., 1976,47, 143. N . Nakatani, K. Fujiwara, M. Matsumoto, and T. Yamada, J . Phys. ( E ) , 1975, 8, 1042. W. Lindner, R. W. Frei, and W. Santi, J . Chromatog., 1975, 111, 365; J. C. Steichen, ibid., 1975, 104, 39. .508 G. J. Trout, D. E. Moore, and J. G. Hawke, J . Phys. Chem., 1975,79,1519. H. Steidl and D. Nowak, 2.phys. Chem. (Frankfurt),1975,94,95. R. D. Kenner and A. U. Khan, Chem. Phys. Letters, 1975, 36, 643. L. J. Allamandola and J. W. Nibler, Chem. Phys. Letters, 1974, 28, 335. 612 S. L. Wellons, R. A. Paynter, and J. D. Winefordner, Spectrochim. Acta, 1974,30A, 21. '@@

tiOD

42

Photochemistry

on excitation at room temperature. Oxygen quenching of these films may be eliminated by simply flushing the sample compartment with dry nitrogen. Detection limits with the filter paper technique have been lowered by one or two orders of magnitude by use of the heavy-atom effect. For example, addition of 1M-NaI increased the phosphorescence of a M solution of 2-naphthalene sulphonate by 40 Conventional phosphorescence spectrometers described include one using two remote1choppingwheels which is claimed to achieve better frequency synchronization than a tuning fork a rotating-can phosphoroscope adapted for use with a Corning-Eel model 244 ~ p e c t r o m e t e r ,and ~ ~ ~an electronically compensated instrument which used a PVA film containing rhodamine B as a quantum counter.51s Phosphorinieters with flash sources usually offer greater sensitivity and time resolution than mechanical systems, and two simple systems (for undergraduate experiments) have been described using low-voltage flash lamp^.^^^^ 518 Scattered light from the photoflash lamp was effectively prevented from affecting the photodetector by use of a camera-type shutter which triggered the lamp about 15 ms before opening.618 Pulsed-source techniques using a gated photodetector have been reviewed 619 and applied to identification of drugs by means of temporal rather than spectral Species with different phosphorescence lifetimes produce signals of differing phase and amplitude relationships ; these have been analysed in a phase-resolved p h o s p l ~ o r i m e t e r . ~Although ~ ~ - ~ ~ ~ quantitative analysis of synthetic binary mixtures was not found to be either accurate or precise with this method, both excitation and emission spectra showing severe overlap can be p h a s e - r e s ~ l v e d . ~ ~ ~ A phosphorescence spectrometer based on a photon-counting technique using a multichannel digital boxcar has been used to study the kinetics of triplet excimer formation in Multichannel scaling with photon-counting has been used for signal processing in studies of the phosphorescence spectra of aralkyl t h i o n e ~ .A~ computer-controlled ~~ laser phosphorimeter using magnetic tape to store phosphorescence spectra as signal-averaged families of decay curves before presenting time-resolved spectra has been reported; an N2 laser was used for excitation and about 100 decay curves were collected at each wavelength.624 High-resolution (2.5-1 5 cm-l) phosphorescence spectra of coronene, 1,12benzperylene, and chrysene at 4.2 K have been determined following excitation with the 489 nm Ar+ laser line.s25 613

P. G. Seybold and W. White, Analyt. Chenz., 1975, 47, 1199.

m J. C. Sutherland, C. P. Yarter, and S. D . Putney, Photochem. andPhotobiof., 1976,23, 141. 616

616 L17 518

61e 520

6a4

623 s24 62c

D. L. Phillips, J. N. Miller, D . T. Burns, and J. W. Bridges, Anafyt.Letters, 1976, 9, 137. M. T. Pailthorpe, J. Phys. (E), 1975, 8, 194. T. R. Dyke and J. S. Muenter, J. Chern. Educ., 1975,52,251. J. L. Charlton and B. R. Henry, J . Chem. Educ., 1974, 51, 753. J. J. Aaron and J. D. Winefordner, Talanta, 1975, 22, 707. K. F. Harbaugh, C. M. O’Donnell, and J. D. Winefordner, Analyt. Chem., 1974, 46, 1206. J. J. Mousa and J. D. Widefordner, Anafyt. Chem., 1974, 46, 1195. T. Takemura, M. Aikawa, H. Baba, and Y . Shindo, J. Amer. Chem. SOC., 1976,98,2205; T. Takemura, M. Aikawa, and H. Baba, J . Luminescence, 1976, 12/13, 819. M. H. Hui, P. De Mayo, R. Suau, and W. R. Ware, Chem. Phys. Letters, 1975, 31, 257. R. M. Wilson and T. L. Miller, Analyt. Chem., 1975, 47, 256. E. I. Al’Shits, R. I. Personov, and B. M. Kharlamov, Chem. Phys. Letters, 1976, 40, 116.

Developments in Instrumentation and Techniques

43 Related publications include descriptions of a ‘popsicle’ technique to eliminate sample cells,526a square Dewar-flask assembly for a Farrand fl~orinieter,~~7 and a novel phosphorimeter in which excitation light was introduced through a space-intermitting pattern (e.g. a fine grid) to produce a spatially inhornogeneous d i s t r i b ~ t i o n .In ~ ~this ~ case, changes in delayed fluorescence from anthracene or diphenylanthracene in ethanol with changing dimensions of the pattern agreed with theoretical expectations based on triplet Raman Spectroscopy.-Raman scattering is inherently a weak effect with the scattering approximately the intensity of the exciting radiation. Although laser sources with high spectral purity and intensity have helped, only a small volume can generally be used and there are obvious constraints on the solid angle of scattered light which may be collected by the detection optics. The inverse Raman effect, however, seems to be one non-linear effect capable of solving sensitivity problems. The light beam is absorbed so that low sample concentrations, e.g. gases, can be compensated by longer absorption paths. Fluorescence does not interfere because absorption is on the short-wavelength side of the excitation source and the use of highly polarized laser beams makes measurement of depolarization ratios more accurate. Studies with liquid benzene 529 using a frequency-doubled dye laser as the excitation source and photographic recording of the absorption clearly showed the effectivenessof this technique. Interference in the measurements from two-photon absorption processes and inhomogerieous laser beams was also discussed. Elimination of fluorescence accompanying excitation of coloured species was accomplished in one arrangement by taking advantage of the polarization of Raman A rotating polarizer replacing the usual chopper and polarization scrambler differentiated between the two light sources. Long-lived (> s) fluorescence in a Raman spectrometer has been suppressed by means of an air-driven dentist drill laser beam chopper at a frequency of 100-400 kHz at a stability of l%.531 Tunable dye laser excitation has been used with gated photon counting to measure Raman scattering and fluorescence 532 and to obtain the resonance Raman spectrum of a stable free radical (DPPH).533 Spectra recorded using an image intensifier coupled to a TV camera and video disc were at a sensitivity 106 times higher than recorded with 3000 ASA Polaroid film.633 The time resolution of Raman spectroscopy is usually limited to the time required to scan a portion of the spectrum with a conventional spectrometer. However, a continuous flow method adapted to acquisition of spectra of reaction intermediates has enabled recording 1 second or later after mixing.634Accessories for b26 527

628

520 630

531 532 693

534

M. Gouterman and P. Sayer, J . Mol. Spectroscopy, 1974, 53, 319. W. C. Neely, J. R. McDuffy, and T. D. Hall, Appl. Spectroscopy, 1976, 30, 363. R. D. Burkhart and J. W. Kenney, Chem. Instruntentation, 1975, 6,37. E. S. Yeung, J. Mol. Spectroscopy, 1974, 53, 379. C. A. Arguello, G. F. Mendes, and R. C. C. Leite, Appl. Optics, 1974,13, 1. 1731, R. J. Nemanich, S. A. Solin, and J. Doehler, Rev. Sci. Instr., 1976,47, 741. M. I. Bell and R. N . Tyte, Appl. Optics, 1974, 13, 1611. R. Wilbrandt, P. Pagsberg, K. B. Hansen, and C. V. Weisberg, Chem. Phys. Letters, 1975, 36,76. W.H. Woodruff and T. G. Spiro, Appl. Spectroscopy, 1974, 28, 576.

44

Photochemistry

Raman spectroscopy included a high-pressure and high-temperature cell 635 and a sample cell for adsorbed species at temperatures up to 450 0C.536 The relative merits of coherent anti-Stokes Raman spectroscopy (CARS) over conventional Raman spectroscopy have been discussed in a publication dealing with the description of an experimental system and its applicability to benzene-toluene An improved design and arrangement for CARS includes a description of first observations of higher-order Raman spectral excitation studies (abbreviated by the authors to HORSES).S38 9 Transient Absorption Spectroscopy

Conventional Flash Photo1ysis.-More than any other factor, the performance of the photolysis flashlamp is critical in determining both the time resolution and sensitivity of microsecond flash photolysis. Although plasma sources have been studied for years, actual lamp characteristics do not always obey theoretical predictions, especially those derived from conventional pulse-forming net works. A recent dynamic treatment 639 on xenon lamps involved observations that the growth of a flash discharge could take a considerable time and was strongly dependent on the initial discharge voltage. In this and other papers (e.g. J. F. Holzrichter and A. L. Schawlow, Ann. New York Acad. Sci., 1970, 168, 703), the light profile is assumed to follow the current profile, which is not the case for short duration (< 100 ps) lamps where the light output lags behind the electrical input. Far better relationships between actual measurements and theoretical predictions have been obtained 640 by assuming that the instantaneous properties of the plasma are determined simply by the energy stored in the plasma itself. Unpublished work by MorrowS4l confirms this assumption and shows that a linear, xenon-filled flashlamp operated at energies between 75 and 230 J in a circuit of 1.04 pH inductance gave flashes whose duration (between 3.3 and 4 ps) agreed with theory to within 10%. The U.V. output of a linear flashlamp (operated up to 500 J at 10-20 kV) was found to be proportional to input energy and independent of xenon pressure above 50 T ~ r r Reproducible . ~ ~ ~ light output, more reliable triggering, increased lamp life, and enhanced U.V. output were all valid claims made for low-power flashlamps operated repetitively with a simmer mode of operation used in a dye A pulsed power system providing a constant pulse energy at up to 5 Hz in a flashlamp-pumped Nd:YAG laser discharged when this voltage had decayed to the pre-set A rotary spark gap to switch 200 J lamps in a dye laser had a jitter time as low as k 0.7 ps,546 and a high voltage (60 kV) parallel triggering 635

637

638 639

640

641 642

548 644

R. S. Hawke, K. Syassen, and W. B. Holzapfel, Rev. Sci. Instr., 1974,45, 1598. G . L. Schrader and C . G . Hill, Rev. Sci. Instr., 1975, 46, 1335. R. F. Begley, A. B. Harvey, and R. L. Byer, Appl. Phys. Letters, 1974, 25, 387. I. Chabay, G . K. Klauminzer, and B. S. Hudson, Appl. Phys. Letters, 1976,28,27. R . H. Dishington, W. R. Hook, and R. P. Hilberg, Appl. Optics, 1974, 13, 2300. W. F. Hug, J. F. Shaw, and R. D. Buhler, Appl. Optics, 1973,12,1331; J. F. Shaw, W. F. Hug, and C. Hains, J. Appl. Phys., 1973,44, 2143. T. Morrow (Queen's University of Belfast) - personal communication. H. J. Baker and T. A. King, J. Phys. (E), 1975, 8,219. A. Marotta and C. A. Arguello, J. Phys. (E), 1974,7,478. C. C. Lo and B. Fan, Rev. Sci. Instr., 1976, 47, 63. C. M. Ferrar, Rev. Sci. Instr., 1974, 45, 1169; Appl. Optics, 1974, 13, 1998.

Developments in Instrumentation and Techniques

45

system employed a mercury thread to conduct this voltage to flash lamp^.^^^ Two other trigger circuits for parallel triggering provided a 30 kV pulse of 1.4 ps to overvolt either a spark gap or a flashlamp and a high current (80 A) short-rise (0.2 ps) pulse for a mercury t h ~ r a t r o n . ~ ~ ' New pulsed sources providing a continuum in the vacuum-u.v. include Cerenkov radiation (62-200 nm) produced by 500 MeV electron irradiation of an improved BRV-Garton vacuum spark source (8-50 nm) 64B and a high-pressure (1-6 atm) high repetition rate (300 Hz) discharge A pinch discharge lamp was claimed to be suitable for following reactions with lifetimes > 1 ps,661and an 18 J transverse discharge flashlamp has been reported to have a 5 ns risetime and 30 ns pulse length.662Exploding wires (produced by a capacitive discharge) have been shown to produce very intense continua extending far into the U . V . ~ ~At~ 220 nm, the peak irradiance of the continuum from a 720 J discharge through 50 mm of 0.08 mm diameter chromel-A was more than lo5 greater than that obtained from a 1600 W xenon lamp. Addition of Zn or Cd to flashlamps can result in a two- to three-fold enhancement in spectral output relative to xenon in the 250-300 nm range but at the expense of a much reduced lamp life (typically 20 shots at 1600 J).664 On the other hand, intense pulses at 228.8 and 326.1 nm with little or no observable background radiation have been generated in a multi-electrode flashlamp filled with 2 Torr of He and the vapour pressure of Cd at temperatures up to 320 0C.655 A multiple-pass monitoring arrangement through a 70 cm cell was required to monitor NH2 in a flash photolysis study of NH2-NO reactions.666 Light at wavelengths not absorbed by NH2 were picked up by a light guide close to the exit slit of the monochromator and used for reference with the pulsed xenon arc lamp. Emission from CH produced by the pulse radiolysis of C2H2 was monitored on a dual beam arrangement in which one monochromator recorded the whole of a molecular band over 7.5 nm (as a reference) and the second only one rotational level in a resolved spectral range of 0.3-0.4 nm.667 Flash photolysis equipment described included a conventional microsecond apparatus,55sa simple cross-beam arrangement to measure triplet absorption in thin polymer a flash spectroscopic system employing a Kerr cell to reduce scattered light for an unjustifiable application,660a vacuum-u.v. apparatus employing a cell with demountable LiF windows,661and a flash pyrolysis apparatus R. E. W. Pettifer, R. G. Flavell, and G. A. Robinson, J . Phys. (E), 1975,8, 875. J. R. Wiesenfeld, Rev. Sci. Instr., 1974, 45, 1465. 64s M. A. Piestrup, R. A. Powell, G. B. Rothbart, C. K. Chen, and R. H. Pantell, Appl. Phys. Letters, 1976, 28, 92. wB A. M. Cantu and G. Tondello, Appl. Optics, 1975, 14, 996. 6 5 0 Z. Ophir, U. Even, B. Raz, and J. Jortner, J. Opt. SOC. Amer., 1974, 64, 1175. 661 N . T. Timofeev and P. A. Shakhverdov, Zhur. priklad. Spektroskopii, 1974, 20, 775. 662 Y . Binur, R. Shuker, and A. Szoke, Rev. Sci. Instr., 1975, 46, 472. 653 D. W. Brinkman and R. D. Sacks, Analyt. Chem., 1975, 47, 1279. M. A. Gusinov, Appl. Optics, 1975,14,2645; Optics Comm., 1975, 15, 190. 665 W. H. Breckenridge and T. W. Broadbent, Chem. Phys. Letters, 1974, 29, 421; W. H. Breckenridge, T. W. Broadbent, and D. S. Moore, J. Phys. Chem., 1975,79, 1233. 6s6 R. Leclaux, P. V. Khe, P. Dezauzier, and J. C. Soulignac, Chem. Phys. Letters, 1975, 35,493. u7 M. Schmidt, H. A. Gillis, and M. Clerc, J. Phys. Chem., 1975,79,2531. Z. P. Zagorski, Z . Zimek, and J. Grodkowski, Jena Rev., 1973, 18, 292. 6sp R. D. Kenner and A. U. Khan, J. Chem. Phys., 1976, 64, 1877. K. P. Ghiggino, C. H. Nicholls, and M. T. Pailthorpe, J. Phys. (E), 1975, 8, 900. 661 M. R. Levy and J. P. Simmons, J.C.S. Faraday II, 1975,71,561. 646

647

3

Photochemistry employing energies up to 14 kJ.662Hydroxyl radical reactions have been studied in a flash photolysis reactor over a temperature range 300-1000 K,663and iodine atom recombinations also at high temperatures in a triple-walled reaction ves~e1.~~~ Triplet quantum yields of aromatic hydrocarbon-inorganic anion solutions have been measured by using an inert all-glass cell having two attached reservoirs.666The solution in the first reservoir could be introduced into the cell compartment and flashed and subsequently mixed with the contents of the second reservoir by opening a break seal and repeating the flash experiment under the same conditions. Delayed fluorescence yields have been determined with a method based on triplet absorption and delayed fluorescence decays,566and a ground-state depletion method has been used to evaluate triplet extinction coefficients of anthracene and 9-bromoanthra~ene.~~~ Among signal-processing equipment for kinetic studies of photometric changes are the logarithmic device referred to earlier,s63 a high-precision ( 0.1 5%) ratioing system with a risetime of 11 ps,668an effective background current subtractorysBB and a digital analyser which records transient processes over more than 8 decades at sweeps from 2 ps to 5 min.670 Many of the reported chemical systems studied with flash photolysis involve atomic species. For example, time-resolved atomic absorption spectroscopy has been used to study TI atoms absorbing at 351.9 nm,671Sb atoms at 259.81 and 252.85 nm,672collisional quenching of I(5p5,2Pa)by I2 at 206.2 nm (using laser and concentrations of I2 and I from alkyl iodides (at 178.3 and 206.2 nm).s74 Hydroxyl radical reactions with N2 have been followed by absorption spectroscopy using an OH microwave source.676 A flashlamp has been used as an excitation source for time-resolved photocurrent measurements on b e n z o p h e n ~ n e . ~ ~ ~ 46

Nanosecond Flash Photo1ysis.-Brief reviews of nanosecond absorption techniques, including descriptions of laser sources and applications, have appeared which cover predominantly selected American 677 and Russian 578 work. An alternative monitoring source to fluorescence in nanosecond flash spectroscopy may be the broad (200nm) continuum generated by non-linear optical ‘a*

ma 66‘

w6 ma 667 668

me .570

671 67z

67s

w4 m6 676

677

678

F. Gans, C. Troyanowsky, and P. Valat, J . Photochem., 1976, 5, 135. R. Zellner and W. Steinert, Znternat. J. Chem. Kinetics, 1976, 8, 397. H. W. Chang and G. Bums, J. Chem. Phys., 1976, 64, 349. A. R. Watkins, J. Phys. Chem., 1974, 78, 1885. F. Tfibel and L. Lindqvist, Chem. Phys., 1975, 10, 471. M. B. Ledger and G. A. Salmon, J.C.S. Furaday ZZ, 1976,72, 883. M. Anson and P. Bayley, Rev. Sci. Znstr., 1976, 47, 370. D. A. Whyte, Rev. Sci. Instr., 1976, 47, 379. R. H. Austin, K. W. Beeson, S. S. Chan, P. G. Debrunner, R. Downing, L. Eisenstein, H. Favenfelder, and T. M. Norlund, Rev. Sri. Znstr., 1976, 47, 445. P. D. Foo, T. Lohman, J. Podolske, and J. R. Wiesenfeld, J. Phys. Chem., 1975,79,414. M. J. Bevan and D. Husain, J. Photochem., 1975,4, 51. D. H. Burde, R. A. McFarlane, and J. R. Wiesenfeld, Chem. Phys. Letters, 1975, 32, 296. T. Donohue and J. R. Wiesenfeld, J. Chem. Phys., 1975, 63, 3130. D. W. Trainor and C. W. von Rosenberg, J. Chem. Phys., 1974,61, 1010. F. K. Dahnke, S. S. Fratoni, and S. P. Perone, Analyt. Chem., 1976,48, 296. J. T. Yardley, in ‘Chemical and Biochemical Applications of Lasers’, ed. C. B. Moore, Academic, New York, 1974, vol. 1. V. Rehak, Sbornik Ved. Pr. Vysok Sk. Chemickstechnol. Pardubice, 1973, 30, 215 (Chem. Abst., 1975, 82, 36 708).

Developments in Instrumentation and Techniques

47

processes in fibre waveguides.57s An N,-laser-pumped dye pulse (from coumarin 120) of 20 kW peak power and 15 nm spectral width is passed through a fibre waveguide, producing a smooth continuum in the visible. The spectral width of the generated continuum depends for fixed fibre lengths on the pump power, and the overall efficiency is very high (> 90%). A time delay of about 95 ns introduced in the system imposes some restriction on time resolution and avoidance of an optical delay in the excitation beam.s7D Cross-sections for absorptions from excited states have been measured using a split-beam arrangement to excite with an N, laser pulse followed by a delayed probe pulse from the same laser.mo More conventional nanosecond apparatus has employed an N, laser with a pulsed xenon monitoring source,581an off-axial (2.5") excitation and monitoring beam arrangement using the fourth harmonic of an Nd laser for excitation,682 and a crossed-beam arrangement using 265 and 353 nm Nd laser radiation to determine triplet quantum yields.583 The latter method measured energy input in relation to standard solutions and relative actinometry and was first used by J. T. Richards and J. K. Thomas (Trans. Faraday SOC.,1970, 66,621). The arrangement for measuring time dependences of transient absorption or emission recorded following a single pulse of radiation (Vol. 6, p. 99) has been improved by scanning the image on a converter camera with a TV camera, storing in a video-disc recorder and transferring, line-by-line, to a Three-dimensional plots of absorbance or emission uersus time and wavelength containing 100 x 100 data points can be displayed. Rate constants for collisional deactivation of CO, (OOO1) in pure C 0 2 were obtained from the temporal decay of 9.4 and 10.4 pm emission immediately following saturated absorption of 10.6 pm CO, TEA laser radiation.685 Laserinduced molecular absorptions have been reported for SF6, CF,Cl, and CF,Br at frequencies about 40 cm-l lower than that of a CO, TEA laser (1-2 ps duration) which was used to excite molecules vibrationally in a low-pressure cell. A frequency-stabilized probe laser operated at C 0 2or NO2 laser frequencies and changes in transmission with time measured photometrically with a monochroniator and HdCdTe detector.586 Chemical applications with more-or-less conventional laser flash photolysis apparatus included studies on internal conversion in gaseous p e n t a ~ e n e , ~ ~ ' transient absorption and emission in stereoisomers of cyanine dyesYKse the decay rate of the uranyl ion as a function of and isomerization of nickel(I1) complexes.6QoA novel method suggested to measure absolute excited singlet absorption spectra of rhodamine involved bleaching the solution C. Lin and R. H. Stolen, Appl. Phys. Letters, 1976, 28, 216. E. Sahar and J. Wieder, Z.E.E.E. J. Quantum Electron., 1974, QE-10,612. T.Hino, H. Akazawa, H. Masuhara, and N. Mataga, J. Phys. Chem., 1976,80, 33. U. Lachish, A. Shafferman, and G. Stein, J. Chem. Phys., 1976, 64, 4205. mS B. Amand and R. Bensasson, Chem. Phys. Letters, 1975,34,44. lia4 K. H. Schmidt, S. Gordon, and W. A. Mulac, Rev. Sci. Znstr., 1976, 47, 356. 686 R. C. Sepucha, Chem. Phys. Letters, 1975,31, 75. IB6 A. B. Petersen, J. Tice, and C. Witting, Optics Comm., 1976, 17,259. 687 B. Soep, Chem. Phys. Letters, 1975, 33, 108. 688 J. T. Knudtson and E. M. Eyring, J. Phys. Chem., 1974,78, 2355. 689 P. Benson, A. Cox, T. J. Kemp, and Q. Sultana, Chern. Phys. Letters, 1975, 35, 195. LsO J. J. McGarvey and J. Wilson, J. Amer. Chem. SOC.,1975,97,2531.

m0

w0 rial

48

Photochemistry

entirely with 530 nm radiation and determining extinction coefficients from the plateau-like absorption signal at very high excitation intensities.591 The lineshape of the coherent anti-Stokes Raman band at 3200 cm-l in water has been measured using two independent but synchronized dye laser pulses (N2 laser pumped) as excitation sources, the pump laser at 450 nm and the second laser tuned from 513 to 540 nm.592 Large temperature rises and uniform sample heating have been obtained in a laser temperature-jump apparatus by incorporating the sample with a (Nd laser) The stimulated Raman effect in high-pressure H2 produced a frequency shift from 1060 to 1890 nm, which is a more suitable absorbing wavelength for excitation and temperature-jump studies of aqueous ~ o l u t i o n ~ .A ~ ~coaxial * monitoring arrangement commonly used in laser flash photolysis has been employed for laser temperature-jump Double absorption photofragment spectroscopy is a new technique developed for observing the time evolution of unimolecular processes.596Molecules isolated from collisions in a molecular beam were prepared in a well-defined intermediate state by an initial laser pulse. As this state evolves, it was monitored by a second probe pulse which, after a chosen delay, photodissociated the molecules. Information about the state to which the molecules had evolved (the iodine B state was taken as an example) was extracted from the mass, density, and angular and energy distributions of the recoiling A photon echo, the optical analogue of spin echo, can be used in a direct method to measure relaxation The photon echo signal was generated by two excitation pulses and appears at the pulse separation time after the second pulse. In a recent study to measure relaxation behaviour of Pr3+in LaF,, a dye laser at 477.6 nm operating at 15 Hz and with a pulse duration of 3 ns was used. The output laser beam was split into two, one delayed by an optical delay line and the other focused into the sample contained in a cryostat. After excitation of the sample, a double-stage Kerr cell shutter blocked off the strong transmitting excitation pulses and prevented detector saturation. The shutter was opened at the echo signal position and the signal recorded p h o t ~ m e t r i c a l l y . ~ ~ ~ Simultaneous monitoring of transient optical absorbance and e.s.r. spectral changes has been obtained with an e.s.r. spectrometer equipped with a colinear dye laser excitation and monitoring beam.598 The arrangement was used to study intermediates in plant and bacterial photosynthesis with a time resolution of 200 ps. X-Band e.s.r. measurements at 77 K using Ar+ laser excitation haw been made of the dependence of the Rh6G triplet state on laser power and U.V. excitation was used in an e.s.r. quencher c o n c e n t r a t i ~ n s . ~ Modulated ~~ 6g1

6e3

694

6s6 6s6 687

69B

G. Dolan, and C. R. Goldschmidt, Chem. Phys. Letters, 1976, 39, 320. I. Itzkan and D. A. Leonard, Appl. Phys. Letters, 1975, 26, 106. J. H. Baldo, B. A. Manuck, E. B. Priestley, and B. D. Dykes, J. Amer. G e m . SOC., 1975,97, 1684. S. Ameen and L. De Maeyer, J. Amer. Chem. Soc., 1975,97, 1590; S. Ameen, Rev. Sci. Instr., 1975,46, 1209. G. Czerlinski and V. Bracokova, Appl. Optics, 1974, 13, 1639. R. K. Sander and K. R. Wilson, J . Chem. Phys., 1975, 63, 4242. N. Takeuchi, J . Luminescence, 1976, 12/13, 743. J. T. Warden and J. R. Bolton, Rev. Sci. Instr., 1976,47, 201. M. Yamashita and H. Kashiwagi, J. Phys. Chem., 1974, 78, 2006.

Developments in Instrumentation and Techniques

49 kinetic study of hydroxyphenoxyl radicals.600 A recent description of a pulsed photolysis-e.s.r. spectrometer system included a criticism of the use of pulsed laser sources for free radical studies.6o1 Steady-state concentrations of radicals are not likely with short pulse lasers, and these excitation sources usually have insufficientpower for U.V. excitation. A pulsed Hg lamp, providing pulse durations from 0.25 to 2.5 ms, was advocated as a more useful source and provided an order of magnitude increase in S/N ratio over a continuous Hg lamp.6o1 Subnanosecond Photophysical Techniques.-Despite the obvious impact which picosecond spectroscopy has made in the past few years on studies of primary processes in chemistry, physics, and biology, this technique is still little used since only a few laboratories are equipped to undertake more than the simplest experiments. Even so, it would be unrealistic in this section to try to cover details of all experimental arrangements published since 1974 since almost every chemical system requires a slightly different arrangement. The reader is referred to a general review on subnanosecond techniquesso2 which covers applications in chemical physics and biophysics and describes the three basic techniques which have evolved, namely probe, optical-gate, and streak-camera. Apart from publications cited here, a collection of papers 603 includes more detailed examples of these techniques. Probe Technique. This is the simplest arrangement for picosecond spectroscopy. A weaker pulse is reflected along a different and variable optical delay path to examine light absorption or scattering as a function of delay time after excitation. The probe pulse may be the fundamental or a harmonic of the excitation laser or frequency-broadened laser pulse (by self-phase modulation). For example, 354 nm laser light was used for excitation and 530 nm light for monitoring to measure the build-up of triplet absorption in nitronaphthalenes 604 and 4-(1-naphthylmethyl)benzophenone.605 A monochromatic probe beam technique with a mode-locked ruby laser was used to monitor the concentration dependence of the stilbene S1lifetime using two-photon absorption to populate this state.606Excitation and monitoring with the mode-locked second harmonic of a ruby laser enabled the triplet lifetime of valerophenone to be measured.6o7 Providing transient lifetimes are shorter than interpulse times, whole trains from mode-locked lasers can be used. The transmission of cryptocyanine as a function of power density was established in this way and the lifetime of this material (estimated to be 16 k 3 ps in methanol) determined with a 300 ps exciting A similar study used 6-7 pulses from a laser train to obtain transient Eo0

eol 602

603

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606

6oo 607 IJO~

K. Loth, M. Andrist, F. Graf, and H. H. Gunthard, Chem. Phys. Letters, 1974,29, 163. P. B. Ayscough, T. H. English, and D. A. Tong, J. Phys. (E), 1976,9, 31. R. R. Alfano and S. L. Shapiro, Physics Today, 1975,28 (7) 30; M. M. Malley in ‘Creation and Detection of the Excited State’, ed. W. R. Ware, Dekker, New York, 1974, vol. 2. ‘Lasers in Physical Chemistry and Biophysics’, ed. J. Joussot-Dubien, Elsevier, Amsterdam, 1975. R. W. Anderson, R. M. Hochstrasser, H. Lutz, and G. W. Scott, Chem. Phys. Letters, 1974, 28, 153. R. W. Anderson, R. M. Hochstrasser, H. Lutz, and G. W. Scott, Chem. Phys. Letters, 1975, 32, 204. E. Heumann, W. Triebel, and B. Wiihelmi, Chem. Phys. Letters, 1975, 32, 589. J. Faure, J.-P. Fouassier, and D.-J. Loughnot, J . Photochem., 1976, 5, 13. J.-P. Fouassier, D.-J. Loughnot, and J. Faure, Chem. Phys. Letters, 1975, 30, 448.

50

Photochemistry

absorptions in a polymethine laser dye.soBPolarized probe beams (Vol. 6, p. 105) have been applied to observation of charge-transfer interactions between anthracene and "-diet hylaniline. 610 Vibrational energy relaxation times of coumarin 6 in CCl, have been measured by populating a well-defined vibrational state and probing with the help of a delayed visible pulse which generated transitions to a fluorescent state. In this case, the measured relaxation time was 1.3 -L 0.3 Recovery times of Rh6G have been measured by a somewhat similar technique by preparing molecules in their excited state and probing their return to the ground state.612 Picosecond pulses at 1060 and 530nm were used in a Raman scattering experiment with aliphatic hydrocarbons in which the i.r. pulses were used to excite the stretching modes. The anti-§tokes Raman light was isolated with a spectrometer and integrated with a p h o t ~ m u l t i p l i e r . ~A~ ~new technique to measure fast recovery times of anisotropic absorption saturation used a short pulse from a cavity-dumped CW mode-locked dye laser at 600 nm to induce dichroism by saturating the absorption of molecules aligned parallel to the pump polarization and a weaker (polarized) probe pulse to measure and time-resolve this induced dichroism. The orientation relaxation time of DODCI in ethylene glycol was found to be 3 f 1 ns,613 which is far longer than previous values obtained using high-power lasers (cf. ref. 614). Photoselection was also reported for Rh6G in ethanol using a similar Optical-gate Technique. This technique is applicable to both transient absorption and emission studies and consists of passing the monitoring light of fluorescence through a CS2 cell situated between a crossed polarizer and analyser. If an intense picosecond pulse is also directed through this cell, a short-lived birefringence occurs, effectively 'opening' the gate for a very short time. By delaying the intense pulse by appropriate amounts, both time and intensity profiles may be obtained (with reflection and transmission echelons). A good example of this technique applied to measurements of fluorescent risetimes of eosin and erythrosin used pre-set delay times to build up intensity-time profiles over a few tens of ps.g16 A similar but more accurate measurement of the risetime of erythrosin was obtained by considering the actual correlation time of the optical These measurements showed that vibrational relaxation occurs in less than 1 ps with these dye molecules (cf. ref. 613) (which gives a groundstate relaxation time between 10-60 ps). Fluorescence risetimes of DMAB in solvents of various polarities were measured with a ps gate following excitation with 265 nm radiation.618 Operation of the CS, shutter with relatively low power, high repetition rate (> lo6s-l) pulses from a mode-locked dye laser has also eoP J.-P. a0 T. J.

Fouassier, D.-J. Loughnot, and J. Faure, Chem. Phys. Letters, 1975, 35, 189. Chung and K. B. Eisenthal, J. Chem. Phys., 1975,62, 2213. 611 A. Laubereau, A. Seilmeier, and W. Kaiser, Chem. Phys. Letters, 1975, 36, 232. @laD. Ricard, and J. Ducuing, J. Chem. Phys., 1975, 62, 3616. P. R. Monson, S. Patumtevapibal, K. J. Kaufman, and G. W. Robinson, Chem.Phys. Letters, 1974, 28, 312. C. V. Shank and E. P. Ippen, Appl. Phys. Letters, 1975, 26, 62. H. E. Lessing, Optics Quantum Electron., 1976, 8, 309. s16 G. Porter, E. S. Reid, and C. J. Tredwell, Chem. Phys. Letters, 1974, 29, 469. G . Mourou and M. M. Malley, Chem. Phys. Letters, 1975, 32, 476. el* W. S. Struve and P. M. Renzepis, Chem. Phys. Letters, 1974, 29, 23.

Developments in Instrumentation and Techniques

51

been reported; a response time of 2.1 k 0.3 ps was found.61g Spontaneous fluorescence of aromatic mixed crystals has been measured with the optical-gate on a spectrograph using a stepped mirror array to display both time- and wavelength-resolved spectra up to 200ps from excitation; the image at the spectrographic plate was amplified with an image intensifier.620 In place of the CS, and crossed-polarizer arrangement it is possible to use a saturable absorbing dye (such as DODCI). A second pulse, at a wavelength within the emission band of the bleachable dye, can rapidly deplete inversion and thus terminate any gain (the first pulse causes population inversion).621 Aperture times can be reproducibly varied from 12 to 60 ps, depending on the recovery time of the dye. Characteristics of this optical amplifier using DODCI as the bleachable dye showed a high gain (> lo2),large extinction ratios (> lo6), and short aperture times.s22 A ‘double-beam’ picosecond spectrometer using two sets of interrogating pulses, one immediately before photolysis and the other coincident with the photolysis single pulse, enabled the sensitivity of real absorbance changes measured with a single pulse to be increased from 0.3 to 0.001 absorbance As an example of the many possible applications of this method, the decay kinetics of tetracene dianions were determined following excitation at 530 nm.623 By far the best known use of the optical-gate technique is simultaneous time and wavelength resolution of transient absorptions using the continuum produced by self-phase modulation as a monitoring source (see Vol. 6, p. 104). Recent studies using the broadband continuum from CCI, involve optical bleaching of solvated electrons 624 and electron localization times.626 Photobleaching and recovery times of DODCI were measured in a similar arrangement with a continuum generated in either H,O or CeH6 by a single 530 nm pulse.626 The novel spectroscopic apparatus described earlier (Vol. 6 , p. 104) which uses a CCI, continuum has been employed to determine excited states of transitionmetal complexes,627transient absorptions in BDN,628and the absorption spectra of excited state of octaethylporphinatotin(1v) dichloride 629 using an OMA to facilitate intensity measurements. A more intense continuum which has very little variation with wavelength has been developed from polyphosphoric acid pumped by the second harmonic of a ruby laser.63o Unlike CCI4 and H20, this N

620

e21 62a

=-

E. P. Ippen and C. V. Shank, Appl. Phys. Letters, 1975,26,92. R. M. Hochstrasser and J. E. Wessel, Chem. Phys., 1974, 6, 19. G. L. Olson and G . E. Busch, Appl. Phys. Letters, 1975, 27, 684. G. E. Busch, K. S. Greve, G . L. Olson, R. P. Jones, and P. M. Rentzepis, Appl. Phys. Letters, 1975, 27, 450.

624

e26

T. L. Netzel and P. M. Rentzepis, Chem. Phys. Letters, 1974, 29, 337. D. Huppert, W. S. Struve, P. M. Rentzepis, and J. Jortner, J. Chem. Phys., 1975, 63, 1205. D.Huppert and P. M. Rentzepis, J. Chem. Phys., 1976, 64, 191; D. Huppert, W. S. Struve, P. M. Rentzepis, and J. Jortner, ibid., 1975, 63,3. G. E. Busch, K. S. Greve, G . L. Olson, R. P. Jones, and P. M. Rentzepis, Chem. Phys. Letters, 1975, 33, 412, 417.

A. D. Kirk, P. E. Hoggard, G. B. Porter, M. G . Rockley, and M. W. Windsor, Chem. Phys. Letters, 1976, 37, 199. D.Magde, B. A. Bushaw, and M. W. Windsor, Chem. Phys. Letters, 1974, 28, 263. D. Magde, M. W. Windsor, D. Holton, and M. Gouterman, Chem. Phys. Letters, 1974, 29, 183.

030

N. Nakashima and N. Mataga, Chem. Phys. Letters, 1975, 35, 487; N. Mataga and N. Nakashima, Spectroscopy Letters, 1975, 8, 275.

52

Photochemistry

material is claimed to produce all wavelengths simultaneously (despite known group velocity changes due to the dispersive medium) with little delay at long wavelengths (cf. Chem. Phys. Letters, 1974,27, 31). A good example of a difficult transient absorption to monitor because of an overlap with a strong fluorescence band is the singlet absorption of anthracene, which was observed with a picosecond spectroscopic arrangement using the continuum from polyphosphoric acid. A distance of 3 m was used between the sample cell and spectrograph to minimize any fluorescence interference without losses on the continuum, which has a low beam divergenceas31A further example of this technique in the visible and near4.r. is the singlet absorption spectrum of 9,9’-bianthryl in h e ~ a n e . ~ ~ ~ Other examples of the use of continuum generation have been reported for two-photon spectroscopy 632 and inverse Raman spectroscopy of a number of organic A fibre optic technique has been used to provide several fixed delays for ps Streak Camera Technique. Ambiguities in results and interpretation in this time-scale can be minimized, for fluorescence measurements at least, by using a streak camera which will produce real-time decay profiles in a single shot. Fluorescence lifetimes of algae, chloroplasts, and chloroplast fragments, following excitation with a train of 530nm pulses, have been recorded on an Imacon streak camera fitted with an optical multichannel analyser in place of the usual photographic plate.635 Other examples include DODCI isomers 636 and car0tenes.637 Under perfect model-locking conditions, pulses from an Nd:glass laser (bandwidth 1&20 nm) should be theoretically about 400 fs duration. In practice, 6 ps pulses are observed which sweep the oscillating bandwidth with time. This ‘chirping’ phenomenon has been explained in terms of the time broadening of a transform-limited pulse by group velocity changes within the dispersive laser medium.638 By using a pair of diffraction gratings to delay the frequency component of the leading edge with respect to that of the trailing edge to compensate for this time distortion, it is possible to shorten pulses to close to their theoretical limit.639,640 With broader-bandwidth lasers, such as CW modelocked dye laser^,^^^^ 641 pulses as short as a few hundred femtoseconds have been Perhaps the first example of transient absorption measurements in this time region was an investigation of haemoglobin and its complexes using optical pulse correlation techniques.643Real-time measurements (with a resolution of 0.5 ps) using these laser pulses and the fast streak camera mentioned earlier 359 would also appear to be possible. N. Nakashima, M. Murakawa, and N . Mataga, Bull. Chem. SOC.Japan, 1976,49, 854. A. Penzkofer, W. Falkenstein, and W. Kaiser, Appl. Phys. Letters, 1976, 28, 319. aS3 S. H.Lin, E. S. Reid, and C. J. Tredwell, Chem. Phys. Letters, 1974,29, 389. 1 3 ~G. ~ S. Beddard, G. Porter, C. J. Tredwell, and J. Barber, Nature, 1975,258,166. 836 M. J. Colles and G. E. Walrafen, Appl. Spectroscopy, 1976,30,358. 636 J. C . Mialocq, A. W. Boyd, J. Jaraudias and J. Sutton, Chem. Phys. Letters, 1976,37, 236. m7 A. J. Campillo, R. C. Hyer, V. H. Kollmann, S. L. Shapiro, and H. D. Sutphin, Biochim. Biophys. Acta, 1975, 387, 533. 838 D.J. Bradley and G. H. C. New, Proc. I.E.E.E., 1974,62, 313. 638 E. B. Treacy, Phys. Letters, 1968, 28A, 34. E. B. Treacy, Appl. Phys. Letters, 1969,14, 112. eP1 C. K. Chan and S. 0. Sari, Appl. Phys. Letters, 1974,25,403. ea2 E. P. Ippen and C. V. Shank, Appl. Phys. Letters, 1975,27,488. 643 C . V. Shank and E. P. Ippen, Science, 1976,193, 50. as1

832

Developments in Instrumentation and Techniques

53

Miscellaneous Applications. Two further descriptions have appeared of stroboscopic picosecond pulse radiolysis 646 including modifications for use of a single pulse as opposed to a train.644 Transient ions produced by the pulsed photolysis of chlorophyll in solution 646 and in thin films 647 have been detected with the aid of photoconductive apparatus, Most studies of the chemistry of free radicals in both flash photolysis and pulse radiolysis are carried out by time-resolved absorption spectroscopy. Although this is a useful technique for kinetic studies, little information is given of structural characteristics. The first report has appeared of a pulsed resonance Raman apparatus to study short-lived free radicals.648 Radical anions of p-terphenyl (produced by electron irradiation) in either tetrahydrofuran (stable) or ethanol-ethylenediamine (transient) were examined by a single dye laser pulse. The resonance Raman spectrum was recorded by an optical multichannel system consisting of an image intensifier coupled to a TV camera.64s Vibrationtranslation relaxation times in liquid O2 and N2 have been obtained using stimulated Raman scattering (with a Q-switched ruby laser) to excite the first vibrational energy level, followed by probing the density change with a CW gas laser using a Schlieren method.649 10 Transient Emission Spectroscopy Instruments and Methods.-Methods for measuring fluorescence lifetimes of biomolecules have been reviewed in an article outlining some (but not all) of the advances in instrumentation during 1974-1 975, methods of analysis, and lifetime standards, and giving examples of studies on proteins and polypeptides, nucleic acids, membranes, and photosynthetic pigments.650 Limitations in accuracy due to drift and fluctuations in excitation light intensity during the course of an experiment, and weak excitation pulses, are drawbacks claimed by some researchers to pulse-sampling and photon-counting methods, respectively, for determining fluorescence lifetimes.651 Convincing evidence was presented to show that a pulse-sampling fluorometer employing a boxcar averager with baseline sampling facilities can produce lifetimes as accurate as other techniques (photon-counting and phase) but with much shorter collection Time-resolved spectra, which are usually reserved for a photon-counting method, have been recorded for various carbonyl compounds using an N2laser for excitation and a (gated) boxcar integrator.652 Although nanosecond resolution is possible, this technique is more suitable and clearly superior to photon-counting (using time-to-amplitude converters) at longer (Ips

646 647

6L1 652

C. D. Jonah, Rev. Sci. Znstr., 1975, 46, 62. C. D. Jonah, J. R. Miller, E. J. Hart, and M. S. Matheson, J. Phys. Chem., 1975,79, 2705. T. Imura, T. Furutsuka, and K. Kawabe, Photochem. and Photobiol., 1975, 22, 129. C. W. Tang, F. Douglas, and A. C. Albrecht, J . Phys. Chem., 1975, 79, 2723. P. Pagsberg, R. Wilbrandt, K. B. Hansen, and K. V. Weisberg, Chem. Phys. Letters, 1976,39, 538. G . Renner and M. Maier, Chem. Phys. Letters, 1974, 28, 614. L. S. Forster, Photochem. and Photobiol., 1976, 23, 445. M. G. Badea and S. Georghiou, Rev. Sci. Znstr., 1976, 47, 314. R. F. Brown, K. D. Legg, M. W. Wolf, L. A. Singer, and J. H. Parks, Analyt. Chem., 1974, 46, 1690; D. Creed, P. H. Wine, R. A. Caldwell, and L. A. Melton, J. Amer. Chem. SOC.,1976, 98, 621.

54

Photochemistry

times, i.e. up to milliseconds. Other examples of apparatus using pulsed lasers with boxcar detection for lifetime measurement 653-655 include applications to OH and OD radicals 654 and LiH and NaH molecular A comparison of fluorometric methods (phase, pulse sampling, and timecorrelated single-photon counting) points out the advantages of the last method for studies on weak absorbers or fluorophors, non-exponential fluorescence decays or time-resolved A detailed description of a gated lamp and nanosecond spectrometer was accompanied by examples of applications in energy transfer and fluorescence quenching and complex formation.656 Accurate measurements of time-resolved spectra require that equal light intensities fall onto the sample during each measurement period. In order to monitor fluctuations in the lamp intensity, a photomultiplier was used to integrate the lamp output and produce a pulse to advance the multichannel analyser by an equivalent wavelength increment.657 A nitrogen-filled, high-pressure, free-running lamp was shown to give a total light intensity 250 times higher than a spark in air at normal pressures 658 with a continuum between 200 and 300 nm. Free-running lamps usually operate at low frequencies (up to 5 kHz), and gated lamps are preferred because their higher operation frequency ( > 10 kHz) facilitates data collection. Unfortunately, gated lamps are relatively weak and their intensity is not helped by discharge in a large volume lamp. By confining the discharge to a gas jet rather than a quartz capillary it may be possible to increase brightness and match the discharge to monochromator slit dimensions. A lamp designed to pump a dye laser at 100 Hz used this gas jet principle to attain a lamp life of lo7 shots and produce a flash duration of 40 ns at 50 mJ input.65QThis principle could be used to advantage in a high-energy, high-repetition rate gated lamp. Radiation produced from a storage ring synchrotron has many of the ideal characteristics for time-correlated single-photon counting. Not only is the radiation an intense continuum extending from the i.r. to X-ray but it is emitted in the form of short pulses (1 ns) at frequencies from 10 to 400 MHZ.~~O Fluorescence decay profiles of NO at narrow band excitation (45 cm-l) and vacuum-u.v. fluorescence of atomic fragments of O2and N2662 are representative examples of such studies. Corrections to fluorescence decay profiles because of the finite duration of the excitation pulse have been discussed in a number of p u b l i ~ a t i o n s . ~Computer ~~-~~~ 663 664 656 666 667

668 669

660

661 663

664

666

ee6

E. Photos and G. H. Atkinson, Chem. Phyx. Letters, 1975, 36, 35; W. Becker, S. Daehne, K. Teuchner, and K. Selinger, Exp. Spec. Phys., 1975, 23, 297. K. R. German, J. Chem. Phys., 1975, 62, 2584. P. J. Dagdigian, J . Chem. Phys., 1976, 64, 2609. M. A. West and G. S. Beddard, Internat. Laboratory, 1975 (5/6), 61. G. S. Beddard and P. Williams, J. Phys. (E), 1975, 8, 720. M. F. Thomaz, I. Barradas, and J. A. Ferreira, J . Luminescence, 1975, 11, 55. C. M. Weysenfeld, Appl. Optics, 1974, 13, 2816. R. Lopez-Delgado, A. Tramer, and I. H. Munro, Chem. Phys., 1974, 5, 72. 0. Benoist D’Azy, R. Lopez-Delgado, and A. Tramer, Chem. Phys., 1975, 9, 325. L. C. Lee, R. W. Carlson, D. L. Judge, and M. Ogawa, J. Chem. Phys., 1974,61, 3261. L. A. Shaver and L. J. C. Love, Appl. Spectroscopy, 1975, 29, 485. G. Hazan, A. Grinwald, M. Maytal, and I. 2. Steinberg, Rev. Sci.Instr., 1974,45, 1602. C. Holzapfel, Rev. Sci. Instr., 1974, 45, 894. B. van Meurs and R. van der Wed, J. Phys. (E), 1974,7,437. A. Heiss, F. Dorr, and I. Kuehn, Ber. Bunsenges Phys. Chern., 1975, 79, 294; A. Gafni, R. Modlin, and L. Brand, Biophys. J., 1975,15,263.

Developments in Instrumentation and Techniques

55

deconvolution is clearly the preferred technique when a fluorescence lifetime is comparable to the pulse source lifetime, and a rule of thumb has established that serious errors in graphical slope-calculated lifetimes will occur if this difference is less than 2 ns.663 For pulsed sources with lifetimes in the 1-3 ns range, the percentage error is 5% or less for fluorophors having lifetimes of 3-5 ns. Instrumental drift resulting in changes of the lamp profile with time has been minimized in an arrangement in which alternate collection of data for the fluorescence and lamp has been adopted.664 This procedure was shown to reduce the effective lifetime of anthracene from 4.17 to 3.97 ns. Statistical methods applied to time-correlated single-photon counting showed that the histogram on the MCA was distorted due to processes of signal sampling. The degree of distortion depended on the count rate and lifetime.666 A 'pile-up' inspector which removes distortion due to the arrival of more than one photon per detection cycle was designed for long decay times (inspection interval from 400 ns to 200 p).s66 Other papers deal with application of Laplace transforms to analyse decay curves.667 Errors in deconvolution introduced because of wavelength effects (Vol. 5, p. 115) have been considered in recent work,668,669 and alternative methods involve the use of a single standard compound of known lifetime. A calibration function for a photomultiplier as a function of wavelength was determined by measuring lifetimes of various short-lived scintillators whose broad emission spectra spanned excitation and emission wavelengths of compounds being A somewhat similar method compared two materials (e.g. anthracene and indole) whose lifetimes were known approximately under the same conditions of bandwidth and excitation and emission wavelength.668Accurate lifetimes were determined by a best-fit condition as the estimated lifetime of one material was altered. The importance of maintaining large container dimensions in studies of long-lived gas-phase molecules has been emphasized in a study made with an integrating-sphere fluorescence cell.670Cell dimensions should be several times greater than the distance travelled by a molecule during its lifetime to avoid instrumental bias on measured fluorescence intensity, yield, and lifetime. Phase fluorometry is not often used for analysing non-exponential decay functions since simultaneous measurement of phase angle and amplitude is difficult. However, an instrument with a continuously variable frequency range (0.5-72 MHz) has been described which can determine phase angles (with an 1") and modulation depth as a function of frequency.671 The accuracy of overall accuracy of the instrument is reported to be +4% for (exponential) decays between 2 and 300 ns, and the time-resolving power is 10-l1 s at 72 MHz. Application of the system to kinetic studies of 2-naphthol-naphtholate prototropic fluorescence changes revealed variations in lifetime with both modulation frequency and acid concentration. The phase fluorometer employing photoncounting described earlier (Vol. 6, p. 108) has been modified with a discriminator to detect the zero-crossing point of the sinusoidal reference signal followed by the TAC.672

+

w8

w9 670

671

wa

A. Britten and G. Lockwood, Mol. Photochem., 1976, 7 , 79. A. G. Szabo, P. A. Hackett, A. E. McKinnon, and D. M. Rayner, J. Photochem., 1976,5,185. G . I. Senuin and S. E. Schwartz, Appl. Optics, 1975, 14, 1143. M. Hauser and G. Heidt, Rev. Sci. Instr., 1975, 46, 70. D. W. Chandler, Rev. Sci. Instr., 1975, 46, 786.

56

Pliotocheniistry

A method described to measure weak luminescence lifetimes with photoncounting does not use a multichannel a n a l ~ s e r .The ~ ~ ~amplitude-normalized decay integral (ANDI) method relies on the area under an exponential decay of the form Ioe-t/Tbeing simply I07,from which T may be determined once I. is known. Photon-counting using scalars can be employed to measure I. for various time intervals. Although examples quoted included lifetimes longer than about 40 ps, the ANDI technique was regarded as applicable to measurement of lifetimes as short as 30 ns.673 Other miscellaneous arrangements for fluorescence lifetime determinations include use of a cavity-dumped argon laser (9 ns duration) and pulse sampling,674 a broadband continuum air spark (whose duration was wavelength-dependent) produced by focusing a ruby laser beam in air to excite materials at 77 K,g76 and a gated detection system to measure decays 50 ns after an electron irradiating An arrangement of little novelty and applicability, claiming to be a 'low cost pulsed tunable U.V. laser' for lifetime measurements, used an N 2 laser (of low repetition rate) and pulse sampling (with a computer of average Phosphorescence lifetimes and non-exponential decays have been determined by a photon-counting arrangement combining multichannel scaling with computer processing.s78 A two-chopper method for measuring microsecond lifetimes has many similarities to a spectroph~sphorimeter.~~~ Radiative lifetimes of individual rotation levels of CN produced by the vacuum-u.v. flash photolysis of C2Nahave been determined using an N,-laser-pumped dye pulse for excitation and a time-of-flight photon-counting system.680 A fluorescence chamber equipped with two Wood's horns was designed to reduce scattered light to 1O-Io for lifetime measurements of OH using a doubled dye laser pulse (282.5 nm) for excitation.681 A similar source has been used to excite propynal vapour in an arrangement to measure transmitted laser intensity and the time dependence of the fluorescence.682 Vibrational relaxation in CO has been studied by observing i.r. fluorescence decays following excitation with a Raman laser (Ha + CO pumped by a ruby laser).ss3 Radiative lifetimes of excited ionic states, such as CO+, N20+, and C 0 2 + ,have been determined using a modified He(1) photoelectron spectrometer and time-correlated single-photon The lifetime of the E -+ B transition in molecular iodine was determined using a technique based on populating the E state by two-photon absorption.685 In the experimental arrangement, two laser beams were sent from opposite directions into a cell containing iodine vapour at 0.25 Torr. One beam was from a CW dye laser (tunable from 560 to 630nm) and induced transitions from the 673 674

676

677 678 6Bo

682

684 Oa6

S. Arnold and N. Wotherspoon, Rev. Sci. Instr., 1976, 46, 751. F. E. Lytle and M. S. Kelsey, Analyt. Chem., 1974, 855. A. Bromberg, D. M. Friedrich, and A. C. Albrecht, Chem. Phys., 1974, 6, 353. D. G . Jameson and B. D. Michael, J. Phys. ( E ) , 1974, 7 , 208. L. J. Andrews, C. Mahoney, and L. S. Forster, Photochem. and Photobiol., 1974, 20, 85. J. Addison, Y. Kumar, G . P. Semeluk, J. Singh, and 1. Unger, J . Photochem., 1976, 5, 185. K. A. Ingersoll, Appl. Optics, 1976, 15, 61. W. M. Jackson, J. Chem. Phys., 1974, 61, 4177. P. Hogen, and D. D. Davis, Chem. Phys. Letters, 1974,29, 555. C. A. Thayer and J. T. Yardley, J . Chem. Phys., 1974, 61, 2487. H. Matsui, E. L. Resler, and S. H. Bauer, J. Chem. Phys., 1975,63,4171. M. Bloch and D. W. Turner, Chem. Phys. Letters, 1975, 30, 344. D. L. Rousseau, J . Mol. Spectroscopy, 1975,58,481.

57

Developments iii Instrumentation and Techniques

X(lZ&) state to the B(377&J state. The second beam, from a krypton laser at 350.7 nm, induced transitions to discrete levels of the E state from the levels of the B state populated by the dye laser. The frequency of the dye laser was then scanned and frequencies of the discrete levels in the B state manifold separated from E state levels by 350.7 nm were located by monitoring E B transitions on a spectrometer. Lifetimes were determined by a delayed coincidence technique using an acousto-optically modulated U.V. laser (100 ns pulses with a rise and decay time of less than 10 ns).685 --f

Applications.-The following examples of photochemical and spectroscopic studies involving time-resolved measurements of fluorescence are to indicate the sensitivity and selectivity of this general technique and, in particular, the applicability and advantages of lasers. High-resolution studies have once again benefited from high-power, narrowbandwidth, tunable lasers used as excitation sources. Single vibronic levels of glyoxal have been excited with an N,-laser-pumped dye laser,6s6and a 0.5 cm-l bandwidth frequency-doubled dye laser has been used to provide dynamic ~? information on vibronic excited states of naphthalene (at 0.07 T ~ r r ) . ~Very narrow bandwidth (1 GHz) dye lasers have allowed lifetime measurements of individual rovibrational levels in the iodine B state688and Ka = 0, 2B1states of Other laser studies include measurement of the Ca Swan band,6go11 common laser rhodamine dyes at low temperature^,^^^ xanthene dyes following biphotonic excitation,693gaseous UF6,694OD and OH chlorophyll a and crude Cross-beam conditions have been used to study fluorescence decays of simple inorganic compounds (BaO, A10, BaC1) resulting from atomic The kinetics of formation of BO* and BCl* produced by pulsed CO, laser irradiation of BCl, + 0,mixtures have been obtained from luminescence pulse shape in the visible region.699 Laser pulse amplitude variations have been corrected for in a ratio circuit where part of the laser pulse was allowed to excite a long-lived phosphor to simplify integration of J of 5-10 ns duration).700 short pulses (typically Conventional time-correlated single-photon counting has been employed to measure lifetimes of selectively populated hyperfine structure levels in 686

eS7 688

689

6B0 6B1

6B2 693

6s4

6B6

697

6B8 6g9

'0°

R. A. Beyer, P. F. Zittel, and W. C. Lineberger, J . Chem. Phys., 1975, 62, 4016. U. Boesl, H. J. Neusser, and E. W. Schlag, Chem. Phys. Letters, 1975, 31, 1. M. Broyer, J. Vigue, and J. C. Lehmann, J . Chem. Phys., 63, 5428. Y. Haas, P. L. Houston, J. H. Clark, C. B. Moore, H. Rosen, and P. Robrish, J. Chem. Phys., 1975, 63, 4195. T. Tatarcyzk, E. H. Fink, and K. €3. Becker, Chem. Phys. Letters, 1976, 40, 136. J. Knof, F.-J. Thiess, and J. Weber, Optics Comm., 1976, 17, 264. F.-J. Thiess and J. Weber, Optics Comm., 1974, 12, 368. G . C. Orner and M. R. Topp, Chem. Phys. Letters, 1975,36,295. P. Benetti, R. Cubeddon, C. A. Sacchi, 0. Svelto, and F. Zaraga, Chem. Phys. Letters, 1976, 40, 240. J. H. Brophy, J. A. Silver, and J. L. Kinsey, Chem. Phys. Letters, 1974, 28, 418. R. Arsenault and M. M. Denariez-Roberge, Chem. Phys. Letters, 1976, 40, 84. R. M. Measures, W. R. Houston, and D. G . Stephenson, Laser Focus, 1974, 10(11), 49. J. G . Pruett and R. N. Zare, J. Chem. Phys., 1975, 62, 2050; P. J. Dagdigian, H. W. Cruse, and R. N. Zare, ibid., p. 1824; G. P. Smith and R. N. Zare, ibid., 1976, 64, 2632. V. N. Bourimov, V. S. Letokhov, and E. A. Ryabov, J. Photochem., 1976,5,46. F. Castelli and R. D. Hefner, Rev. Sci. Instr., 1976, 47, 509. F. Paech, R. Schmiedl, and W. Demtroder, J. Chem. Phys., 1975, 63, 4369.

58

Photochemistry

Na(4,P) produced by 100 kHz vacuum-u.v. flash photolysis of NaI v a ~ o u r , ~ O ~ and 9,lO-diphenylanthracene with both excitation and emission monochromators to eliminate scattered light from the decay curve.7o3Fluorescence reabsorption in anthracene single crystals has been determined on a conventional apparatus using a 2.5 kHz hydrogen lamp.7o4 Fluorescence measurements made on small molecules and atomic species have included the determination of rate constants for deactivation of O(l0) O(lS), N2(20), and N2(A3C,+)decays from the (1 p)vacuum-u.v. flash photolysis of N20,?06the lifetime of NH, by multichannel scaling,7o7and the radiative lifetime of NH (bl+, u1 = 0) following vacuum-u.v. flash p h o t o l y s i ~ , ~ ~ ~ In the last example, the stability and linearity of the detection system were checked with attenuated light from a green LED in an RC circuit. A 30cm diameter cell has been used in a time-resolved fluorescence study of SO, following excitation with a frequency-doubled dye laser (tunable from 260 nm to 325 nm).?OO A time-resolved phosphorescence study on the same molecule in the presence of dichloroethylenes used a similar laser source.71o A modulated microwave source (1 p s duration) was used in a study on imprisonment and absorption of argon resonance radiation at 130 nm with a channeltron as a detector.?ll Ionizing radiation in the form of a 70 keV electron beam has been used to investigate the lifetime of a new cerium pentaphosphate s ~ i n t i l l a t o r . ~ A~ ~ combination of a pulsed dye laser with a pulsed ion cyclotron resonance spectrometer was the basis of a powerful experimental method for studying laserinduced ionic processes.713 A real-time method for measuring the fluorescence of toluene induced by 330 ps pulse from a 3 MeV electron accelerator used either a 75 ps risetime photodiode or an 850 ps risetime photomultiplier for detection followed by sampling to improve the S/N ratio.714 Fluorescence of the ketyl radical produced by the laser photolysis 718 and pulse radiolysis ?17 of benzophenone solutions has been characterized. Consecutive two-photon (353 + 530 nm) fluorescence excitation 716 was shown to be more sensitive than absorption spectroscopy for studying this radical product in a flash photolysis experiment. 11 Signal Processing Computer control of spectroscopic instruments is now quite common, and a revolution in instrument control and data reduction and analysis is now happening ?Oa ?Oa ?04 ?06

?06 ?07 ?08 ?09

?ll 718

713 71* 716

R. Bersohn and H. Horwitz, J. Chem. Phys., 1975, 63, 48. D. J. S. Birch and R. E. Imhof, Chem. Phys. Letters, 1975, 32, 56. R. J. Bateman, R. R. Chance, and J. F. Hornig, Chem. Phys., 1974,4,402. J. A. Davidson, C. M. Sadowski, H. I. Schiff, G. E. Streit, C. J. Howard, D. A. Jennings, and A. L. Schmeltekopf, J. Chem. Phys., 1976, 64, 57. G. Black, R. L. Sharpless, T. G. Slanger, and D. C. Lorents, J. Chem. Phys., 1975, 62,4266. J. B. Halpern, G. Hancock, M. Lenzi, and K. H. Welge, J. Chem. Phys., 1975,63,4808. B. Gelernt, S. V. Filseth, and T. Carrington, Chem. Phys. Letters, 1975, 36, 238. L. E. Brus and J. €2. McDonald, J. Chem. Phys., 1974, 61, 97. F. B. Wampler and J. W. Bottenheim, Internat. J. Chem. Kinetics, 1976, 8, 585. M. J. Boxall, C. J. Chapman, and R. P. Wayne, J. Photochem., 1975, 4, 281. D. Bimberg, D. J. Robbins, D. R. Wight, and J. P. Jeser, Appl. Phys. Letters, 1975,27, 67. J. R. Eyler and G. H. Atkinson, Chem. Phys. Letters, 1974, 28, 217. G . Beck, J. T. Richards, and K. J. Thomas, Chem. Phys. Letters, 1976, 40, 300. R. Mehnert, 0. Brede, and W. Helmstreit, 2. Chem., 1975, 15, 448. M. R. Topp, Chem. Phys. Letters, 1976, 39, 423. B. W. Hodgson, J. P. Keene, E. J. Land, and A. J. Swallow,J. Chem. Phys., 1975,63,3671.

Developments in Instrumentation and Techniques

59

because of applications of microprocessor^.^^^ These are large-scale integrated circuit chips which have much of the capability of minicomputers because of memory units known as RAMS or PROMS. It is a safe prediction to make that many u.v.-visible and i.r. absorption spectrometers and fluorescence spectrometers will soon incorporate these devices to facilitate scan control, signal processing, and correction. At present, however, mixed-order kinetics must be handled by mathematical analysis 71g or, in the case of simple exponentials, by an analogue device 720 or equivalent transient recorder.721 A digitizing system uses enlarged back-projected images of photographic records and a minicomputer.722 Lock-in amplifiers are among the most common and effective instruments in a photochemistry laboratory for measurements of noisy signals. A design of one of these instruments with a phase-locked loop is claimed to offer enhanced performance for low construction A related publication giving a formalized approach to phase-sensitive detection is an overview of the subject written to illustrate both the theory and application.72P J. Petruzzi, Analyt. Chem., 1974, 46, 915A; R. E. Dessey, P. Janse-Van Vuuren, and J. A. Titus, Analyt. Chem., 1974, 46, 917A; R. E. Dessey, J. A. Titus, and P. Janse-Van Vuuren, ibid., p. 1055A. A. R. Watkins, 2.Phys. Chem. (Frankfurt), 1975,96, 125; Mol. Photochem., 1976,7, 171. 7 a o C. K. Chang, Appl. Spectroscopy, 1976, 30, 364. 721 P. H. Daum and P. Zamie, Analyt. Chem., 1974, 46, 1347. 7ea 1. D. G. Macleod and M. R. Siegrist, J. Phys. ( E ) , 1975, 8, 896. 723 G . Horlick and K. R. Betty, Analyt. Chem., 1975, 47, 363. la* D. P. Blair and P. H. Sydenham, J. Phys. (E), 1975, 8, 621. 718

2 Photophysical Processes in Condensed Phases BY K. SALISBURY

1 Introduction Progress has been reported on a wide front in this area of photochemistry. Welldefined standards for the measurement of
fluorescence lifetimes (q),oscillator Table 1 Fluorescence quantum yields (Of), strengths (f), and radiative rate constants (kfo) for 9,l O-diphenylanthracene in solution at room temperature a 3-Methylpentane Cyclohexane Benzene 0.93 0.86 0.82 7.88 7.58 7.34 -rf/ns k*O/l08s-1 1.13 1.13 1.12 0.176 0.176 0.175 f a, ( n ) 0.95 0.96 0.95 ktO(n)x 10' s-' 1.30 1.21 1.25 a Values in parentheses determined at 77 K. Superscript n denotes the application of a refrac-

a*

Ethanol 0.95 8.19 (7.95) 1.16 0.175 0.95 1 16

tive index correction. k f o = (Dflrf.

'

The importance of concentration effects was also emphasized. Although the general agreement for both Tf and Ofbetween this and earlier work is reasonable, I believe that it is becoming clear that at this time the reproducibility of @f and Tf values from one laboratory to another is nowhere near as good as quoted error limits might indicate. Thus DPA may be given a value of 0.9 f 0.1 for
J. V. Morris, M. A. Mahaney, and J. R. Huber, J. Phys. Chem., 1976, 80,969. L. J. Cline Love and L. A. Shaver, Analyt. Chem., 1976, 48, A364.

60

Photophysical Processes in Condensed Phases

61

data d i s c ~ s s e d*. ~ ~ Appropriate correction factors for variable temperature studies of solution emission have been analysed in detail, and it turns out that if a correction of n-, is applied to radiative lifetimes an error of less than 10%is likely to result. On the other hand, the assumption of a constant value for kf may lead to large errors (up to 100%) in derived q~antities.~ In recent years there has been a steady increase in the number of molecules identified as exhibiting S, -+So fluorescence and interest in this area has increased. However, the assertion that in all molecules in which the energy gap between the lowest vibrational levels of the first two excited singlet states is small and comparable with kT, a considerable repopulation of S, and thus S2-+ So fluorescence should be expected, remains to be proved. For anthracene and 4,4’-dimethylstilbene samples in stretched polyethylene films, linear dichroic measurements have revealed that So -+ S2and So -+ S1absorptions contribute to the total fluorescence. Furthermore, it is suggested that the application of the simple techniques of polarized absorption and emission spectroscopy, in revealing dual (S, -+ So and S1-+ So)emission may help to rationalize some of the anomalies in some systems where (as is usual) one has assumed that a single transition is involved in the fluorescence process.s Some complications arising from restricted rotatory Brownian motion in the analysis of fluorescence detected circular dichroism have been analysed and attributed to photo~election.~A general theory of fluorescence correlation spectroscopy (FCS), including the effects of translational and rotational motions and chemical reactions in ideal solutions has been derived.8 Explicit equations for the calculation of time resolved emission spectra for molecules undergoing collisional vibrational relaxation have been presented, and these same equations may be used to describe the steadystate (frequency-resolved) emission spectra as a function of added vibrational quencher gas.g The use of time-resolved spectroscopy and travelling wave dye lasers in studying liquid state relaxation processes of electronically excited fluorescent molecules has been described.1° On the other hand, a much simpler device using a high pressure (11 atm) N, spark lamp and a conventional pulse fluorometer system has also been used to obtain time-resolved emission spectra. Although the use of high pressures in the lamp leads to a very great increase in light output per pulse, particularly in the 250-300nm region, compared with the normal 0.5-1 .O atm lamps, the pulse width is usually increased ( 37 ns).I1 The azobenzene system has been developed as a convenient actinometer for the determination of quantum yields of photochemical reactions.12 The nature of the non-radiative processes in benzene have been discussed from a theoretical point of view.13

* lo

l1 la

l3

L. A. Shaver and L. J. Cline Love, Appl. Spectroscopy, 1975, 29, 485. A. Britten and G. Lockwood, Mol. Photochem., 1976,7, 79. J. Olmsted, Chem. Phys. Letters, 1976, 38, 287. L. Margulies and A. Yogev, Chem. Phys. Letters, 1976, 37, 291. B. Ehrenberg and I. Z . Steinberg, J. Amer. Chem. SOC.,1976, 98, 1293. S. R. Aragon and R. Pecora, J. Chem. Phys., 1976, 64, 1791. G. R. Fleming, 0. L. J. Gijzeman, K. F. Freed, and S. H. Lin, J.C.S. Faraduy ZZ, 1975,71,773. C. P. Keszthelyi, Spectroscopy Letters, 1975, 8, 931. M. F. Thomaz, I. Barradas, and J. A. Ferreira, J. Luminescence, 1975, 11, 55. G. Gauglitz, J . Photochem., 1976, 5, 41. S. J. Formosinho and J. Dias da Silva, Mol. Photochem., 1974, 6, 409.

Photochemistry

62

A brief survey of the current state of knowledge of the electronic relaxation processes in benzene and related molecules has been given14 and Carroll and Quina have given a detailed account of their new method of determining intersystem crossing quantum yields.15 Scheme 1 outlines the important processes lA0

'A,

lA1

+C lA,

lAl

+X 3A

3A+ C

3 c

___+

---+

____+

lA, 'A0

+h

v ~

lA0 + A lA0

+C

3A 3A + X lA,, 3C

l&+

1&+C

c

- T C = cis-pent-2-ene; T

=

trans-pent-2-ene;X

=

xenon.

Scheme 1

considered for the cases where A is a benzene derivative. The relationship between [C] and its fluorescence quenching effect is given by equation (12), and F0/F =

7fo = 1 + k p ~ I o [ C ] T*-l

-1 Fo/F = Tfl -Ti

+ ka

Til[x]

(1 2)

(13)

the effect of added X to a solution also containing C is given by equation (13). The effect of added X on the yield of T is given by equation (14) where YT/YT' = the

ratio of T formed with added X to that in the absence of X. Combining (12), (13), and (14) gives equation (15).

)

Thus plots of (F) Fo (F F 1 - 1 versus lP l6

(Fi

- 1 allow (Disc to be determined.

K. F. Freed, J. Luminescence, 1976, 12, 339. F. A. Carroll and F. H. Quina, J. Amer. Chem. SOC.,1976, 98, 1.

63

Photophysical Processes in Condensed Phases

Table 2 gives the values of (Disc determined in this work. The agreement with previously determined values for benzene is good. The availability of these values for (Disc has made it possible to analyse trends in kist as a function of benzene substitutionla and as in the case of kf17 a qualitative dependence on the symmetry of methyl substitution is found. Table 2 Values Compound Benzene Toluene o-Xylene p - X ylene m-Xylene 1,2,3-Trimet h y1benzene 1,2,4-TrimethyIbenzene Mesitylene lY2,4,5-Tetramethylbenzene 1,2,3,5-Tetramethylbenzene 1 ,2,3,4-Tet ramethy1benzene PentamethyI benzene

@iBC

0.25 0.51 0.58 0.64 0.58

0.31 0.55 0.55 0.60 0.44 0.35 0.17

& 0.02 rf: 0.03 zk 0.03 rf: 0.03 f. 0.03 f. 0.02 f 0.03 k 0.03 k 0.03 k 0.03 k 0.02 & 0.01

An analysis of the fluorescence and phosphorescence spectra of toluene and deuterium-substituted toluenes at 77 K in polycrystalline methylcyclohexane has given some information on the geometry of the S, and Tl states. For the tripletstate emission, progressions in the totally symmetric ring carbon-carbon vibration are interpreted to signify a planar, non-regular hexagonal ring structure for the T' states of toluenes: the fluorescence has been interpreted in terms of an expanded regular hexagonal ring for the S1state.l* The vacuum U.V. irradiation of benzene and C6H6 and C,D, in argon and nitrogen matrices leads to fluorescence and phosphorescence and also to thermoluminescence (phosphorescence observed after irradiation of the matrix had stopped and the matrix warmed up).l@ The irradiation of benzene in aqueous solution at 214 nm (lAIS + lBIu) gives a similar major product with a similar yield as does irradiation at longer wavelengths, e.g. at 254 nm. These results correlate well with the wavelength dependence of fluorescence and support the view that, unlike fluorescence, the photochemistry emanates from a nonthermalized state.eo The interest in the photochemistry and photophysics of valence isomers of aromatic compounds continues. Photoexcited 1,4-Dewar naphthalene (DN) rearranges to excited naphthalene in both the singlet and triplet manifolds. DN on direct excitation at 77 K gives three emissions, fluorescence from D N and fluorescence and phosphorescence from naphthalene. No phosphorescence from DN was identified and acetophenone sensitization of DN resulted in naphthalene phosphorescence. At room temperature D N does not fluoresce but l6

l7

2o

F. H. Quina and F. A. Carroll, J. Amer. Chem. SOC.,1976, 98, 6. A. Reiser and L. J. Leyshon, J . Chern. Phys., 1972, 56, 1011. V. J. Morrison and J. D. Laposa, Spectrochim. Acta, 1976, 32, 443. C. Hellner and C. Vermeil, J . Mol. Spectroscopy, 1976, 60, 71. Y . Tlan, M. Luria, and G . Stein, J. Phys. Chem., 1976, 80, 584.

64 Photochemistry undergoes non-radiative relaxation partly to naphthalene. One interesting aspect of this work is the excitation wavelength dependence of the ratio of @ ~(DN) f and Of(naphthalene) which has been taken to indicate the presence of a rapid vibrational energy-dependent photochemical reaction which can compete with thermal relaxation.21 Figure 1 gives a qualitative picture of the interconversion 200 I

I

Reaction Coirdi no It'

Figure 1 State correlation diagram for the inter-conversion of Dewer-naphthalene and naphthalene. Line 1, conversion via singlet excited state; line 2, conversion via triplet state; line 3 , thermal conversion (Reproduced by permission from Chem. Phys. Letters, 1976, 39, 57)

processes (the figure labelling is self-evident): it accounts for the lack of phosphorescence from DN and suggests an adiabatic conversion of 3DN to 3naphthalene. In flexible molecules such as biphenyls and polyphenyls significant differences between ground-state and excited-state equilibrium geometrics often manifest themselves in large Stokes shifts. Another interesting observation is the lack of structure in absorption but considerable structure in fluorescence. The latter observation may be understood in terms of the difference in the steepness of the potential energy curves of Soand S1.2a The photoionization of pyrene in acetonitrile has been examined and two routes to the pyrene radical-cation suggested. One route involves the interaction of pyrene in its first excited singlet state with a ground-state pyrene (without involving the excinier state); the other route involves the population of an upper singlet electronic state from which ionization occurs Pressure effects on intersystem crossing from the S, state of anthracenes have been reviewedY2* 21 22

2s

2p

R. V. Can, B. Kim, J. K. McVey, N. C. Yang, W. Gerhartz, and J. Michl, Chem. Phys. Letters, 1976, 39, 57. K. Razi Naqvi, J. Donatsch, and U. P. Wild, Chem. Phys. Letters, 1975, 34, 285. A. R. Watkins, J. Phys. Chem., 1976, 80, 713. F. Tanaka, Rev. Phys. Chem. Japan, 1974,44, 65.

65

Photophysical Processes in Condensed Phases

the narrow line fluorescence spectra of perylene and the excited singlet states of fluoranthene examined using a number of different techniques (e.g. linear dichroism).26 Further spectroscopic and photochemical studies of the theoretically interesting pleiadenes have been An examination of the photophysical parameters of a series of helicenes highlights some of the important differences and similarities between the helicenes < 1.0 for the linear and linear acenes.28 (i) Whereas above tetracene (Disc acenes for all the helicenes studied (up to 9-helicene), (Disc Of z 1.0. (ii) Trends in kisc('S-Tn) for the helicenes indicate the involvement of a 3Lb state. (iii) Unlike the linear acenes a plot of log kisc(Tl-S'o) versus ET,- Eu/q, where q = (no. of hydrogen atoms) (no. of hydrogen atoms no. of carbon atoms)-l gives a good straight-line relationship for the non-planar helicenes as well as planar acenes (9-helicene is an exception). Weak monomer [@f(cyclohexane) = 2 x 1 0 7 and excimer [O,f(cyclohexane) = 5 x 1 0 7 fluorescence from [2,2]metacyclophanehas been observed and the low yields attributed to the rapid radiationless deactivation involving the two benzene rings.29 Ab initio configuration interaction calculations carried out on trimethylene methane have indicated that the S1state has a twisted geometry whereas the ground state is a planar triplet Although several studies of the effect of temperature and solvent viscosity on (Di and cD(geom. isomerization) for stilbenes have been carried out, little data indicating how these environmental changes effect q have been reported. In a recent study a combination of measured Of and q and literature values for R ~ for trans-stilbene has given the temperature dependence of kF, ~ N and k(t-c).31 The ~ N Ris mostly composed of the component k(robbion, z 2k+c,. Using the potential energy diagram of Orlandi and Siebrand 32 the temperature dependence of the rate constant for internal conversion may be seen as the result of the energy barrier between S1(8 = 0 "C) and S1(8 = 90 "C). The rate constant for internal conversion ( z 2k(t-,,) at higher temperatures - 90 to - 60 "C is consistent with an internal conversion involving an energy gap of z 14 000 cm-l and thus it seems that in this temperature range in methylcyclohexane-iso-hexane geometric isomerization is a singlet-state process [from S, (twisted) to So (twisted)]. No evidence for a temperature dependence of intersystem crossing was found. The same workers summarized the kinetics of photo-geometric-isomerization of stilbenes, pointing out the need to analyse the isomerization of these and related molecules using the full kinetic scheme; the reversible population of the twisted singlet state from the planar cis and trans isomers, intersystem crossing from the planar isomers, reversible population of the twisted triplet state from both planar isomers, and non-reversible population from the twisted singlet and

+

+

+

26

2e 27 28

30

s1 32

I. I. Abram, R. A. Anerlach, R. R. Birge, B. E. Kohler, and J. M. Stevenson, J. Chem. Phys., 1975, 63, 2473. E. W. Thulstrup, M. Nepras, V. Dvorak, and J. Michl, J, Mol. Spectroscopy, 1976, 59, 265. J. Kolc, J. W. Downing, A. P. Manzara, and J. Michl, J. Amer. Chem. Soc., 1976, 98, 930. M. Sapir and E. Vander Donckt, Chem. Phys. Letters, 1975, 36, 108. H. Shizuka, T. Oyiwara, and T. Morita, Bull. Chem. Soc. Japan, 1975, 48, 3385. J. H. Davis and W. A. Goddard, J. Amer. Chem. SOC.,1976,98, 303. D. J. S. Birch and J. B. Birks, Chem. Phys. Letters, 1976, 38, 432. G. Orlandi and W. Siebrand, Chem. Phys. Letters, 1975, 30, 352.

66

Photochemistry

population of the twisted ground state from the twisted singlet or twisted triplet states. In addition, internal conversion (S,-+ So) and intersystem crossing (T,-+ So)from either planar isomer may be i m p ~ r t a n t .The ~ ~ consequences of solvent influences on the photochemical cis-trans isomerizations of substituted stilbenes have also been The effects of molecular overcrowding on the fluorescence spectra (Stokes shifts) and quantum yields on a series of 4- and 4,4’-substituted and a,a’-substituted stilbenes have been examined. Although all cis-stilbene derivatives exhibit a small Stokes shift, the a,a’-dimethyl derivative shows a huge shift (16 500 cm-l). The Stokes shift studies indicate a large difference between the Franck-Condon and relaxed S, and Sostates. This is perhaps not surprising when the ground-state and excited-state bond orders are examined since a large change takes place resulting in large differences in preferred geometries. As observed in other aryl olefins, increases sharply with decreasing temperature and increasing viscosity. Furthermore, with increasing overcrowding the temperature range over which fluorescence is observed is shifted downwards and the magnitude of the viscosity effect is increased. Although no detailed information is yet available to indicate which of the possible non-radiative processes from the Sl states of sterically hindered stilbenes is most affected by temperature and viscosity and could therefore account for changes in (I a+ reasonable , correlation between calculated Franck-Condon factors for the non-radiative transitions and was obtained using the harmonic oscillator a p p r o x i m a t i ~ n . ~ ~ Earlier work has shown that 4-cyano-4’-methoxystilbeneundergoes geometric isomerization from the S’ state, whereas 4-nitro-4‘-methoxystilbene reacts from its triplet state. A more recent study has concentrated on the solvent- and temperature-dependence of this pair.36 While the activation energy for deactivation of the cyano derivative is independent of solvent polarity this is not the case for the nitro-derivative. This study concludes that the intersystem crossing is negligible in the case of the cyano derivative and the activation barrier = 2.8 kcalmol-l in toluene) is attributed to the twisting process from the planar to the orthogonal singlet state; but in the case of the nitro-derivative, intersystem crossing predominates. These findings are very much in line with the earlier conclusions for the mechanisms of geometric isomerization for the two compounds. Further developments have occurred in the use of electronic overlap population analysis to analyse the reactivity of cis-stilbene derivatives in the photochemical electrocyclic ring closure processes. Thus the formation of only two of the possible three ring closure products (4a, 4b-dihydrophenanthrene derivatives) from 1,2-di-p-naphthylethylenemay be understood in terms of MO and strain energy minimili~ation.~’Electronic overlap population has also been applied as a reactivity measure in the photocyclization of pentahelicene~.~~

(a

z.~ J. B. Birks, Chem. Phys. Letters, 1976, 38, 437. 84

s6 86 87

88

D. Schulte-Frohlinde and D. V. Bent, Mol. Photochem., 1974, 6, 315. G. Fischer, G. Seger, K. A. Muszkat, and E. Fischer, J.C.S. Perkin ZZ, 1975, 1569. M. N. Pisanias and D. Schulte-Frohlinde, Ber. Bunsengesellshaft Phys. Chem., 1975, 79, 662. K. A. Muszkat, S. Sharafi-Ozeri, G. Segar, and J. A. Pakkanen, J.C.S. Perkin IZ, 1975, 1515. A. W. A. Tinnemans, W. H. Laarhoven, S. Sharafi-Ozeri, and K. Muszkat, Recueil, 1975,94, 239.

Photophysical Processes in Condensed Phases

67 The ‘free rotor’ effect on triplet-state reactivity of molecules having the potential to undergo a di-n-methane rearrangement has been further investigated. Thus in the series (l), (2), and (3), although the quantum yields of rearrangement

8 Ph Ph

8 B

Ph Ph

Ph Ph

from the singlet state are very similar (zO.ll), the quantum efficiencies for triplet-state reaction vary in a way which suggests molecular flexibility control (i.e. the ratio of quantum efficiencies for rearrangement is (1) : (2) : (3) = 0 : 1 : 120.39aConformational effects on the di-n-methane rearrangement have been tested39band a full report, expanding the earlier communication on the effect of ring size on the photophysics of a series of l-phenyl-cycloalkenes, has appeared.39c Isoindenes, proposed as intermediates in the rearrangement of 1,l-diarylindenes, have now been observed using flash p h o t o l y ~ i s .The ~ ~ first-order decay of these transients allows information about the 1&hydrogen shifts to be obtained (Scheme 2).

Scheme 2

Of a wide range of substituted j?-nitrostyrenes studied, only o- and p-N(Me),substituted derivatives exhibited fluorescence. These and other observations related to the luminescence properties of p-nitrostyrenes have been analysed in terms of the relative insensitivity of the n,n* singlet and triplet states toward substituents. The charge-transfer (NO, +- Ar)r,rr* state, sensitive to substituents and solvent, is seen to control the changes in photophysical properties through the series.41 A short review of the photophysical and photochemical properties of some biologically important polyenes, including retinol, retinal, and some of their derivatives, has been p~blished:~, see also Part VI. Those workers interested in the photochemistry of a,&-diphenylpolyenesmay be interested to learn of the synthesis of a range of substituted olefins in this series.43 (a) H. E. Zimmermann, F. X. Albrecht, and M. J. Haire, J. Amer. Chem. Soc., 1975,97,3726; (b) H. E. Zimmermann and L. M. Tolbert, J. Amer. Chem. Soc., 1975, 97, 5497; (c) H. E. Zimmermann, K. S. Kamm, and D. P. Werthemann, J. Amer. Chem. Soc., 1975, 97, 3718. 40

41

4a 43

J. J. McCullough and A. J. Yanvood, J.C.S. Chem. Comm., 1975,485. D. J. Cowley, J.C.S. Perkin IZ, 1975, 1576. E. J. Land, Photochem. and Photobiol., 1975, 22, 286. G. M. Peters, jun., F. A. Stuber, and H. Ulrich, J. Org. Chem., 1975, 40,2243.

68

Photochemistry

The dramatic decrease in the quantum yields of geometric isomerization and electrocyclic ring closure to methylcyclobutene observed on changing the wavelength for exciting cis- or trans-penta-1,3-diene from 254 nm to 228.8 nm has been qualitatively rationalized. It is suggested that 228.8 nm excitation populates the S, hypersurface at a nuclear configuration unfavourable for the molecular motions responsible for the reactions observed on 254 nm e x ~ i t a t i o n .Ground~~ state conformational effects have been shown to have an influence over the photochemistry of hexatrienes 45 and selection between the two possible conrotatory electrocyclic ring openings of cy~lohexa-1,3-dienes.*~ A further report on the very interesting fluorescent s-trans-steroidal dienes has appeared. The singlet-state energies obtained from the fluorescence spectra are reported, together with triplet-state parameter^.^^ A general theoretical approach to the understanding of the photochemistry of saturated hydrocarbons based on INDO calculations has been developed. The model correctly predicts that the process involving secondary CH bond cleavage [e.g. MeCH,Me -+ (Me),C: H,] will ~ r e d o m i n a t e . ~ ~ Recently there has been considerable interest in the effects of excitation wavelength on the photochemical and photophysical processes of polar molecules such as aromatic amines and phenols. A study of phenol in neutral aqueous solutions has been carried out and both the formation of hydrogen atoms and eCaq)- are more efficient at higher excitation energies. The increase in quantum yields of H and e(aq)-with decreasing wavelength does not, however, fully account for the decrease in @F. Using a heavy atom (Cs+) perturbation technique it was shown that at least two pathways for electron formation exist: one competes with internal conversion to the fluorescent (relaxed S,) state, and another occurs from a state populated via the fluorescent state, Table 3 gives an indication of

+

Table 3

Quantum yields for the formation of e(aq),H atoms and of fluorescence obtained by excitation of phenol at 254 nm (S,) and 229 (S,) in neutral aqueous solution Quantum yields X/nm

H atoms

254 229

G 0.002 0.08 & 0.002

e(aq)Fluorescence 0.030 k 0.003 0.12 0.06 k 0.02 0.085

the size of the wavelength effect.49 The nature of the excited states of 2-naphthol and l-anthrol hydrogen-bonded with pyridine has been examined.60 2-Naphthol dissolved in micellar aqueous sodium-dodecyl sulphate is distributed between the two phases. However, while the naphthol in the water shows the well known prototropic fluorescence change, the naphthol dissolved in the micellar phase does not show the expected anion emission. These obser44 45 48 47

4s

D. Vanderlinden and S. Boue, J.C.S. Chem. Comm., 1975, 932. P. Courtot and R. Rumin, Tetrahedron, 1976, 32, 441. P. Courtot and J. Y . Satatin, J.C.S. Chem. Comm., 1976, 124. J. Pusset and R. Beugdmans, Tetrahedron, 1976, 32, 791. P. M. Saatzer, R. D. Koob, and M. S. Gordon, J. Amer. Chem. SOC.,1975, 97, 5054. J. Zechner, G. Kohler, G. Grabner, and N. Getoff, Chem. Phys. Letters, 1976, 37, 297. S. Yamamoto, K. Kikuchi, and H. Kokubun, Chem. Letters, 1976, 65.

69

Photophysical Processes in Condensed Phases

vations and the effects of detergent concentration can be quantitatively explained in terms of the Nernst distribution law.51 A report of some novel observations on the emission from acetonitrile solutions of acetone has appeared. On irradiation, a peak (at 515 nm) assigned to biacetyl phosphorescence is observed: thus the total emission is a composite of acetone fluorescence and phosphorescence, and biacetyl phosphorescence. While this observation is perhaps not surprising, the fluctuations in the eniission intensity at 515 nm with irradiation, amounting to ca. 25% of the total emission intensity, most certainly is. Thus this may present an example of a case of an illuminated system undergoing oscillations.62 However, a detailed mechanism to account for the oscillations must await further work. While pinacol is the major product from the photolysis of acetone in isopropanol at room temperature, at -70 "C the major products are those resulting from the addition of acetone to its enol isomer. This is almost certainly the result of the longer lifetime of the enol at - 70 "C (> 500 s) compared with room temperature ( 14 s ) . ~The ~ role of enol intermediates in the photoreduction and Type I cleavage reactions of aliphatic aldehydes and ketones has also been reviewed 54 as have general energy-wasting processes in ketone photo~hemistry.~~ Two closely related pieces of work on the competition between intramolecular y-hydrogen abstraction and other processes emanating from the n,n* excited 67 singlet states of alkanones have been In a continuation of their application of time-resolved laser spectroscopy to studies of aromatic ketone photophysics, Singer and co-workers 58 have obtained information about thermal and p-type delayed fluorescence as well as phosphorescence efficiencies from a series of 4,4'-disubstituted benzophenones. Variable temperature studies on the thermal delayed fluorescence has allowed the S,--T, energy differences to be determined and these, together with other useful data are shown in Table 4 (ksqis the self quenching rate constants and the superscript lim denotes values obtained by extrapolation to zero concentration, i.e. in the absence of self quenching). It is proposed that the self quenching process (16), involves exciplex formation in which the half-filled n orbitals of TI is directed towards the .n-electron density of an aromatic ring of So. N

Although log k,, correlate well with a+ values ( p = - 1.7), a much wider range of substituents, notably electron-deficient substituents, must be introduced in order to probe fully the variety of exciplex interactions possible. Michler's ketone has almost completely anisotropic fluorescence when measured for ethanol solutions at room temperatures and this is explained by the unusually short singlet lifetime.69 61 52

63 6p

66 67

s8 69

U. K. A. Klein and M. Hauser, Z . phys. Chem. (Frankfurt), 1975, 96, 139. T. L. Nemzek and J. E. Guilett, J . Amer. Chem. SOC.,1976, 98, 1032. A. Henne and H. Fischer, Helv. Chim. Acta, 1975, 58, 1598. B. Blank, A. Henne, G. P. Laroff, and H. Fischer, Pure Appl. Chem., 1975, 41, 475. D. I. Schuster, Pure Appl. Chem., 1975, 41, 601. J. C. Dalton and R. J. Sternfels, Mol. Photochem., 1974, 6, 307. M. V. Encina and E. A. Lissi, J. Photochem., 1975, 4, 321. M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer, and J. H. Parks, J . Amer. Chem. SOC., 1975, 97,4490. W. Liptay, H. J. Schumann, and F. Petzke, J. Luminescence, 1976, 12, 793.

1.2 1.3 (1.0 1.1 2.1 1.3

f 0.2 k 0.3 0.2p A 0.1 k 0.4 t 0.3

afre1 Q

(25 "C) 1.3 0.1 x 10-3 0.93 & 0.09 x 2 0 k 0.2 x 10-3 1.3 f 0.1 x 10-3 9.9 f 1.0 x 10-3

@;lm

"C)/

3.0 _+ 0.3 2.1 k 0.2 7.7 & 0.8 5.5 k 0.6 43 & 4 25 i-2 1

PS

gPlLm (25

f 0.4 k 0.4 t 0.4 k 0.2 k 0.2 2.5 k 0.3

3.7 3.4 4.4 1.9 2.1

k,,/M s-l x lo5 x 105 x lo5 x los x 107 x 108g -l

A Es, 3) kcal mol-1 3.9 ,+ 0.4 4.1 k 0.4 4.9 t, 0.5 4.5 2 0.5 5.1 k 0.5

ETICI kcal mol-l 68.9 66.8 67.6 67.8 68.1

a Essentially given by (kf/ki,)x/(k,/ki,)B, where X = substituted benzophenone and B is benzophenone. Measured relative to quinine sulphate and corrected for differences in refractive indices. At room temperature as measured from the midpoint of the phosphorescence (0-0) transition. Absolute @f (prompt) % 106-10-6 from a comparison of the integrated areas under the prompt fluorescence and phosphorescence emissions and using QD Z lo-'. "To be compared with k,, = 1.6 x los M-As-l. f To be compared with (25 "C)= 27 ps. To be compared with k,, = 1.25 x los M-'.'-S

System 4,4'-Difluoro4,4'-DichloroBenzophenone 4,4'-Dimet hyl4,4'-Dimethoxy4,4'-Bis(dimethy1amino)-

Table 4 Some photophysical parameters of the benzophenones in benzene

Photophysical Processes in Condensed Phases

71

Studies of coumarins, some of which are important for use in dye-lasers, have continued with investigations of the fluorescences of 4-methyl unbelliferone and related coumarins 6o and some 7-substituted coumarins.sl In the last few years, more attention has been paid to the photochemistry of aldehydes, and some interesting comparisons can now be made between aldehyde and ketone photochemistry. Table 5 shows the relative fluorescence quantum Table 5 Relative fluorescence quantum yields and fluorescence lifetimes of some alkanals Alkanai

Propanal 1-Butanal 1-Pentanal 2-Met hylpropanal 2,2-Dimethylpropanal

@[)F

1 .o 0.98 0.55 0.82 0.49

78

x 10-Os

2.3 1.7

1.o 1.4 0.7

yields and the fluorescence lifetimes of a series of alkanals. If the assumption is made that intersystem crossing is unlikely to be affected by distant alkylation and remembering that the radiative rate constants (Sl-So) are very low (- lo5 s-l) for alkanals then the changes in 7f-l observed may be understood in terms of the increasing efficiency of intramolecular y-hydrogen atom abstractions and other photochemical processes. Taking the rate constant for y-hydrogen abstraction in propanol as zero, the corresponding rate constants for butanal and pentanal can be calculated as - 1 x lo8 s-l and 5-6 x lo8 s-l respectively. These rate constants are comparable with those obtained for primary and secondary hydrogen atom abstraction for ln,n* alkanones and like alkanones are greater than the corresponding rate constants for %,n* reaction. The increase in 7f-l with a-methylation (cf. propanal and 2,2-dimethylpropanal) may be the result of the increasing efficiency of a-cleavage.62 Some interesting comparisons can also be made between the above results obtained for solutions and those obtained by Hansen and Lee for some of the same alkanals in the gas In order to compare the electronic and steric effects in alkanals with those of alkanones, quenching studies using triethylamine (TEA) and trans-dicyanoethylene were also carried out. The most important observation made was that alkanal ln,n* states are 5-10 times more reactive towards TEA quenching than alkanones (e.g. k, x lo01 mol-1 s-l = 1.7 for 2-butanone and 9.3 for l-butanal). Attempts to relate this difference to electronic factors failed and it was concluded that steric effects (replacing an alkyl group by hydrogen) were involved.62 The quenching of glyoxal fluorescence by a magnetic field has been attributed to an enhancement of intersystem crossing.s4 The visual chromophore, retinal, continues to provide interesting results for those studying its photochemical and photophysical behaviour. Thus while the values for @'isc for the all-trans and 9 4 s isomers are independent of excitation 6o 61

6a

64

S. C. Haydon, Spectroscopy Letters, 1975, 8, 815. J. Hinohara, K. Amano, and K. Matsui, Nippon Kagaku Kaishi, 1976, 247. J. C. Dalton, M. W. Geiger, and J. J Snyder, J. Amer. Chem. SOC.,1976, 98, 398. D. A. Hansen and E. K. C. Lee, J. Chem. Phys., 1975,63, 3272. A. Matsuzaki and S. Nagakura, Chem. Phys. Letters, 1976, 37, 204.

72

Phtochenzistry

wavelength, the values for the 11-cis and 13-cis isomers show a marked increase in @isc at 353 nm compared with 265 nm excitation 65 (see also Part VI). Both intramolecular and intermolecular hydrogen-bonding can have a profound effect on the luminescence properties of molecules capable of hydrogenbonding. Such effects, similar to those previously shown to exist for methyl ~ a l i c y l a t e ,have ~ ~ recently been shown to be present in methyl 2,6-dihydroxybenzoate.67 Thus the short wavelength fluorescence (387 nm) selectively promoted on excitation at 330nm is the result of emission from the solventstabilized enol form of the dihydroxybenzoate, whereas the 500 nm fluorescence is the result of emission from an excited species formed by the excitation of the intramolecularly bonded ground-state molecule followed by proton transfer. Emission yields the ground-state enol form which undergoes proton transfer again ( Scheme 3).67 OMe I

OMc I

OMe I

H

0

I1

Scheme 3

Substituent and solvent effects on the photophysics of the 1,2- and 9-carbomethoxy (-COOMe) derivatives of anthracene have been investigated.68 A controversy which arose over the interpretation of the selectivity of certain 66

67

R. Bensasson, E. J. Land, and T. G. Truscott, Photochem. and PhotobioZ., 1975, 21, 419. W. Klopffer and G. Naundoff, J. Luminescence, 1974, 8, 457. E. M. Kosower and H. Dodiuk, J . Luminescence, 1976, 11, 249. T. C. Werner, T. Matthews, and B. Soller, J. Phys. Chem., 1976, 80, 533.

73

Photophysical Processes in Condensed Phases Ph I

Ph

I

sensitizers to sensitize only the geometric isomerization of the lactones (4) and ( 5 ) seems to have been resolved. Ullman and Baumann had suggested that matching of the orbital symmetries of sensitizer and acceptor would lead to selective excitation of the lactone to its n,n* triplet to cause the selective reaction, while a lack of matching would lead to inefficient unselective energy transfer and resulting unselective reactions (hydrogen abstraction and geometric isomerizat i ~ n ) .It~has ~ now been shown that (4)efficiently quenches the triplet sensitizers, regardless of Ullman's classification, and the extent of reabsorption by fluorescence emitted from the sensitizers is seen to govern the reaction course.7o The considerable differences between the geometries of the ground states and relaxed S1states of flexible aliphatic amines have been used to interpret the results of a study of the temperature dependence of from flexible and rigid amines. Thus dimethylethylamine shows an increase, by a factor of 77, in its fluorescence between - 100 "C and 100 "C. It is envisaged that the initially formed nonfluorescent state undergoes a geometry change having a small activation (e.g. 4.3 kcal mol-1 for triethylamine) to the relaxed fluorescent state. In contrast the rigid amine l-azabicyclo[2,2,2]octane shows a temperature-independent intensity.'l An analysis of vibronic coupling and intersystem crossing in aromatic a r n i n e ~and , ~ ~an electron impact investigation of electronic excitations in furan, thiophene, and pyrrole have been Continuing investigations of the photophysics of highly polar aromatic compounds provide useful quantitative data, which although difficult to interpret, does provide empirical correlations which are undoubtedly useful in interpreting the photochemistry and photophysics of more complex analogues of these molecules. Table 6 provides some photophysical parameters for the cyanoaniline~.'~Emission from nitroanilines has also been inve~tigated.'~ The controversy over the origins of the two emissions observed on excitation is still with us. Four models have been presented of N,N-dialkyl-p-cyanoanilines to account for the observations so far made. (i) Solvent reorientation model. The short-wavelength fluorescence, which predominates in non-polar solvents, is attributed to radiation from unsolvated molecules whose lowest-energy excited singlet is lLb (in the Platt notation). The long-wavelength fluorescence which predominates in polar solvents, is attributed to radiation from solvated molecules whose lowest-energy excited singlet state is lLa. In slightly polar solvents, the 69

70 71 72

73

74

E. F. Ullman and N. Baumann, J. Amer. Chem. SOC.,1970,92, 5892. H. Sakuragi, I. Ono, N. Hata, and K. Tokumaru, Bull. Chem. SOC.Japan, 1976, 49, 270. A. M. Halpern and D. K. Wong, Chem. Phys. Letters, 1976, 37, 416. H. J. Haink and J. R. Huber, J. Mol. Spectroscopy, 1976, 60, 31. W. M. Flicker, 0. A. Mosher, and A. Kuppermann, J. Chem. Phys., 1976, 64, 1315. Y. H. Lui and S . P. McGlynn, J. Luminescence, 1975, 10, 113. 0. S. Khalil and S. P. McGlynn, J. Luminescence, 1976, 11, 185.

74

Photochemistry

Table 6 Solvent and temperature efects on absorption bands a Shift of band maximum

Molecule o-Cyanoaniline m-Cyanoaniline p-Cyanoaniline

Transition lLb +- IA

298 K 2MP --+ EPA -1130

77 K 3MP - 1650

298

-f

298

WF,) --+

77

K

298 -+ 77 K

EPA -970

3MP +2020

EPA +190

IA

- 1250

- 1720

-930

+1330

+250

lL, + lA

- 1400

- 2380

-980

+3038

+180

‘Lb +

a Energies are cited in wave numbers (cm-l); the experimental error is f 5 0 cm-l; Ai+ is the half-band width; A(Ai;,) is the effect of temperature on half-band width.

two fluorescence processes are supposed to co-exist. (ii) The excimer model. The short-wavelength fluorescence is assigned to a monomer emission and the long-wavelength fluorescence to an excimer emission. The excimer is presumed to possess a configuration in which the static dipole moments of both components are nearly parallel. (iii) Excited-state isomerization model. The shortwavelength fluorescence is attributed to a planar excited-state /I-isomer, and the long-wavelength fluorescence to an excited-state a-isomer in which the -N(Me)2 group is more or less perpendicular to the @-CNplane. (iv) The exciplex model. The short-wavelength fluorescence is a ’&molecular fluorescence, the longwavelength one originates from a molecule-solvent excited state complex, an ‘exciplex’, in which the emitting state has a ‘La parentage. Intensification of the long-wavelength fluorescence in concentrated solutions of slightly polar solvents is due to formation of ‘solvates’ in which the excited NN-dimethyl-p-cyanoaniline is solvated by ground-state ones through formation of an excited-state 1:l ‘electrostatic loose self-complex’. Khalil 76 has analysed the reported observations in terms of the excimer model and has rationalized the apparent lack of concentration dependence of fluorescence intensities in terms of two considerations. (i) Since the orientation of the transition dipoles was found to be critical for own-polar excimers, it is probable that only a small proportion of collisions lead to excimer formation. (ii) While the excimer has a large dipole moment ( 23 D) suggesting a near parallel dipole configuration, this arrangement is opposite to that found for the ground-state dimers. Thus, an increase in concentration increases dimer formation and thus the number of ground-state molecules available for excimer trapping is not linear with concentration. Studies by Kosower et a1.” reach rather different conclusions. In this case a very extensive study of solvent effects leads to the conclusion that the shortwavelength emissions are the result of fluorescence from both the planar and perpendicular (a-isomer) states depending on solvent. The long-wavelength emission in proton-donating solvents is assigned to a protonated molecule (6) on the basis of a deuterium isotope effect. The dimer emission is also identified. Using derivatives of NN-dialkyl-p-cyanoaniline so as to impose either coplanar or twisted conformations of the NMe, group, another study has been carried out.

-

76 77

0. S. Khalil, G e m . Phys. Letters, 1975, 35, 172. E. M. Kosower and H. Dodiuk,J. Amer. Chem. SOC.,1976,98, 924.

Photophysical Processes in Condensed Phases

6 4 $ v

Me,+

,Me

Me\

,Me

Me\

,Me

Me\

Me

C

CN

II

(7)

NH

75

N-C\H2

CN

CN

(9)

(6) In non-polar solvents (7), (8), and (9), in which a planar conformation is possible or imposed, the short-wavelength fluorescence is emitted by a state other than that mainly responsible for the first absorption band in (7).78 The longwavelength fluorescence is ascribed to emission from a state populated by relaxation from the state responsible for the short-wavelength fluorescence and is a highly polar rotamer in which the NMe, is perpendicular to the aromatic ring, The aminonaphthalene sulphonates have already provided a wealth of interesting results concerning the environment dependence of their emissions, and two new reports in this area add further to the experimental observations and interpretations of the behaviour of these molecules. High energy (360nm) and low energy (400 nm) excitation of N-methyl-N-ethyl and N-(Zhydroxyet hyl)-2-Nphenylamino-6-naphthalenesulphonatein glycerol lead to different emissions (Amx = 405 nm for high energy and Amx = 468 nm for low energy excitation). While the latter is the expected emission, the former is ascribed to fluorescence from a form in which a proton has been transferred to the 1-position of the naphthalene ring.79 An analysis of the solvent effects on the fluorescence of l-(dimethylamino)-5-naphthalenesulphonicacid and related compounds,8oaand fluorescence and flash photolysis studies of l-hydroxynaphthalene-2-sulphonate and l-hydroxy-naphthalene-4-sulphonateions,8obhave been reported. Tryptophan and the parent compound indole continue to attract attention, and this is hardly surprising in view of the controversies which still exist over the photochemistry (particularly photoionization) of indole and its derivatives and the importance of tryptophan in protein photochemistry. Table 7 shows a useful compilation of fluorescence yield values. A good correlation between Stokes shifts and Kosower’s 2 values for indole and the solvent and wavelength effects on CDt. may indicate an increased efficiency of solvent-induced internal conversion (S, -+ S,) with increase in solvent polarity.81 The photoionization of indole in alkaline solution at 77 K has also been examined.*, On excitation to its S, state indole in concentrated ( > 5 M) NaOH undergoes photoionization (loss of H+) and the resulting emission at 375 nm is 78

’@

82

K. Rotkiewicz, Z . R. Grobowski, A. Krowerzynski, and W. Kuhnle, J. Luminescence, 1976, 12, 877.

H. Dodiuk and E. M. Kosower, Chem. Phys. Letters, 1975, 34, 253. (a) Y. H. Li, L.-M. Chan, L. Tyer, R . T. Moody, C . M. Himel, and D. M. Hercules, J. Amer. Chem. Soc., 1975, 97, 3118; ( b ) R . M . C. Henson and P. A. H. Wyatt, J.C.S. Furuduy IZ, 1975, 71, 669. I. Tatischeff and R . Klein, Phorochem. and Photobiol., 1975, 22, 221. S. Yamashita, M. Yoshida, and G. Jomita, Z . Nafurforsch., 1976, 31a, 361.

76

Photochemistry

Table 7 Fluorescence quantum yields of indole in aerated andlor deaerated solvent systems at excitation wavelength A. = 280 nm a Solvent

Water Acetonitrile Methanol n-Butanol Cyclohexane Purified cyclohexane n-Hexane n-But anol-w ater

(@i)air

0.274

0.008(7) 0.32 0.23-0.27-0.32 0.26 0.26, 2 0.00, (2) 0.16

99: 1

0.34 0.33-0.35 0.45 A 0.00,(4)

0.49 0.42

0.34

Ethanol-water 0.95 : 99.05 19 : 81 47.5 : 52.5 95 : 5 Ethanol-(water)-cyclohexane 9.5 : (0.5) : 90 0.95 : (0.05) : 99

0.24 0.33 0.35 0.28, i-0.00, (2)

0.39 0.40 0.45

Tryp (2 x M) in water is taken as standard of fluorescence quantum yield with air at A. = 280 nm. b Number in brackets indicates the number of measurements and stated accuracy corresponds to the mean measured deviation: without number, measurements are single ones. a

(@&fa

= 0.13

tentatively ascribed to the indole anion fluorescence. A series of somewhat contradictory reports on the role of electron ejection subsequent to excitation of tryptophan and its derivatives have appeared. Bryant et aLE3using a 265 nm laser-flash technique have concluded that in neutral aqueous solution three important primary products are formed, the neutral tryptophan radical (as a result of N-H bond fission), the triplet state, and the hydrated electron. It is suggested that after excitation the formation of a loose complex (Scheme 4)

Scheme 4

competes with relaxation to the fluorescent singlet state and that the complex may give back the neutral molecule or dissociate with deprotonation. Table 8 gives the quantum yields of e(aq) formation. The reason for the high values relative to earlier conventional flash results must be due to the rapid radicalcation electron recombination (within the complex). Bent and H a y ~ n , using * ~ a similar laser-flash system, but measuring the by the absorption method without a standard (the previous workers used a ferrocyanide reference), obtained much lower values (Table 8). Although it is not immediately obvious why the discrepancies exist, if the back reaction of the electron and radical cation is as fast and as important as claimed, measure@)e(,q)

83 84

F. D. Bryant, R. Santus, and L. I. Grossweiner, J. Phys. Chem., 1975, 79, 2711. D. V. Bent and E. Hayon, J. Amer. Chem. SOC.,1975, 97, 2612.

77

Photophysical Processes in Condensed Phases

Table 8 Photochemical electron quantum yields jor tryptophan and tyrosine Tryptophan 0.25 (neutral aqueous solution) Tryptophan 0.08 (pH = 6.0)

Tyrosine (neutral aqueous solution) Tyrosine

0.29 a 0.095

(pH = 7.5)

ments of e(aq) production may be expected to be both time and technique sensitive. Although in the latter two papers electron ejection was considered to be the result of a monophotonic reaction from the excited singlet state, other workers now dispute this. The effect of oxygen on the radical yield points to a triplet-state precursor.85 The initial decay rate of the isothermal fluorescence from tryptophan or phenolate as a result of radical-cation electron recombination increases with decreasing photoionization energy. This is probably the result of an increase in the electron-cation separation as the photoionization energy increases and thus imparts more kinetic energy to the electron.ss The fluorescence of tryptophancontaining peptides on paper or silica gel after different treatments may be used as a detection method in chr~matography.~~ A somewhat selective review of protein luminescence has appeared.88 The fluorescence of hydroxypyridines 89 has been discussed, and other workers have reported the fluorescence characteristics of substituted 2-methyl-l-isoquinolines,vo alkylated phenazinium ion-phenazyl radical v1 systems, some azaphenanthrenes,v2 and 4-pyridoxic acid and its 1act0ne.~~ 3,3’-Diethyloxadicarbocyanine iodide (DODCI) is important because of its use in the mode-locking rhodamine 6G dye-laser. However, some uncertainty exists as to the fluorescence lifetime of DODCI and its photoisomer (efficiently formed on photolysis). Using single and multiple picosecond pulse techniques, a value of 1.2 ns, in good agreement with some existing values, has been obtained for the lifetime of DODCI, while a value of 420 ps has been given to the lifetime of the excited singlet state of the p h o t o i ~ o m e r . ~ ~ Using sub-picosecond pulses from a mode-locked C.W. laser, and fitting the data to an equation having two exponentials, the ground-state recovery kinetics of malachite green have been measured, equation (17). In methanol a single exponential time constant of 2.1 ps is measured, while in more viscous solvents 86 87

8B

s2

93 gq

H. Templer and P. J. Thistlethwaite, Photochem. and Photobiol., 1976, 23, 79. K. K. Ho, J. Moan, and L. Kevan, Chem. Phys. Letters, 1976, 37,425. L. I. Larsson, F. Sundler, and R. Hakanson, J. Chromatography, 1976, 117, 355. R. E. Dale and L. Brand, Photochem. and Photobiol., 1975, 21,459. A. Weisstuch, P. Neidig, and A. C. Testa, J. Luminescence, 1975, 10, 137. R. A. Henry, C. A. Hiller, and D. W. Moore, J. Org. Chem., 1975,40, 1760. W. Rubaszewska and Z. R. Grabowski, J.C.S. Perkin ZI, 1975, 417. F. Masetti, U. Mazzucato, and J. B. Birks, Chem. Phys., 1975, 9, 301. N. P. Bazhulina, M. P. Kirpichnikov, Y. M. Morozov, F. A. Savin, R. M. Khomutov, and V. 0. Chekhov, Mol. Photochem., 1974, 6, 337. J. C. Mialocq, A. W. Boyd, J. Jaraudias, and J. Sutton, Chem. Phys. Letters, 1976, 37,236.

4

78

Photochemistry

+

R(r) = exp (- f / r , ) a exp ( - t / r &

(17)

a fast decay process converts S, to a high vibrational level of So, giving rise to partial recovery of the absorption. Subsequently, this hot ground state thermalizes at a slower rate for complete recovery of a b ~ o r p t i o n .The ~ ~ hexamethylindotricarbocyanine fluorescence spectrum and quantum yield are sensitive to

H20

Nd

NaTl

Figure 2 Schematic illustration of an AOT inverted micelle with an aqueous core @ = sulphosuccinate head group (Reproduced by permission from J. Amer. Chem. Soc., 1976,98, 2391)

solvent and t e m p e r a t ~ r e . ~The ~ fluorescence characteristics of aridine IIYe7 4-pyrones, 4-thiopyronesYand 4-pyridonesYe8some alkaloids and a d ~ e n a l i n e , ~ ~ and some aromatic thioketones (S, emission) loohave been examined. A study of the amphiphile, di-iso-octyl sodium sulphosuccinate (AOT), capable of forming inverted micelles (Figure 2) using the fluorescent probes anilinonaphthalenesulphonate (ANS), pyrene-sulphonic acid (PSA), and rhodamine B E. P. Innen, C. V. Shank, and A. Bergman, Chem. Phys. Letters, 1976, 38, 611. A. Eranian and 0. de Witte, Compt. rend., 1975, 281, 505. 97 J. 0. Williams, B. P. Clarke, and M. J. Shaw, Chem. Phys. Letters, 1976, 39, 142. N. Ishibe, H. Sugimota, and J. B. Gullivan, J.C.S. Faruduy IZ, 1975, 71, 1812. F. Nachtmann, H. Spitzy, and R. W. Frei, Analyt. Chim. Acta, 1975, 76, 57. l o o M. Mahaney and J. R. Huber, Chem. Physics., 1975,9, 371. 96

Photophysical Processes in Condensed Phases

79

has been carried out. The interest in this system stems from the fact that the inverted micelle is capable of containing water clusters in the central polar area of the micelle and it is of interest to determine the nature of these water clusters. ANS was found to be very sensitive to the size of the solubilized water clusters in that both its fluorescence yield and lifetime decreased with increasing radius of the aqueous micelle core. The microviscosity of the AOT micelles was examined using fluorescence polarization experiments and from the strength of rhodamine B fluorescence polarization in the absence of water it may be concluded that the micelle has a very rigid core. The microviscosity of the micelle core decreases with increase in the size of the water cluster. While the diffusion of ionic fluorescence quenchers is dependent on the water cluster size, oxygen may diffuse efficiently with or without the presence of water.lol The fluorescence behaviours of some fluorescent probes in aqueous solutions of cationic,lO*anionic,lo3 and non-ionic lo* surfactants have also been studied. Guanines undergo optical changes at low temperatures in diol-containing solvents by irradiation at A < 300 nm. Although the natures of the primary photoproducts can only be speculated upon, nevertheless, the observation of the well defined optical changes may be used as a semi-quantitative probe for the population of the excited states of guanines. The possibility of the involvement of these low temperature products in biological damage is not entirely excluded; but the special conditions of their formations and their thermal instability render this unlikely.lo5 Bilirubin has attracted a great deal of attention from photochemists because of its importance in the phototherapy of neonatal hyperbilirubinemia, and interest in the photophysical properties has also developed recently. A novel fluorescence emission (TP < 5 ns; , ,A 525 nm) obtained by cooling an EPA-dimethylformamide solution of bilirubin to 77 K has been obtained and has been assigned in a dangerously loose way to a second ‘species’of bilirubin which is in equilibrium and is favoured at low temperature. (This emission is not seen at room temperature.) lo6 Protoporphyrin IX dimethyl ester on pulse radiolysis produces an emitting singlet state (TF = 23 ns) and a non-emitting triplet state (TT > 240 ~ S ) . ~lo8 O ~The * fluorescence spectra and quantum yields have been obtained for a series of free base tetra-arylporphins and their Zn derivatives in which substituents were at the 2, 3, and 4 positions of the phenyl rings. Halogen substitution resulted in a decrease in as a result of induced intersystem crossing. Zn tetraphenylporphins exhibit an emission at 560 nm, which perhaps surprisingly has been assigned as hot-band fluorescence.log The recently isolated ‘large’

-

M. Wong, J. I<. Thomas, and M. Gratzel, J. Amer. Chem. SOC.,1976, 98, 2391. I. Abe, J. Koga, and N. Kuroki, Nippon Kagaku Kaishi, 1976, 523. lo3 I. Abe, J. Koga, and N. Kuroki, Nippon Kagaku Kaishi, 1976, 342. lo4 I. Abe, J. Koga, and N. Kuroki, Nippon Kagaku Kaishi, 1975, 879. lo5 J. P. Morgan and P. R. Callis, Photochem. and Photobiol., 1976, 23, 131. lo* R. Bonnett, J. Dalton, and D. E. Hamilton, J.C.S. Chem. Cumm., 1975, 639. lo’ S. J. Chantrell, C. A. McAuliffe, R. W. Munn, A. C. Pratt, and E. J. Land, J.C.S. Chem. Comm., 1975, 470. lo* S. J. Chantrell, C. A McAuliffe, R. W. Mum, A. C. Pratt, and E. J. Land, J. Luminescence, 1976, 12, 887. log D. J. Quimby and F. R. Longo, J. Amer. Chem. Soc., 1975,97, 5111.

lol

loa

Photochemistry

80

phytochrome (Pr), which retains its apoprotein undergraded, shows a marked temperature-dependent at. Thus while Q remains constant at 0.01 between 14 K and 200 K, it decreases rapidly above 200 K, and is unmeasurable at room temperafure.ll0 The literature covering luminescence phenomena is littered with claims for 'new' luminescence from molecules which eventually prove to be due to impurities. Although such claims may sometimes be an indication of the quality of work being produced by a particular group, they more frequently reflect the very great difficulties of purifying materials, particularly large complex molecules, to the level necessary for emission studies. A new examination of carefully purified chlorophyll a in EPA has revealed that an emission previously assigned as a fluorescence hot band is in fact due to an impurity. Furthermore, for concentrations up to M there is no evidence for dimer emission.lll The fluorescence lifetimes exhibited in vivo by chlorophyll a have also been examined.ll2 N

Ionic and Radical Phenomena.-Interest in those molecules which undergo dissociation to ionic fragments (e.g. proton + anion or radical-cation + electron) on electronic excitation has increased recently and therefore this subsection has been devoted to a discussion of recent observations in this area and also some free radical process. Although many attempts to obtain excited state pK* values by fluorimetry have been made, it was pointed out some time ago that if one of the excited species in equation (18) is non-fluorescent (i.e. T < 10-l'~) then the reaction in which it AH*

+B 7 A-* + BH+

(18)

takes part (deprotonation or protonation) cannot proceed to any appreciable extent during its short lifetime.l13 Thus, under these conditions, a pH versus fluorescence intensity curve does not represent an excited-state equilibrium and therefore cannot give p&*, equation (18). This situation applies whether B is H 2 0 or a buffer acid. An analysis of the data given by fluorescence intensity us. pH plots for the cases of near equilibrium and non-equilibrium has been presented by other The solvent dependence of absorption and the results of solvent-dependent photocurrent measurements have indicated that the spontaneous formation of free solvated electrons from the vertical Rydberg-like states of tetra-aminoethylenes takes place in polar The use of pulse radiolysis-generated solvated electrons to produce interesting free radical and ionic intermediates in chemistry is well known. This technique has recently been used to generate carbanions in solution 116[equation (19)]. A flash-photolysis study of a-naphthol e,110 lla 113 114

116 116

+ (PhCH,),Hg

-

PhCH,-

+ PhCH,Hg

(19)

p.-S. Soag, Q. Chac, and W. R. Briggs, Photochem. and Photobiol., 1975, 22, 75. A. W.-H. Mau, Chem. Phys. Letters, 1976, 38, 279. G . Hervo, G . Paillotin, J. Thiery, and G. Breuze, J . Chim. phys., 1975, 72, 761. N. Lasser and J. Feitelson, J. Phys. Chem., 1975, 79, 1344. S. G. Schulman and A. C. Capomacchia, J. Phys. Chem., 1975, 79, 1337. (a) Y . Nakato, A. Nakane, and H. Tsubomura, Bull. Chem. SOC.Japan, 1976, 49, 428; (b) Y . Nakato and H. Tsubomura, J . Luminescence, 1976, 12, 845. L. M. Dorfman and B. Bockrath, J. Phys. Chem., 1975, 79, 3040.

Photophysical Processes in Condensed Phases

81

in liquid solutions confirms the presence of the naphthoxyl radical as the major transient. In the presence of proton acceptors Et,O, dioxan, or THF, the major transient is the triplet of a-naphthol which, unlike the wnaphthoxyl radical, decays u n i m ~ l e c u l a r l y . ~ ~ ~ It is of interest to compare the above results with those obtained by Richardson et al. in their study of the relative gas phase photodetachment of electrons from phenoxides and thiophenoxides.lls Chandross has been very critical of a report on the photochemical nitrosation of 2-naphthol in a buffered (pH 7.0) solution of sodium nitrite. The authors claimed that the enhanced ionization of the naphthol in the excited state increases the nitrous acid concentration which is the crucial rate-determining factor. However, a number of rather straightforward considerations make it clear that this first claimed example of a reaction, which takes advantage of the increased acidity of phenols in their first excited state, cannot be accepted. Perhaps the most important general criticism made should be remembered by us all: any be it emission or reaction, must be process with a quantum yield of analysed with a great deal of caution.l19 The fluorescence properties and acid-base equilibria of naphthylpyridylethylenes,120a rapid radiationless deactivation process competing with proton transfer in the S, state of 2,4-bis(dimethylamino)-6-(2-hydroxy-5-methylphenyl)s-triazine,121 and the pH-dependence of fluorescein fluorescence have been discussed.122 Although a large amount of lifetime data on the fluorescence from benzyl and related radicals are available, no fluorescence quantum yield has been available. Recent measurements on EPA matrices (Table 9) now make it possible to obtain l ) ~ N R ( T N R - ~data. ) The radiative lifetimes are small (< s) accurate k ~ ( ~ -and as expected for a forbidden transition.lZ3 Excitation of the benzyl radical near the long-wavelength limit of the absorption spectrum with a laser has resulted in the identification of some new emission bands.124 The interesting use of ketyl radical fluorescence to monitor its production and decay in the pulse radiolysis of benzophenone has been reported. The decay of the ketyl radical as observed by the fluorescence decay matched that obtained by absorption Fluorescence from the excited ketyl radical has also been studied using a combined pulse radiolysis-laser photolysis technique.126 The emission spectra of seven polyfluorobenzene radical cations have been obtained by using controlled electron impact. The emissions lie in the 400500 nm region.127 K. Hara and H. Baba, J.C.S. Furaday ZZ, 1975,71, 1100. J. H. Richardson, L. M. Stephenson, and J. I. Brauman, J. Amer. Chem. SOC.,1975, 97, 2967. E. A. Chandross, J. Amer. Chem. SOC.,1976, 98, 1053. l Z o G . Favaro, F. Masetti, U. Mazzurato, and P. Bortolus, J. Phys. Chem., 1975, 74, 2785. 121 H. Shizuka, K. Matsui, T. Okamura, and L. Tanaka, J. Phys. Chem., 1975, 79, 2731. lZ2 M. M. Martin and L. Lindquist, J. Luminescence, 1975, 10, 381. laa T. Okamura and I. Tanaka, J. Phys. Chem., 1975,79,2728. 12* Y . Ono, T. Ikeshoji, and T. Mizuno, Chem. Phys. Letters, 1975, 39, 451. 126 B. W. Hodgson, J. P. Keene, E. J. Lane, and A. J. Swallow, J. Chem. Phys., 1975, 63, 3671. lzCR. Mehnert, 0.Bredi, and W. Helmstreit, Z. Clzem., 1975, 15, 448. 12' M. Allan and J. P. Maier, Chem. Phys. Letters, 1975, 34, 442. 117

118

T,/ns 173.8 192.3 92.5 167 153.4 192.3

Radical Benzyl [2H,]Benzyl p-Methylbenzyl m-Methylbenzyl o- Met hylbenzyl 3,5-Dimethylbenzyl 2,4,5-Trimethylbenzyl

The @f data include a 20 % (maximum) deviation, and those of T

Parent molecule Toluene [2H,]Toluene p-Xylene m-Xylene o-Xy lene Mesitylene Durene ~ T, ~

k 0.08 i- 0.20 k 0.09 i-0.04 f: 0.07 f 0.02 f: 0.02

a

@f

0.39 0.47 0.18 0.38 0.59 0.64 0.42

3.3 6.3 6.7 2.0 1.3 0.88 1.2

Trip

and ~ f, include a f30 % (maximum) deviation.

1.28 2.97 1.21 0.75 0.79 0.56 0.52

TflPS

Table 9 Lifetimes and quantum yields of the benzyl radical and its derivatives

f 0.00098 0.00050 0.00049 0.0017 0.0026 0.0040 0.0030 Solvent cyclohexane.

2.1 5.6 1.5 1.2 1.9 1.6 0.90

T a J p

Photophysical Processes in Condensed Phases

83

The ground- and excited-state acid dissociation constants of protonated alltrans retinal Schiff’s base have been measured and compared with theoretical

predictions,12*and the absorption and emission spectra of cyclo-octatetraene and its radical-anion examined.12g Excimers.-Perhaps the most important use of the photolysis of aromatic hydrocarbon dimers to produce sandwich pairs is in the study of the polarization of excimer fluorescence without concentration depolarization. By employing this

E

Figure 3 Schematic potential energy diagrams, rationalizing the emission characteristics of sandwich pairs of naphthalene and anthracene. The dashed curves represent solvent potential which prevents the dissociation of the molecular pair into two monomers (Reproduced by permission from Chem. Phys. Letters, 1975, 32, 503)

technique it has been shown that the lowest energy excimer singlet state for anthracene is derived from the molecular exciton state. The absence of monomer phosphorescence suggests that the triplet state is bound in the sandwich pair. These and other results allow the construction of a qualitative P.E. diagram and Figure 3 compares this with the situation for na~htha1ene.l~~ The state of polarization of monomer and excimer emission from pyrene has also been examined.131 Although there is evidence to show that in the solid state, excimer formations precede the photodimerization of anthracene, for the solution photodimerization no direct evidence for the involvement of an excimer existed (but see below). An energy transfer technique (to rhodamine B) has been utilized to detect and obtain a lifetime (1-1.511s) for the non-fluorescent anthracene excimer in toluene-ethanol mixtures and also shows that dimer formation proceeds through the e x ~ i m e r . l ~Another ~ interesting method of obtaining excimers involves the formation of dimer cations by radiolysis at 77 K followed by warming to allow electron cation neutralization. Using this method, excimer fluorescence from trans-stilbene (Amx = 430 nm) and diphenylacetylene (Amax = 390 nm) have been 0 b ~ e r v e d . I ~ ~ lz8 lzS

I3O ls1 lsa

A. M. Schaffer, T. Yamaoka, and R. S. Becker, Photochem. and Photobiol., 1975, 21, 297 V. Dvorak and J. Michl, J. Amer. Chem. SOC.,1976, 98, 1080. P. C. Subudhi, N. Kanamaru, and E. C. Lim, Chem. Phys. Letters, 1975, 32, 503. A. S. Ghosh, D. Gupta, and S. Basu, J. Photochem., 1975. 4 227. M. D. Cohen, Z . Ludmer, and V. Yakhot, Chem. Phys. Letters, 1976, 38, 398. B. Brocklehurst, D. C. Bull, M. Evans, P. M. Scott, and G. Stanney, J. Amer. Chem. SOC., 1975, 97, 2977.

84

Photochemistry

Intramolecular excimer formation continues to be of interest and the detection of conformational control on excimer photophysics is of particular importance. Thus dianthrylethane (DAE) and dianthrylpropane (DAP) show different excimer emissions at room temperature and at 77 K. The Type I excimer is observed on excitation at room temperature, while the Type I1 excimer is formed by the photocleavage of the intramolecular photodimers in a rigid glass at 77 K. The efficient formation of the dimers from the Type I1 excimer may be the reason for lack of Type I1 fluorescence at room temperature.134 An interesting series of papers concerned with the photochemistry and photophysics of the photodimerization of acenes has appeared.13+13' The production of phenanthrene triplet excimers and their modes of decay have been at the centre of some controversy over the past few years. New evidence, using a time-resolved spectroscopy method, indicates that the triplet excimer exists and phosphoresces even in fluid In the concentration range used in this study, the triplet excimer is probably formed as indicated by equation (20). The applicability 3Ph,

+ lPb

-

3(Ph.-.Ph),

(20)

of the relative excimer yield equation to electrogenerated chemiluminescence has been discussed 13Q and the kinetics of formation of the excimer of 3,4-benzo[a]pyrene a n a 1 ~ s e d . l ~ ~ Singlet Quenching by Energy Transfer.-Simple approximate analytical solutions based on an earlier developed method of treating the statistics of long-range energy transfer have been described. Numerical solutions obtained using this method appear to compare very well with other calculations in accounting for experimental Excitation transfers, in a rigid medium, of the type described by equations (21) and (22) have been examined using a Q-switched Sl T'

+ Sl + TI

-

So

+ Sn

(21)

So

+ Tn

(22)

ruby laser. For process (21) pyrene was used while perdeuteriophenanthrene was used to examine process (21). The experimental luminescence decay curves in each case may be matched to a theoretical decay function using Foster's dipole-dipole coupling It is known that for energy-transfer processes which involve electron exchange and therefore close contact between donor and acceptor, steric factors are important. This situation has been confirmed for triplet energy transfer to a series of azo compounds from aromatic hydrocarbons and ketones. For exo134

J. Hayashi, T. Suzuki, N. Mataga, Y. Sakata, and S. Misumi, Chem. Phys. Letters, 1976, 38, 599.

lSs 136 13'

A. Castellan, R. Lapouyade, and H. Bouas-Laurent, Bull. SOC.chim.France, 1976, 201. A. Castellan, R. Lapouyade, and H. Bouas-Laurent, Bull. SOC.chim. France, 1976, 210. A. Castellan, G. Dumartin, R. Galante, and H. Bouas-Laurent, Bull. SOC.chim. France, 1976, 217.

138 139 140

141 142

M. Aikawa, J. Takemura, and H. Baba, Bull. Chem. SOC.Jupan, 1976, 49, 437. J. T. Maloy and A. J. Bard, Spectroscopy Letters, 1975, 8, 97. F. Hafner, R. Frey, M. Hauser, and G. Heidt, Z . Nuturforsch., 1975, 30, 1049. V. Kosele, M. Hauser, V. K. A. Klein, and R. Frey, Cliem. Phys. Letters, 1975, 34, 519. N. Nakashima, Y. Kume, and N. Mataga, J. Phys. Chem., 1975, 79, 1788.

85

Photophysical Processes in Condensed Phases

thermic energy transfer singlet energy transfer was found to be less sensitive to steric effects than triplet energy transfer, apparently because diffusion rather than energy transfer is rate determining in the former case. The effect of steric hindrance on triplet energy transfer can be illustrated by the ratios of k, for azo-n-butane and azo-t-butane. For the series of donors used, this ratio varies from 3.6 to 10.7.143In contrast, triplet energy transfer from 3(acetone),produced by the thermal decomposition of tetramethyldioxetane, to a range of structurally different ketones is apparently insensitive to steric factors. The values of k , vary only by a factor of two from 1 x lo61 m 0 l - l . l ~Structure ~ reactivity factors in the fluorescence quenching of a-diketones 145 and the importance of singlet to triplet energy transfer equation (23) for all-trans-retinal, chlorophyll a, anthracene,

Si

+ To

7 ' 1

+ So

(23)

and rhodamine G have been examined, see also Part VI. The large values of the radius of influence, approximately 21 8, for all-trans-retinal suggests that this process may be important under certain circumstance^.^^^ Fluorescence quenching by foreign absorbing substances 14' and theoretical calculations concerning singlet energy transfer between aromatic amino-acids and nucleic acid bases 148 have been discussed and the possibility of intramolecular photosensitization of the peptide by a phenyl chromophore examined.149 A model to explain the kinetics of fluorescence and energy transfer in thin films of poly(vinylcarbazole) has been proposed 160 and a teaching experiment based on the measurement of excitation and emission spectra of Ru(bipyridyl),2+ and the quenching of emission by a range of quenchers 0ut1ined.l~~

aoST,

Exciplexes and Electron Donor-Acceptor Complexes.-In a review of systems where fluorescence quenching does not follow simple kinetics, complicating effects such as ground-state complexing, transient effects in diffusion-controlled kinetics and feedback from an exciplex have been discussed. The authors have concentrated in the main on the use of the analysis of emission decay curves to provide information on such systems but have also emphasized the importance of temperature studies. The kinetic complexities of exciplex-containingsystems were discussed in most detail and using Scheme 5 , the kinetic equations were derived. Scheme 5 is described by equations (24) and (25), where k, = k, + k6 + k,. d[A*]/dt = k4[AQ*]

- ( k , + ka

d[AQ*l/dt = k,[Q1 [A*]

+ k, Q) [A*] + Za(t)

+ (k4 + kp) ([AQ*I)

(24) (25)

The driving function I . ( t ) can be either time-independent as in steady-state experiments or time-dependent as in transient studies. Steady-state analysis of 1*3

C. C. Wamser, R. T. Medary, I E. Kochevar, N. J. Turro, and P. L. Chang, J. Amer. Chem. SOC.,1975, 97, 4864.

Schuster and N. J. Turro, Tetrahedron Letters, 1975, 2261. B. M. Monroe, C. G . Lee, and N. J. Turro, Mol. Photochem., 1974, 6, 271. R. S. Knox and V. J. Ghosh, Photochem. and Photobiol., 1975, 22, 149. 14' A. Kauski and M. Ston, 2. Naturforsch., 1975, 30, 1611. ld8 T. Montenay-Garestier, Photochem. and Photobiol., 1975, 22, 3 . H. Morrison and F. Palensky, Photochem. and Photobiol., 1975, 21, 367. lb0 G. E. Venikouas and R. C. Powell, Chem. Phys. Letters, 1975, 34, 601. lS1 J. N. Demas, J. Chem. Educ., 1975, 52, 677. lGL4 G. 146

86

Photochemistry

A -I- hvV

c A

J.

A

+Q

A + Q tkv,

Scheme 5

equations (24)and (25) gives a Stern-Volmer equation, while the kinetic description of the transient behaviour allows other parameters to be determined.152 A theoretical framework for the understanding of radiationless processes in excimers and exciplexes has been presented 153 and applying a semi-empirical SCF-MO-CI calculation to the 9,lO-dicyanoanthracene-naphthalene EDA system, routes (based on energy-contour maps) from the Franck-Condon excited state of the EDA complex to the exciplex state have been Observations of absorbance changes in the nanosecond time domain of pyreneNN-diethylaniline system in a variety of solvents provide the first direct evidence for solvent-induced changes in the electronic structure of a polar intermolecular exciplex. The absorption spectrum of the exciplex varies continuously with solvent polarity becoming identical with that of the separated radical-ions in polar A report of solvent effects on exciplex emission from pyrene-tertiary aromatic amine systems has also appeared.166uIn order to elucidate the geometrical requirements for fluorescence quenching and fluorescent exciplex formation, Mataga and co-workers have investigated the intramolecular exciplex forming systems, p-(CH,)2N-C6H,-(CH2),-(l-pyrenyl) and p-(CH3)2N-C6H4(CH2),-(9-anthryl). Fluorescence rise and decay curves, S, -+ S, and T, -+ T1 spectra of the pyrenyl system were analysed in detail as a function of solvent polarity. The results indicate considerable changes in the exciplex electronic structure with changes in solvent polarity and the exciplex is seen to become more polar in solvents of high polarity. Furthermore, like the situation for some e ~ c i m e r s the , ~ ~sandwich ~ structure is not necessary for intramolecular charge transfer .lSBb The interactions between polar molecules and the intramolecular exciplexforming naphthalene derivative l-hydroxy-l-(2-naphthyl)-3-(N-piperidinyl)propane (HNPP) may be considered as static (ground-state association) and dynamic (polar molecule-l(HNPP), interactions). The extent of ground-state complexing is small as indicated by the weak dependence of quenching (decay) on the excitation wavelength. The time-resolved emission spectra obtained for cyclohexane-ethanol and benzene-acetonitrile systems show a progressive red shift in the exciplex emission with time. Such an effect is expected if the diffusion of the polar molecule to the exciplex is responsible for the red shift.157 Excited-state l-j2

153

Is* lS5 156

lS7

W. R. Ware, Pure Appl. Chem., 1975, 41, 635. S. J. Formosinho, Mol. Photochem., 1976, 7, 41. T. Mimura, M. Itoh, T. Ohta, and T. Okamoto, Bull. Chem. SOC.Japan, 1975, 48, 2245. N. Orbach and M. Ottolenghi, Chem. Phys. Letters, 1975, 35, 175. (a) D. Gupta and S. Basu, J. Photochem., 1975,4,307; (b) N. Mataga, J. Okada, H. Masuhara, N. Nakashima, Y . Sakata, and S. Misumi, J. Luminescence, 1976, 12, 159. G. S. Beddard, S. E. Carlin, and C. Lewis, J.C.S. Faruduy ZZ, 1975,71, 1894.

87

Photophysical Processes in Condensed Phases

interactions between aromatic hydrocarbons and 1,2-di(tertiary-amino) ethanes and a new emission, assigned as exciplex fluorescence, have also been examined lSB has been obtained from the interaction of the excited singlet state of 2-aminopyridine and ground-state p-nitr0ani1ine.l~~ Acrylamide Quenching of indole and N-acetyltryptophanamide proceeds both by static and dynamic processes and is proving to be a good topological probe for protein studies. The tryptophan units may be located using the acrylamide probe, which appears to behave as a perfectly neutral probe in model micelle systems, unlike ionic or hydrophobic quenchers. Acrylamide quenching rate constants in water for a range of indole derivatives are all 6 x loB1 mol-1 s-l.160 The synthetic uses of exciplexes is an area which has aroused some interest recently and a study of 1- and 2-cyanonaphthalene interactions with tetramethylethylene gives some indication of the factors important in maximizing the formation of products. The results of this study may be summarized as follows.lel (i) Exciplex formation precedes cycloaddition in benzene solution. (ii) Different factors control the formation and collapse of the exciplexes. (iii) In polar solvents (methanol and acetonitrile) electron transfer predominates over exciplex formation. 2,5-Dimethylhexa-2,4-diene(DMHD) forms exciplexes with arenes, and in the case of a number of substituted anthracenes and octafluoronaphthalene these exciplexes are fluorescent. The relationship between the thermodynamics of exciplex formation and the polarity of the exciplex and product formation in arene DMHD and arenecyclohexa-l,3-diene systems have been 163 A more detailed study of one of the diene exciplex systems, namely the 9,lO-dichloroanthracene (DAC)-DMHD system, has been carried out by SaltieI and co-workers. Good linearity of the Stern-Volmer plot (up to diene concentrations of 0.32 M) for monomer fluorescence quenching allows transient, ground-state complexing, or other complicating effects to be eliminated in this concentration range.164 On the other hand, at higher concentration both the appearance of a new emission and the quenching kinetics point to the formation and decay of a triple exciplex as indicated by equations (26) and (27). N

'(DCA

- DMHD)

+ DMHD

'(DCA - DMH,)

-

(DCA

- DMHD,)

(26)

DCA

+ 2DMH + h

(27)

The formation of a triple exciplex of a different kind has been the subject of an equally interesting report that 1,3-dinaphthylpropane (a$ and /3,/3) (DNP) and 1,4-dicyanobenzene(DCB) form a triple exciplex, DCB-DNP-. DCB. Thus the fluorescence spectrum of a dioxan solution of /3,/3-DNP and DCB consists of three separate contributions; intramolecular excimer fluorescence (A, 400nm; 7-f = 52 ns), /3,/3-DNP-DCB exciplex (Arnx 420nm; Tf = 9 ns), N

N

15*

lS8 le0

lel lG3

164

A. Zweig and J. B. Gallivan, MoE. Photochem., 1974, 6, 397. J. Wolleben and A. C. Testa, J. Phys. Chem., 1975, 79, 1137. M. R. Eptink and C. A. Ghiron, J. Phys. Chem., 1976, 80, 486. J. J. McCullough, R. C. Miller, D. Fung, and W.-S. Wu, J. Amer. Chem. Sac., 1975,97, 5942. N. C. Yang, D. M. Shold, and J. K. McVey, J. Amer. Chem. SOC.,1975, 97, 5004. N. C. Yang, K. Srinivasachar, B. Kim, and J. Libman, J. Amer. Chem. Soc., 1975, 97, 5006. J. Saltiel, D. E. Townsend, B. D. Watson, and P. Shannon, J. Amer. Chem. SOC.,1975, 97, 5688.

88 Photochemistry and triple exciplex, ,A,( 490 nm; ~i = 110 ns). From the lack of excimer lifetime dependence on DCB concentration it was concluded that the triple exciplex is formed from the exciplex (by interacting with the other naphthyl fragment) and not from the e ~ c i p 1 e x . l ~ ~ The complete kinetic analysis of exciplex formation and decay is still relatively rare. For this reason a report on the analysis of a range of exciplexes is particularly welcome. The time development of emission at particular wavelengths and time-dependent emission spectroscopy were both used to obtain relevant kinetic data. Two reactive (i.e. product-producing) systems, 9-cyanophenanthrene-pbutenylanisole and 9-cyanophenanthrene-trans-anethole, and two unreactive systems, anthracene-diethylaniline and plienanthrenefumaronitrile, were examined. Thermodynamic behaviour seems to differ little for exciplex formation between the unreactive and reactive systems and, perhaps not surprisingly, it may be concluded that different factors control the collapse of the exciplexes to products.166*167 The interaction between the excited singlet state of phenanthrene and dimethyl fumarate is now confirmed to lead to an interesting system of exciplexes. Thus, the formation of a weakly emitting exciplex leads to the formation of the dioxetane (10) and the cyclobutane (1 1). In addition, intersystem crossing N

8: \

Me0

COOMe

/

@ \

I

0,Me

"CO,Me

@,-c%Me \

"CO,Me

/

~

produces a triplet exciplex. The triplet exciplex produces a mixture of the cyclobutane isomers (1 1) and (12), and decays to ground-state phenanthrene and fumaronitrile or triplet phenanthrene and ground-state fumaronitrile.168 The photodimerization of styrenes in the presence of tetracyanobenzene is considered to involve an electron-transfer process,169and the fluorescent states of inter- and intramolecular exciplexes and excited donor-acceptor complexes of 9,lO-dicyanoanthracene and alkylbenzenes may be described as states in which the excitation is mostly localized on the electron acceptor.170 The analysis of results from the determination of the ionic photodissociation yield of pyrenequencher systems indicates that the ionic dissociation may be outlined as shown in Scheme 6. Thus after formation of the collision complex, other processes compete with solvent relaxation to the solvated ion-pair.171 (a) T. Mimura and M. Itoh, J. Amer. Chem. Soc., 1976,98, 1095; (b)T. Mimura and M. Itoh, J . Luminescence, 1976, 12, 839. D. Creed, P. H. Wine, R. A. Caldwell, and L. A. Melton, J. Amer. Chem. Soc., 1976, 98, 621. le7 R. A. Caldwell, D. Creed, and H. Ohta, J. Amer. Chem. SOC.,1975, 97,3246. lR8 S. Farid, S . E. Hartman, J. C. Doty, and J. L. R. Williams, J. Amer. Chem. SOC., 1975, 97, 3697. leg T. Asanuma, M. Yamamoto, and Y. Nishijima, J.C.S. Chem. Comm., 1975, 608. 170 M. Itoh, Y. Kumano, and T. Okamoto, Bull. Chem. SOC. Japan, 1976, 49, 42. 171 T. Hino, H. Akasawa, H. Masuhara, and W. Mataga, J. Phys. Chem., 1976, 80, 33. le6

89

Photophysical Processes in Condensed Phases p* 4- Q

-

-.--IL

(p* ...Q) ____

P + Q

I- ,--

-+

(p+ ...Q-)*

reaction

-&

(p+-.-QL)q-------3

11

p,+

+ Qs-

3P + Q Scheme 6

The relative efficiencies of photodissociation to radical-ions in the two systems pyromellitic dianhydride--perylene and tetracyanoethylene-perylene have also been examined. In these cases population of the triplet state of donor and acceptor can be ruled out. However, the complication of exciplex production by both direct excitation of the ground-state complex and as a result of excited perylene ground-state acceptor interaction was also considered and the yields of radicalions produced in both ways A closely related study of the tripletstate ionic photodissociation of the weak charge-transfer complexes formed between pyromellitic dianhydride (PMDA) and naphthalenes has been and the effect of bromine substitution on the charge-transfer emission from the complex of phenanthrene and tetracyanobenzene 17* examined. The photochemical behaviour of 1-cyanonaphthalene in the presence of phenylacetic acid has also been examined.175 The triplet-triplet spectra of polycyclic hydrocarbons have been observed for many molecules. However, when (Disc is low for a particular molecule it is often difficult to obtain good T1-T'. spectra, especially when the corresponding transition is forbidden. These difficulties are to some extent overcome by use of a ground-state complex of the hydrocarbon (donor) and an acceptor (e.g. chloranil). It is assumed that the spectrum of the short-lived species obtained on flashing the complex approximates to that of the Tl state of the donor. Using this assumption the well resolved spectra obtained for perylene and other hydrocarbons can be split up into Tl-T, and T1-T3 contributions. One shortcoming of the technique is the uncertainty which exists over the magnitude of the perturbation from the isolated hydrocarbon spectra which must result on complexing the The competition between sensitized charge transfer of an excited oxocarbocyanine dye adsorbed on the surface of a molecular crystal of chloranil and energy transfer to a metal layer has been utilized to measure the rate constant for charge transfer (kCT = (4 f 2) x los s-1),177 and the photoreaction of tetracyanobenzene with tetrahydrofuran has been examined.178 The initial interaction between electronically excited ketones and amines usually results in electron transfer from the filled n-orbital of the amine to the half-filled n-orbital on the ketone oxygen. Such an interaction produces an 172

173 174 176

176

177 178

P. Hentzschel and A. R. Watkins, J. Phys. Chem., 1976, 80, 494. Y . Achiba and K. Kiniura, J. Phys. Chem., 1975, 79, 2626. K. Chum and B. R. Henry, J. Mol. Spectroscopy, 1976, 60, 150. J. Libman, J. Amer. Chem. SOC.,1975, 97, 4139. M. A. Slifkin and A. 0. Al-Chalabi, Spectrochim. Acta, 1976, 32, 661. H. Killesreiter, J. Luminescence, 1976, 12, 857. M. Ohashi and K. Tsujimoto, Chem. Letters, 1975, 829.

90 Photochemistry exciplex which may yield photoreduction products on collapse to ground-state reactants. A solution phase study of the fluorescence quenching of 11 acyclic, cyclic, and bicyclic alkanones by diethylamines (DEA) and triethylamine has been published. Steric effects suggest that a specific orientation of alkanone and amine in the encounter complex is required for efficient quenching. It appears that the formation of exciplexes involving the alkanone S, state and alkylamines is sufficiently exothermic that the amine oxidation potential and solvent polarity have little effect on the observed k , values. Consistent with this are the much lower k , values for n,r* triplets (about an order of magnitude) when the AG values are of course much 10wer.l'~ Weak charge-transfer interactions are invoked to account for the formation of an intramolecular excited complex in #?-vinyl phenyl ketones (Ph-COCH,CH,-CH=CHR). The decay modes of these complexes were discussed lE0as was the quenching of exciplex fluorescence by an electric field in poly(N-vinylcarbazole) film doped with dimethyl terephthalate.lE1 In addition to the production of an exciplex by the absorption of light, the direct interaction of radical-anion radical-cation pairs generated electrochemically may also lead to exciplex formation. However, in the past it has been assumed that in the polar solvents used (e.g. acetonitrile) to produce the radicalions, exciplex emission would not be observed. Bard and co-workers, during electrogenerated chemiluminescence studies, have now observed exciplex emission from systems of tri-p-tolylamine (TPTA) with a range of ketone and hydrocarbon donors in acetonitrile and tetrahydrofuran as solvents and from naphthalene and five amine donors in acetonitrile. Studies of the dibenzoylmethane-TPTA system in a range of solvents where only exciplex emission is observed demonstrates the importance of solvent dielectric on exciplex emission intensify.lE2 Using as the basis for their analysis the previously described master equation to describe the time-dependent behaviour of vibrational relaxation,183a Lin et al. have analysed the effects of temperature and quencher concentration on vibrational relaxation in condensed media.1E3bA theoretical analysis of the quenching of excited aromatic molecules by paramagnetic species has also been given.lS4 Biacetyl fluorescence quenching by inorganic ionslE6and a range of oxygenquenching processes have been examined.186-188 The observation that oxygen, the universal quencher of luminescence from organic molecules, can under certain conditions act to enhance luminescence was reported and discussed in previous Volumes of these Reports. A more detailed study of the oxygen-enhanced fluorescence has now been published. Two mechanisms have been presented to account for the enhancement (observed in polymer matrices at low temperatures): (i) the trivial mechanism whereby 302 quenches the triplet states of molecules which have very long lifetimes under the 179 180

J. C . Dalton and J. J. Snyder, J. Amer. Chem. SOC.,1975, 97, 5192. H. Morrison, V. Tisdale, P. J. Wagner, and K.-C. Lin, J. Amer. Chem. SOC.,1975, 97, 7189.

hl. Yokoyama, Y. Endo, and H. Mikawa, J. Luminescence, 1976, 12, 865. S. M. Park and A. J. Bard, J . Amer. Chem. SOC.,1975,97, 2978. 189 (a) S. H. Lin, J. Chem. Phys., 1974,61,3810; (6) S. H. Lin, H. P. Lin, and D. Knittel, J. Chem. Phys., 1976, 64, 491. 184 S. J. Formosinho, Mol. Photochem., 1976, 7 , 13. 186 P. Bortolus and S. Dellonte, J.C.S. Furaday ZZ, 1975, 1338. 180 T. Sakata, T.Okai, and H. Tsubomura, Bull. Chem. SOC.Japan, 1975, 48, 2207. 187 M. W. Geiger and N. J. Turro, Photochem. and Photobiol., 1975, 22, 273. la8 M. R. Faith and P. G. Squire, Photochem. and Photobiol., 1975, 21, 439.

Is1

Photophysical Processes in Condensed Phases

91

experimental condition used; this reduces the extent of ground-state depopulation and thus increases the number of molecules available for direct excitation to the S,state; (ii) the singlet oxygen feedback mechanism depicted in Figure 4. Using polycyclic hydrocarbons as guest molecules, it has been demonstrated that the singlet oxygen feedback mechanism makes an important contribution to the (b) enhancement effect, while the trivial mechanism plays only a minor r01e.189(a)

f

Figure 4 A schematic of the interaction of organic triplet state (T,)with singlet oxygen (lagor lCg+)to generate excited singlet state (S,)of the organic molecule in the feedback mechanism

Heavy Atom Quenching.-Last year we reported on the case of bromocyclopropane as a heavy atom perturber for use in mechanistic studies. Because of the absorption spectrum of this compound, it cannot be used at wavelengths < 300 nm. It has recently been demonstrated that xenon has some potential for solution phase mechanistic studies in that it is non-absorbing at 200 nm and is chemically inert. Its use to increase the quantum yields of triplet-state processes (by inducing intersystem crossing) may be limited by the lifetime of the triplet state, in that triplet quenching by xenon of long-lived triplets may give rise to interpretation problems.19oFrom studies of the effect of heavy atom perturbation on the photochemistry and photophysics of anthracene and 9,lO-dibromoanthracene, the following conclusions have been reached.lal (i) Heavy atoms quench the excited singlet states much more effectively than they quench the triplet states. (ii) A non-radiative process characteristic of external heavy atom involvement is competing with triplet relaxation (T1+ So)and with intersystem crossing to the triplet manifold (S, + T,J. As part of the above study, the triplet quantum yields of anthracene (0.74) and 9,lO-dibromoanthracene (0.79) were determined. The increase in the apparent molecular fluorescence of 9,lO-dibromoanthracene in fluid solution on addition of ethyl bromide has been examined in detail and ascribed to collision-induced (a) R. D. Kenner and A. U. Khan, J. Chem. Phys., 1976,64,1877; (6) R. D. Kenner and A. U.

Khan, Chem. Phys. Letters, 1975, 36, 643. H. Morrison, T. Nylund, and F. Palinsky, J.C.S. Chem. Comm., 1976, 4. lQ1 R. P. De Toma and D. 0. Cowan, J . Amer. Clzem. SOC.,1975, 97, 3283. lQ0

92 fluorescence, equation (28).lQ2The internal and external heavy atom effects on the properties of the individual magnetic sublevels of quinoxaline, naphthalene, and benzene derivatives have been studied at 1.6 K (using molecular crystals). The PMDR method employed gave evidence for an out-of-plane distortion in the geometry of the triplet states of bromobenzenes. The results on the external heavy atom effect indicate that the main spin-orbit coupling singlet state is the charge-transfer state of T molecule-o* perturber type.lQ3(See also ref. 194.) The decay functions of phosphorescence and of triplet-state populations, in a system where heavy atom perturbation is present, have been analysed lQ5and deuterium isotope effects on the radiative decay of heavy-atom-substituted aromatic molecules reported.lQs The photophysical parameters of a series of chargetransfer complexes of hexamethylbenzene (an electron-donor) with phthalic anhydride and its monohalo- and tetrahalo-derivatives, measured at 90 K, indicate that changes of decay times and fluorescence to phosphorescence quantum yield ratios are due to the heavy atom effect on the charge-transfer phosphore~cence.~~~ The fluorescence of all-trans-retinal is enhanced by halide salts. This effect, which contrasts with the heavy atom quenching of aromatic molecules, has been attributed to an enhancement of the fluorescence radiative rate constants as a result of interactions between the cation of the halide (e.g. K+) and the retinal.1a* Heavy atom effects on the photodimerization of acenaphthalene,lQQthe photochemical reduction of N-acetyldiphenylketimine by toluene and the corresponding addition,200the photochemistry of dixanthylidene,201the triplet states of dimethyl- and dihalo-xanthones,202and the photorearrangement of quinoline-l-oxide 203 have been reported.

3 The Triplet State Radiative and Non-radiative Processes.-Quantum yields of intersystem crossing or triplet quantum yields are probably the most difficult of the basic photophysical parameters to obtain, although there are several techniques available. A development of an earlier technique 204 has been used to obtain OT values for a wide range of aromatic hydrocarbons and some other compounds. The technique depends upon the use of a standard to act as the actinometer. Thus the standard of known triplet quantum yield is subjected to laser flash. The triplet concentration is measured by absorption (knowing A at the monitoring wavelength). The R. P. De Tome and D. 0. Cowan, J. Amer. Chem. SOC.,1975, 97, 3291. C. T. Lin, J. Luminescence, 1976, 12, 375. ls4 S. Yamauchi, L. Matsuzaki, and T. Azumi, J , Luminescence, 1976, 12, 369. ls6 J. Najbar, J. Luminescence, 1976, 11, 207. ls8 J. Friedrich, J. Vogel, W. Windhager, and F. Dorr, 2. Naturforsch., 1976, 31, 61. 197 M. Gronkiewicz, B. Kozankiewicz, and J. Prochorow, Chem. Phys. Letters, 1976, 38, 325. P A . Song, Q. Chae, M. Fujita, and H. Baba, J. Amer. Chem. SOC.,1976, 98, 819; P . 4 . Song and Q. Chae, J. Luminescence, 1976, 12, 831. lQs J. C. Kozlar and D. 0. Cowan, J. Amer. Chem. SOC.,1976,98, 1001. N. Toshima, A. Asao, and H. Hirai, Chem. Letters, 1975, 451 ; S. Asao, N. Toshima, and H. Hirai, Bull. Chem. SOC. Japan, 1976, 49, 224. so1 P. Konenstein, K. A. Muszkat, M. A. Slifkin, and E. Fischer, J.C.S. Perkin II, 1976, 438. 208 H. J. Downall and I. Granoth, J. Phys. Chern., 1976, 80, 508. 20s G. G. Aloisi and G. Favaro, J.C.S. Perkin II, 1976, 456. zoo J. T. Richards and J. K. Thomas, Trans. Faraday SOC.,1970, 66, 621. ls2 lg3

Photophysical Processes in Condensed Phases

93

compound of unknown OT is then subjected to the same laser flash and thus, knowing X for the compound of unknown @T, the value of @T may be A comparative study of the excited singlet yields in the T-T annihilation of naphthalene and anthracene has been reported.206 From an analysis of the microwave-induced delayed phosphorescence signals for coronene in n-octane at 1.35 K, the relative radiative rate constants for the triplet sub-levels have been E.s.r. investigations of the triplet state of naphthalene in plastic hosts,208and the application of the stretched polymer film method to the e.s.r. study of phosphorescent triplets 209 have proved very interesting. Several other reports of the triplet-state studies of polycyclic aromatic compounds have appeared.210-216 Using the electron-impact-energy-loss technique, the singlet-triplet spectra of methylated ethylenes have been obtained. The transition to the T,T* triplet seems well characterized. However, no evidence for the singlet-triplet or singletsinglet Rydberg transitions was found. This report contains a useful compilation of singlet-triplet energies of alkenes.216A review of the current state of knowledge of the triplet states of stilbenes has appeared (with particular emphasis placed on the results from triplet-quenching One important point made in this review is that stilbene triplets have a twisted (30") geometry in benzene solutions and a lifetime of 120 ns at 30 "C. For a review of stilbene photophysics up to 1973 see ref. 218. Intersystem crossing Tl -+ S, and geometric isomerization have also been considered by Orlandi and M a r c ~ n i . ~ l ~ Phosphorescence lifetimes for a series of monohalogenated ring-substituted phenylacetylenes reveal a marked decrease in T~ with the mass of the halogen atom except for fluorine which results in an increase in T~ (over the parent phenylacetylene).220Flash kinetic spectrophotometry using a series of sensitizers has enabled the triplet energy of azulene to be set near 39 kcal mol-l, the triplet energy of /%carotenebetween 21 and 25 kcal mol-1 and ferrocene between 38 and 41 kcal mo1-1.2a1 The reactions of benzaldehyde, acetophenone, and benzophenone at 77 K under high-intensity irradiation take place from upper triplet states produced by biphotonic processes. In contrast with the TI states under N

206 206 207

208

208

2l0

211 212 815 214

216

*16 217

218 218

221

B. Amand and R. Bensasson, Chem. Phys. Letters, 1975, 34, 44. F. Tfibel and L. Lindqvist, Chem. Phys., 1975, 10, 471. K. Ohno, N. Nishi, M. Konoshita, and H. Inokuchi, Chem. Phys. Letters, 1975, 33, 293. F. B. Bramwell, M. E. Laterza, and M. L. Spinner, J . Chem. Phys., 1975, 62, 4184. J. Ito, J. Niguchi, and T. Hoshi, Chem. Phys. Letters, 1975, 35, 141. R. W. Shaw and M. Nicol, Chem. Phys. Letters, 1976, 39, 108. K. Itoh, T. Zentoh, and H. Kawakanii, J. Luminescence, 1976, 12, 397. M. A. El-Sayed, Ann. Rev. Phys. Chem., 1975, 26, 235. T. Takemura, M. Aikawa, and H. Baba, J. Luminescence, 1976, 12, 819. M. Zander, Z. Naturforsch., 1973, 30, 1097. C. Rulliere, E. C. Colson, and P. C. Roberge, Canad. J. Chem., 1975, 53, 3269. W. M. Flicker, 0. A. Mosher, and A. Kuppermann, Chem. Phys. Letters, 1975, 36, 56. J. Saltiel, D. W. L. Chang, E. D. Megarity, A. D. Rousseau, P. T. Shannon, B. Thomas, and A. K. Uriante, Pure Appl. Chem., 1975, 41, 559. J. Saltiel, J. D'Agostino, E. Dennis, L. Metts, K. R. Neuberger, M. Wrighton, and 0. C. Zafiriou, 'Organic Photochemistry', Marcel Dekker, New York, 1973, p. 1. G. Orlandi and E. C. Marconi, Chem. Phys., 1975, 8, 458. H. Singh and J. D. Laposa, Chem. Phys. Letters, 1975, 36, 639. W. G. Herbstroeter, J. Amer. Chem. SOC.,1975, 97, 4161.

94

Photochemistry

these conditions, the upper triplet states produce ketyl radicals.2az The triplet states of carotenoids from photosynthetic the Arrhenius parameters for the photocleavage of butan-2-one triplets,224and the results of direct measurement of carbonyl triplet lifetimes in polymer solutions have been examined.225 For a small energy gap between Tland T,, the sparse density of TI vibronic levels isoenergetic with the T2 vibrationless level leads to slow Tl + 7 ' ' internal conversion. This situation exists in some aromatic carbonyl compounds and leads to dual phosphorescence. A non-interactive density-of-states model has been used to examine the situations when TI is n,r* in character and T2is r,r* in character and vice versa. Interesting temperature effects on phosphorescence are predicted.226The triplet lifetimes of benzophenone in the aqueous solutions have been obtained for the pH range 9.3 to 1.3. Reaction of triplet benzophenone with H30+ ions gives a short-lived These results should be compared with those produced in a study of benzophenone phosphorescence in acetic Light-absorbing transients formed in the photolysis of benzophenonehydrogen donor systems have a profound effect on the kinetics of the system in that they not only absorb light but are diffusion-controlled quenchers of triplet benzophenone (cf. benzpinacol k, = 4 x lo61 11101-1 s-l). The precise nature and lifetimes of the light-absorbing transients will vary a great deal for different aromatic ketones and solvents and so these results should be interpreted with care.,,* A bond-order-bond-energy method which assumes a pure radical mechanism (no charge-transfer involvement) has been used to calculate the kinetic parameters for the reactions of carbonyl triplets with various substrates having X-H bonds (where X is not carbon). One point of importance in this study is that abstraction of a hydrogen atom from N-H and O-H may be more important than previously The synthetic utility of intramolecular hydrogen atom abstraction by benzophenone derivatives has received further attention.230 In a related study, a method for using the phosphorescence of benzophenones as a probe for the conformation of hydrocarbon chains in polar and protic solvents has been developedZ3land the extraction of Faradic signals from flash photocurrent measurements of benzophenones has been Vibronic effects in the triplet states of anth hone,^^^ an analysis of the absorption and emission spectra of the lowest n,r* triplet state of 9,lO-anthraq~inone,~~~ and the appearance of a-diketone phosphorescence from ozone-olefin reactions have been discussed.236 2Z2

223 824 226 226

227

Ban 229

230 231 23a

233 a3Q 236

H. Murai and K. Obi, J. Phys. Chem., 1975,79,2446. R. Bensasson, E. J. Land, and B. Maudinas, Photochem. and Photobiol., 1976, 23, 189. E. Alwin and E. A. Lissy, J. Photochem., 1976, 5, 65. J. Faure, J. P. Fouassier, and D.-J. Lougnot, J. Photochem., 1976, 5, 13. S.-Y. Chu and L. Goodman, Chem. Phys. Letters, 1975, 34, 232. (a) G . Favaro and G. Bufalini, J . Phys. Chem., 1976, 80, 800; (b) M. A. Winnik and C. K. Lee, Mol. Photochem., 1974, 6, 477. J. Chilton, L. Giering, and C. Steel, J. Amer. Chem. SOC.,1976, 98, 1865. C. M. Previtali and J. C. Scaiano, J.C.S. Perkin 11, 1975, 934. R. L. Wire, D. Prezant, and R. Brislow, Tetrahedron Letters, 1976, 517. M. A. Winnik, A. Lemine, D. S. Saunders, and C. K. Lee, J. Amer. Chem. SOC.,1976,98,2000. K . F. Dahnke, S. S. Fratoni, and S. P. Perone, Analyt. Chem., 1976, 48, 296. H. J. Pownall, Mol. Photochem., 1974, 6, 425. K. E. Drabe, H. Veenuliet, and D. A. Wiersona, Chem. Phys. Letters, 1975, 35, 469. U. Schurath, H. Gusten, R.-D. Penzhorn, J. Photochem., 1976,5, 33.

Photophysical Processes in Condensed Phases

95

Electron-impact spectroscopy has been used to determine the triplet energy levels of thiophene, furan, and pyrrole. The values obtained 3.99 and 5.22 eV for furan, 3.75 and 4.62 eV for thiophene, and 4.21 eV for pyrrole show that the lowest triplets have energies close to that of benzene (3.95 eV) and quite well separated from the cyclopentadiene triplet (3.1 eV). These facts fit the model where the lone pair of electrons on the heterocyclic atom are donated into the ring 7r-electron While the main decay mode of tryptophan triplets in poly(viny1 alcohol) is almost certainly the result of triplet-triplet interactions, for wool reactions in the presence of air, tryptophan triplets are mainly deactivated by interaction with The triplet states of 1- and 2-methylindazole have similar absorption spectra with maxima at 420 and 405 nm respectively, and lifetimes of s.238 The diazines : pyrazine, pyrimidine, pyridazine, quinoxaline, and phthalazine, have been the subject of a laser-flash investigation. Table 10 summarizes some of the important results obtained. Although triplet N

Table 10 Quantum yields and extinction coefficients of triplet states of diazines in water at 25 "C E / M cm-l Diazine aim hfnm Pyrazine 0.87 & 0.1 260 4.9 x 103 700 1.1 x 105 Pyrimidine 1.0 0.2 260 -3.0 x 103

*

Pyridazine Quinoxaline

<0.02 0.67

-

600

5.3 x lo2

270

~14.9x 103 ~7.4x 103

413

432

660

~ 7 . 2x 103 a . 4 x 103

lifetime studies give one method of determining the nature (n,n* or 7r,n*) of the lowest triplet state, quenching studies are also revealing. Thus %,v*triplets are efficiently quenched by hydrogen atom donors and by certain inorganic ions which exhibit CTTS character. The quenching of %,v* triplets is on the other hand inefficient.239 Because of the very small absorption which occurs on flashing pyridazine, little information on this molecule was obtained from the above study. However, other workers have concentrated on pyridazine, and using naphthalene-d8 as the acceptor and emitter have obtained the So-T absorption spectrum of pyridazine with the sensitized phosphorescence technique.240 A photochemical determination of a new acid-base equilibrium of thionine in its triplet a study of the triplet states of morphine and d i a r n ~ r p h i n e an ,~~~ analysis of the reactivity at different sites of the triplet states of 4-acylpyrimi d i n e ~ and , ~ ~ studies ~ in the 2,3,7,8-tetramethoxythianthreneseries revealing a novel ground-state triplet di-cation 244 have been reported. 238 237

W. M. Flicker, 0. A. Mosher, and A. Kuppermann, Chem. Phys. Letters, 1976, 38, 489. K. P. Ghiggino, C. H. Nicholls, and M. T. Pailthorpe, Photochem. and Photobiol., 1975, 22, 169.

2y9

240

a41

242 243

*44

J. P. Ferris, K. V. Prabhn, and R. L. Strong, J. Amer. Chem. Soc., 1975, 97, 2835. D. V. Bent, E. Hayon, and P. N. Moorthy, J. Amer. Chem. Soc., 1975, 97, 5065. K. Yamamoto, T. Takemura, and H. Baba, Bull. Chem. SOC.Japan, 1975,48,2599. R. Bonneau, J. Pereyre, and J. Joussot-Dubien, Mol. Photochem., 1974, 6, 245. A. Bowd and J. H. Turnbull, J.C.S. Chem. Comm., 1975, 651. E. C. Alexander and R. J. Jackson, J. Amer. Chem. Soc., 1976,98, 1609. I. B. Goldberg, H. R. Crowe, G. S. Wilson, and R. S. Glass, J. Phys. Chem., 1976, 80, 988.

96

Photochemistry

Models for the triplet-state geometric isomerization of azomethine dyes have been presented. Two pathways are available in general, (i) torsion about the azomethine bond, and (ii) inversion about the nitrogen atom. The mechanism for isomerization in any dye is largely dependent upon the nature of the aromatic s u b s t i t ~ e n t s .The ~ ~ ~triplet states of EDA complexes of aromatic hydrocarbons with a range of electrophilic olefins have been studied using phosphorescence, e.s.r., T-T absorption spectroscopy, and microwave-induced delayed phosphorescence The radiation-induced formation of naphthalene singlet and triplet states in hydrocarbon solvents has been examined.247 Triplet Quenching and Triplet Energy Transfer.-The identification and characterization of triplet excimers is still relatively novel and for this reason a recent report on the kinetics of triplet excimer formation in the naphthalene and l-chloronaphthalene systems is interesting. Figures 5 and 6 together give an

1

I’M1

M

Figure 5 A plot of relative quantum yield of the excimer phosphorescence against the concentration of naphthalene in iso-octane at 293 K (0). The full line refers to the theoretical prediction (Reproduced by permission from J. Amer. Chem. Soc., 1976, 98, 2205)

interesting picture of the concentration and time-dependence of excimer formation. These and other results can best be explained by Scheme 7. Where kl’[lMo] is the triplet-state quenching (by a ground-state molecule) rate.248 Comparison of the interactions of ground-state oxygen and p-carotene with stilbene triplets suggests that electronic excitation is not transferred to oxygen. Rather, an encounter complex between stilbene and oxygen leads to ground-state oxygen and twisted stilbene ground-state molecules. Thus in the case of triplet quenching of the twisted stilbene triplet state by /3-carotene, trans-stilbene is produced almost exclusively, whereas oxygen (having the same lowest S-T energy gap) produces a photostationary state of close to 50 : 50 cis : trans isomers. The implications of these findings may be important for other flexible molecules having triplet state P.E. minima close to ground-state P.E. maxima, since in these circumstances only weak perturbations may be necessary to induce ~pin-exchange.~~~ Reports of the uranyl-ion-sensitized isomerization of stil246 248 247 248

249

W. G. Herbstroeter, J. Amer. Chem. SOC.,1976, 98, 330. H. Hayashi, M. Yagi, and N. Nishi, J. Luminescence, 1976, 12, 169. P. O’Neill, G. A. Solmon, and R. May, Proc. Roy. SOC.,1975, 347, 61. T. Takemura, M. Aikawa, H. Baba, and Y . Shindo, J. Amer. Chem. SOC.,1976, 98, 2205. J. Saltiel and B. Thomas, Chem. Phys. Letters, 1976, 37, 147.

PhotophysicalProcesses in Condensed Phases

97

b e n e ~ , ~the ~ Oisomerization of 2-butenes sensitized by adsorbed acetone,251and a method of studying energy transfer between like molecules using isotopic mixtures 252 have appeared.

*

-

4

-

!! 1.0 -a C P,

? I

; 0.5 n 0

2

1

C

, msec Figure 6 Some representative l ( t ) curves for naphthalene in iso-octane at 293 K. The monitoring wavelengths are (1) 470 nm, (2) 510 nm, and (3) 340 nm (Reproduced by permission from J . Amer. Chem. SOC.,1976, 98, 2205) t

From a study of energy-transfer kinetics and isomerization efficiencies on sensitizing a number of alkenes with acetophenone, it has been concluded that some kind of steric effect exists which, while not preventing energy transfer, interferes with geometric isomerization. Although some suggestions to account kd

PMoI

3D*

3M*

'MO Scheme 7

for this observation have been presented, it is quite clear that these are not very satisfactory and that further work is In a related study, it has been found that whereas trans-piperylene has unit efficiency as a triplet acceptor for naphthalene triplets, cis-piperylene is only 0.76 as efficient. However, in the latter case, 3(naphthalene)-cis-piperylene encounters lead to naphthalene quenching without energy transfer.254 Further work on intramolecular triplet energy transfer between indanone and naphthalene groups has been reported and the earlier work Measurements of the time-dependent phosphorescence decay curves of carbazole in the presence of varying concentrations of naphthalene (triplet 260

262

254 266

R. Matsushima, T. Kishimoto, and M. Suzuki, Bull. Chem. SOC.Japan, 1975, 48, 3028. K. Otsuka and A. Morikawa, Bull. Chem. SOC.Japan, 1975, 48, 3021. A. Inoue and N. Ebara, Chem. Letters, 1975, 1137. A. Gupta and G. S. Hammond, J. Amer. Chem. SOC.,1976,98, 1218. J. R. Kelly, A. Gupta, and G. S. Hammond, Photochem. and Photobiol., 1975, 21, 275. K. Schaffner, W. Amrein, and I. M. Larssen, Israel J. Chem., 1975, 14, 48.

Photochemistry

98

acceptor) in a rigid glass have enabled the Dexter mechanism for energy transfer to be measured. Good exponential dependence of the rate of triplet-triplet energy transfer on intermolecular separation is obtained with energy transfer occurring over distances greater than 10 A.266 Although the spectroscopic triplet-state energy levels of alkenes can be obtained in a number of ways, the energy levels of relaxed triplets and particularly the triplet lifetimes have proved very difficult to measure. However, some useful results in this area have now been obtained from the pulse radiolysis of a benzene solution of norbornene containing some anthracene. The principle behind the experiment is that triplet benzene produced by the pulse transfers energy to norbornene which acts as an energy carrier to sensitize anthracene triplet formation. Thus measuring the time dependence of anthracene triplet formation and assuming that the rate of delayed triplet formation is controlled only by equations (30) and (31), the relationship (29) holds. The above allows the triplet lifetime of k,, = k,

Norbornene" (TI) Norbornene* (Tl)+ anthracene (So)

+ k, [anthracene]

___+

(29)

norbornene (So) (30) norbornene (So) + anthracene' (Tl) (31)

norbornene to be evaluated: it is 250 ns. Using a range of aromatic hydrocarbon acceptors, the triplet energy of norbornene was bracketed between that of chrysene (ET= 19 800 cm-l) and naphthalene (ET = 21 200 cm-l), and this value is at least 5000 cm-l below the energy of the spectroscopic (vertical A comparative study of the interactions of a number of aliphatic and aromatic ketone triplets with aliphatic amines in the gas-phase and solution has been carried out. While reactivity in solution correlates well with the amine ionization potentials, rate constants for the gas-phase interactions are largely dependent upon the presence of N-H bonds, and presumably reflect the reactivity of these Laser-flash photolysis has been employed to confirm the earlier reported effects of amine structure and solvent on the reactions of triplet benzophenone with aromatic a m i n e ~ . ~ ~ ~ Although theoretical calculations have predicted the presence of the triplet states of aliphatic hydrocarbons 1-2 eV below the first singlet state, little attention has been paid to the experimental problem of investigating the former. Energy transfer from upper triplets of aromatic hydrocarbons to the aliphatic hydrocarbons has been used to estimate triplet levels (e.g. for methylcyclohexane the triplet state is between 47 800 and 48 450 cm-l while trans-decalin has a lower energy triplet, 47 320-47 800 cm-1).260 The triplet energy levels of a range of azomethine dyes have been obtained using an energy-transfer technique 261 and the effects of donor structure on the 266

257 268 269

260

G . B. Strambini and W. C. Galley, J. Chem. Phys., 1975, 63, 3467. A. J. G. Banvise, A. A. Gorman, and M. A. J. Rodgers, Chem. Phys. Letters, 1976, 38, 313. E. B. Abuin, M. V. Encina, E. A. Lissi, and J. C. Scaiano, J.C.S. Furaduy I, 1975, 1221. S. Arimitsu, H. Masuhara, N. Mataga, and H. Tsubomura, J. Phys. Chem., 1975, 79, 1255. V. A. Smirnov, V. B. Nazarov, V. I. Gerko, and M. V. Alfimov, Chem. Phys. Letters, 1975, 34,500.

261

W. G. Herbstroeter, J. Amer. Chem. SOC.,1975, 97, 3090.

Photophysical Processes in Condensed Phases

99

triplet-triplet energy transfer from styrylpyridines to biacetyl 262 examined. The quenching of benzophenone triplets by naphthalene has been used as the basis of a physical-organic teaching experiment.263A number of observations relating to triplet quenching by inorganic species have recently been recorded. Thus quenching by tris(/?-diketonato) complexes of iron(@, ruthenium(m), and a l u m i n i u m ( ~ ~by ~ ) ,f~e ~r r~o ~ e n e ,and ~ ~ ~by Group V organometallics are of interest. Finally, electron transfer from the triplet state of phenothiazine to metal ions has been examined.267 4 Two-photon Processes The detection of a novel two-photon process, in which benzene radical cation C6H2 photodissociates upon irradiation from an intense argon ion laser source, provides evidence that internal conversion competes favourably with fluorescence 362

37.2--,---

I

T

s3

Figure 7 Energy level diagrams for (a) rhodamine 6G and (b) rhodamine B, calculated on the basis of pre-existing absorption data, and both pre-existing and new fluorescence data. We draw only the Franck-Condon maxima for clarity: - energy levels corresponding to Franck-Condon maximum radiative transitions; - - - energy levels reached by available laser frequencies. The notations $, cpl, and cp2 correspond to equilibrium nuclear configurations in S,, S1,and S2 respectively. Wavelengths of laser excitation and fluorescent transitions are in nm (Reproduced from Chem. Phys. Letters, 1975,36,295)

following 2E,,-+ 2A2uexcitation. The state to which internal conversion occurs and from which photodissociation takes place may be the 2Elustate or a vibrationally excited ground state.a68Fluorescence spectra, originating from the upper excited singlet states of three xanthene dyes, have been obtained using biphotonic pulsed laser excitation. Figure 7 combines some old data with those obtained in this study on the important dyes rhodamine 6G and rhodamine B.269 2fi2

2R:' 2n4 281'

268

26!'

G. Favaro, G. Bartocci, and D. Bortolus, Z . phys. Chem. (Frankfurt), 1975, 96, 161. P. Natarajan, J. Chem. Educ., 1976, 53, 200. F. Wilkinson and A. Farmilo, J.C.S. Faraday IZ, 1976, 72, 604. A. Farmilo and F. Wilkinson, Chem. Phys. Letters, 1975, 34, 575. R. H. Lema and J. C. Scaiano, Tetrahedron Letters, 1975, 4361. S. A. Alkaitis, G. Beck, and M. Gratzel, J. Amer. Chem. SOC.,1975, 97, 5723. B. S. Freiser and J. L. Beauchamp, Chem. Phys. Letters, 1975, 35, 35. G. C. Orner and M. R. Topp, Chem. Phys. Letters, 1975, 36, 295.

100

Photochemistry

A review of coherent two-photon processes,27oin which transient and steadystate cases are discussed, has appeared, and biphotonic processes in aniline 271 and in weak charge-transfer complexes 272 have been investigated. 5 Photo-oxidation In order to conserve space this section has been reduced in size compared with previous years. For other important references in this area see Part 111, Chapter 5. The role of the superoxide radical anion 0,' in photo-oxidation mechanisms has been of considerable interest in recent years, and some of the kinetic parameters involving this species are now becoming available. In a study designed to probe the interactions of superoxide and singlet oxygen (lAg), two species which may be found together in biological systems, the superoxide was found to quench lo2in aprotic solvents with a k , of 3.6 f 0.1 x lo71 mol-1 s-l. This relatively high rate constant may be related to the low binding energy of the extra electron in the superoxide radical anion (0.43 f 0.03 eV) by analogy with '~ other electron-rich singlet oxygen quenchers (e.g. amines and s ~ l p h i d e s ) . ~These results also tend to rule against a suggested mechanism for the formation of lo, by the disproportionation of the superoxide, equation (32). Since the dispro2H20

lo, + H202 + TOH

+ 0,' + 0,'

portionation reaction is slower than quenching, process (32) can hardly be an important source of lo2.More recent work on the above quenching process in the same solvent acetonitrile gave a much higher value for k , (7 f 6 x lo9 1 mol-1 s-l). The difference may be accounted for by an efficient decay of 0 , ' which occurs in acetonitrile. In dimethyl sulphoxide, k , was found to be 1.6 x lo9 I mol-1 s-l, which is only a factor of ten below the value for /3-carotene. Interesting as these studies are, it is apparent that in biological systems neither the disproportionation of superoxide nor superoxide quenching of lo2can be of great importance, since the concentration of superoxide in aqueous solution can never be very The photo-oxygenation of triethylamine sensitized by Rose Bengal has been shown to involve both singlet oxygen and radical intermediates (Scheme 8). The

Dye (TI)

lAg

+Radicals

O2

---+

oxidation products

------Scheme 8

importance of each of the two routes can be controlled by the concentration of amine. Thus in competitive experiments with 2,5-dimethylfuranYthe decrease in the rate of oxidation of the furan with increased concentrations of triethylamine 270 271 27a

273 274

R. G . Brewer and E. L. Hahn, Phys. Rev. (A), 1975, 11, 1641. G. Perichet, R. Chapelon, and B. Pouyet, Mol. Photochem., 1976, 7 , 1. Y . Achiba and K. Kimura, J . Luminescence, 1976, 12, 871. I. Rosenthal, Israel J. Chem., 1975, 13, 86. H. J. Guirand and C. S. Foote, J. Amer. Chem. Soc., 1976, 98, 1984.

Photophysical Processes in Condensed Phases

101

can be accounted for by the amine quenching the singlet and triplet states of the dye.275 Some important parameters for the photo-oxidation of bilirubin and biliverdin, of interest because of their role in the phototherapeutic treatment of neonatal hyperbilirubinemia, have been obtained:27sk , (physical quenching of '0,by bilirubin in CC14) = 2.3 x log1 mol-1 s-l; k, (physical quenching of lo, by biliverdin in CHCl,) = 3.3 x logI mol-1 s-l: k (reaction of bilirubin with lo2) = 1.7 x lo8 lmol-ls-l; k (reaction of biliverdin with 1 0 2 d 3 x lo61 mol-1 s-l. Low temperature studies of intermediates in the singlet oxygen oxidation of indene 277 and 1,2-dihydronaphthalene 278 have been carried out, and the importance of N-formylkynurenin as a triplet sensitizer in the studies of tryptophan photo-oxidation emphasized.27g Two reports 281 on novel photo-oxidation products of indole derivatives have appeared, and the diffusion coefficient D* for diffusion of 0,(lAJ in ground-state oxygen reported.2R2 280s

6 Chemiluminescence

A number of papers dealing with various aspects of dioxetane decompositoni and the ensuing luminescence on energy-transfer processes have appeared this Year.283-288 Another important chemiluminescent process in the benzene series has been reported. Thermolysis of 3,3'-biscyclopropenyl (BCP) at 63 "C in acetonitrile leads to the formation of a triplet xylene (TT = 10 ps) probably via a dimethyl Dewar-ben~ene.~~~ Hydrazide derivatives of some paracyclophanes are chemiluminescent under oxidative conditions and chemiluminescence is activated by tetramethylthiurandisulphide in radical reactions.2g1 Benzophenone triplet emission in fluid solution is observed when an electrogenerated chemiluminescence technique is applied to the benzophenone-thianthrene system and related mixed systems. The presence of benzophenone triplets was confirmed by experiments involving energy transfer to naphthalene.2g2 Chemiluminescence and energy transfer in systems of electrogenerated anions and benzoyl and the role of R. S. Davidson and K. R. Trethewey, J.C.S. Chem. Comm., 1975, 674. B. Stevens and R. D. Small, jun., Photochem. and Photohiol., 1976, 23, 33. 277 P. A. Burns, C. S. Foote, and S. Mazur, J. Org. Chem., 1976, 41, 899. 278 P. A. Burns and C. S. Foote, J. Org. Chem., 1976, 41, 908. 279 P. Walrant, R. Santus, and L. I. Grossweiner, Photochem. and Photobiol., 1975, 1, 63. 280 I. Saito, M. Imuta, S. Matsugu, and T. Matsuma, J. Amer. Chem. SOC.,1975, 97, 7191. 281 G. Stohrer, J. Heterocyclic Chem., 1976, 13, 157. 282 P. H. Vidaud, R. P. Wayne, and M. Yaron, Chem. Phys. Letters, 1976, 38, 306. 293 H. E. Zimmerman and G. E. Keck, J. Amer. Chem. SOC.,1975, 97, 3527. 284 P. Lechtken and H.-C. Steinmetzer, Chem. Ber., 1975, 108, 3159. ms C. S. Foote and T. R. Darling, Pure Appl. Chem., 1975, 41, 495. 286 W. Adam, N. Duran, and G. A. Simpson, J. Amer. Chem. SOC.,1975, 97, 5464. 287 W. H. Richardson, F. L. Montgomery, P. Slusser, and M. B. Yelvington, J. Amer. Chem. SOC.,1975, 97, 2819. 288 T. Wilson, D. E. Golan, M. S. Harris, and A. L. Baumstock, J. Amer. Chem. SOC.,1976,98, 1086. 288 N. J. Turro, G. B. Schuster, R. G. Bergman, K. J. Shea, and J. H. Davis, J. Amer. Chem. SOC.,1975, 97, 4758. 290 K.-D. Gundermann and K.-D. Raker, Annalen, 1976, 140. 291 J. Rychly, A. Andrasik, E. Staudner, and J. Beniska, J. Luminescence, 1976, 11, 173. 292 S. M. Park and A. J. Bard, Chem. Phys. Letters, 1976, 38, 257. 193 T. D. Santa Cruz, D. L. Akins, and R. L. Birke, J. Amer. Chem. SOC.,1976,98, 1677. 275

276

Photochemistry

102

hydrogen peroxide and peroxides in chemiluminescence reactions 294 have been discussed, and the chemiluminescence of 2,2-azobisisobutyronitrile and cumene hydroperoxide in the presence of bis( - )-ephedrine copper(i1) chelates reported.Zs6

7 Photochromism The piezo-chromism and the photochromism as a function of pressure have been studied for two photochromic spiropyrans 296 in polystyrene and poly(methy1methacrylate) films. The spectral and electrical properties of some s p i r ~ p y r a n s , ~ ~ ~ the thermal stabilization of some benzothiazoline s p i r o p y r a n ~ and ,~~~ quantum yields of coloration and thermal stability of coloured forms for some spiropyran layers have all been investigated.2s9 In the past there have been a number of disputes over the mechanisms of photochromism of the dianthrone system. In particular it has been disputed whether the coloured B isomer is formed via the triplet state of dianthrone or directly from the singlet state. New evidence that B is indeed formed via the triplet state has been obtained by a flash-photolysis study of the tetramethyl derivative TMD (13).300 Although the photochromic properties of crystalline 0

2,3,4,4-tetrachloro-l-keto-l,4-dihydronaphthalene (p-TKN) have been known for 75 years, questions still remain concerning the mechanism of the photochromism of this molecule. An e.s.r. study of p-TKN has been carried out to investigate the nature of a triplet species detected some time ago on photolysis of this molecule, and it has been concluded that the triplet state is the ground state of a radical pair.3o* The thermal bleaching of the coloured form of the photochromic 2-hydroxy-2’,4,4’-trimethoxytriphenylmethanol 302 in acetonitrile, the autoxidation and photochromism of some a r y l h y d r a z o n e ~ ,the ~ ~ ~photo294 295

2D6 297

2n8

K. D. Gundermann, Chem.-Ztg, 1975,99,279. L. Rychla Matisova, V. Horanska, and J. Barton, J. Luminescence, 1975, 10, 129. D. G. Wilson and H. G. Drickamer, J. Chem. Phys., 1975, 63, 3649. A. A. Darshutkin and V. A. Krongauz, Mol. Photochem., 1974, 6,437. A. Samat, J. Kister, F. Garnier, J. Metzger, and R. Guglielmetti, Bull. SOC.chim. France, 1975, 8, 2627.

ang

300

301 302 303

H.-P. Vollmer, 2. Naturforsch., 1975, 30, 1425. T. Bercovici, R. Korenstein, G . Fischer, and E. Fischer, J. Phys. Chem., 1976, 80, 108. F. P. A. Zweegers and C. A. G . 0. Varma, Chem. Phys., 1976, 12, 231. S. Hamai and H. Kokubun, Bull. Chem. SOC.Japan, 1975,48, 1848. G. E. Lewis and G. L. Spencer, Austral. J. Chem., 1975, 28, 1733.

Photophysical Processes in Condensed Phases 103 chromism of some quinoylhydraxones and the photochromic behaviour of some phenothiazine molecular complexes in the absorbed state have been examined.

Ph N-%

N-ko

Ph

N-ko

Ph Scheme 9

E-(2)-Isopropylidene-3-(a or p-naphthylmethylene) succinic anhydrides and N-phenyl imides undergo reversible photochemical ring closure to form orange on red 4,4a-dihydrophenanthrene derivatives. Attempts to observe the double ring closure of the potentially multiphotochromic compound (14) failed.306 8 Some Low Temperature and Crystal Studies Differences in the electronic structure of benzene in two different crystalline modifications of cyclohexane host have been and two new studies of energy transfer in naphthalene, one of energy transfer in molten naphthalene 908 and one in isotopically mixed naphthalene c r y ~ t a l have ~ , ~ ~been ~ reported. A 304

306

306 307

308

J. L. Wong and M. F. Zady, J. Org. Chem., 1975, 40, 2512. M. Bereiter, W. Winde, and G. Krause, Z. Chem., 1975, 15, 373. R. J. Hart, H. G. Heller, R. M. Megit, and M. Szewczyk, J.C.S. Perkin I, 1975, 2227. P. J. Vergragt and J. H. van der Waals, Chem. Phys. Letters, 1975, 36, 283. R. B. Kellogg and A. Prock, J . Chem. Phys., 1975, 63, 3161. H. Post, D. Vogel, and H. C. Wolf, Chem. Phys. Letters, 1975, 34, 23.

104 Photochemistry number of photophysical studies of crystalline anthracene have been carried O U ~ , and ~ ~investigations ~ - ~ ~ ~of the solid-state photophysics of crystalline p y ~ e n e pyrene , ~ ~ ~ in an n-heptane anthracene-doped ~ h a n a n t h r e n e , ~ ~ ~ coronene and perylene in n-heptane solid t e t ~ a c e n e thin , ~ ~ ~layers of p e n t a ~ e n e ,and ~ ~ ~doped fluorene crystals 320 have been reported. Novel work on the isotopically selective photochemistry in molecular crystals has been initiated.321 The structural influence on excimer emission from crystalline ~ t i l b e n e , ~ ~ ~ the anisotropy of the fluorescence from biphenyl the triplet state of acenaphthenequinone energy transfer and spin alignment in the triplet manifold of p-chloroaniline single the solid benzophenone sensitized isomerization of 1,3-pentadiene326 and the vibrational relaxation and photochemistry of cyclic ketones in low temperature matrices 327 have been the subjects of recent investigations. Matrix isolation, which has already proved itself to be a powerful technique for the isolation and identification of metastable intermediates produced in photochemical reactions, has been applied to a number of problems this year. Thus the matrix photolysis of 1,2,3-thiadiazole has indicated the possible involvement of thiirene as an intermediate.328 The matrix photolysis of 4-phenyl1,3,2-oxathiazolylio-5-oxide329 has been carried out, and methyl and phenyl radicals have been identified by i.r. spectroscopy following the matrix photo1ysis of acetyl benzoyl 310 311

312 313

314 315 318 317

318 319 320 321 322 323 324 326

326 327 328 328

330

J. Ferguson, Chem. Phys. Letters, 1975, 36, 316. S. Iwashima, H. Honda, M. Kuramachi, T. Sawada, M. Takekawa, S. Fujisawa, and J. Aoki, Nippon Kagaku Kaishi, 1975, 746. E. Glockner and H. C. Wolf, Chem. Phys., 1975, 10,479. J. 0. Williams, B. P. Clarke, J. M. Thomas, and M. J. Shaw, Chem. Phys. Letters, 1976, 38, 41. H. Port and K. Mistelberger, J. Luminescence, 1976, 12, 351. V. D. Tuan, U. P. Wild, M. Lamotte, and A. M. Merle, Chem. Phys. Letters, 1976, 39, 118. S. H. Tedder and S . E. Webber, Chem. Phys., 1976, 12, 253. M. Lamotte, A. M. Merle, J. Joussot-Dubien, and F. Dupuy, Chem. Phys. Letters, 1975, 35, 410. H. Muller and H. Bassler, Chem. Phys. Letters, 1975, 36, 312. I. Hornyak, J. Luminescence, 1976, 11, 241. R. Furrer, J. Gromer, A. Kacher, M. Schwoerer, and H. C . Wolf, Chem. Phys., 1975, 9, 445. R. M. Hochstrasser and D. S . King, J. Amer. Chem. SOC.,1975, 97, 4760. R. Cohen, Z. Ludmer, and V. Yakhot, Chem. Phys. Letters, 1975, 34, 271. A. Brce, M. Edelson, and R. Zwarich, Chem. Phys., 1975, 8, 27. M. Sano, T. Narisawa, and Y. J. I’hara, Bull. Chem. SOC.Japan, 1975, 48, 3469. N. Nishi and M. Kinoshita, J. Luminescence, 1976, 12, 383. J. S. DeGuzman and G . R. McMillan, J. Phys. Chem., 1975, 79, 1377. L. T. Molina and E. K. C . Lee, J. Phys. Chem., 1976, 80, 244. A. Krantz and J. Laureni, J. Amer. Chem. Sac., 1976, 98, 641. I. R. Dunkin, M. Poliakoff, J. J. Turner, N. Harrit, and A. Holm, Tetrahedron Letters, 1976, 873. J. Pacansky and J. Bargon, J. Amer. Chem. SOC.,1975, 97, 6896.

3 Gas-phase Photoprocesses ~~

~~

BY D. PHILLIPS

1 Introduction The trend noticeable in recent years of an increasing volume of published work being concerned with small molecule and atomic systems, particularly those of interest in aeronomy and atmospheric chemistry, has continued this year. For this reason, the final section of this chapter contains a very large number of references, although lack of space has prevented expansion of this section. The book ‘Chemistry of the Atmosphere’ by M. J. McEwan and L. F. Phillips (Edward Arnold, London, 1975) provides a useful review of photochemical processes in the atmospheres of Earth and other planets. 2 Alkanes, Alkenes, and Alkynes There have been comparatively few papers in this area this year, so separate headings for the different chromophores have not been retained. Chemiluminescence from CO+(x21T)has been observed in a beam experiment through reaction (l), and evidence was given that the CO+(L211)is formed with abnormally high rotational excitation for collision energies of 3.6 eV.l

+

-

c+ 0 2 C O + ( P r I )+ 0 (1) Fluorescence analysis of the CH and CD(X~A+ x211)systems arising from the pulsed radiolysis of acetylene has allowed estimation of the initial rotational energies in the CH fragments as 5.5 kcal mol-1 (CH) and 2.1 kcal mol-1 (CD) corresponding to rotational temperatures of 2780 and 1200 K, respectively.2 Addition of 2 Torr He increases the CH rotational temperature to 4400 K, but for high pressures of He, rotational relaxation was shown to be complete in less than 500 ns. The radiative lifetimes of the N’ = 3-15 rotational states of the BE- state of CH vary only slightly, between 300 and 400 ns, increasing slightly with increase in N’ up to N’ = 14;3 the N’ B 15 levels have much shorter radiative lifetimes. Emission from the X3n,,, binu, and z3zg-states of C , produced in the reaction of hydrogen atoms with halogenated compounds (CH2-nX2+n, X = C1, Br, or I) shows that the initial relative populations of these states, produced by reaction (2) are 10 : 400 : lOOO.* C 1 2

*

+ CX

____+

C2*+ X (X = halogen or hydrogen)

Ch. Ottinger and J. Simonis, Phys. Rev. Letters, 1975, 35, 924. M. Schmidt, H. A. Gillis, and M. Clerc, J . Phys. Chem., 1975, 79, 2531. R.A. Anderson, J. Peacher, and D. M. Wilcox, J. Chem. Phys., 1975,63, 5287. s. J. Arnold, G. H. Kimbell, and D. R. Snelling, Canad.J. Chem., 1975,53, 2419.

105

(2)

Photochemistry

106

A theoretical method for the evaluation of energy distributions in products from the photoionization of CH4 and CD4 gives reasonable agreement with experiment.s Reactions (3)-(5) were investigated. The photoacoustic detection CH, 3- hv

-

+e

(3)

CH3++H+e

(4)

CH,+ CHa+

+ H, + e

(5)

spectroscopic method with dye laser excitation has been applied to the 619 nm band of CH, and the 645 nm band of NH,.S Evidence has been presented that in the photolysis of CH4 and CD, in the vapour phase at 123.6 and 104.8 nm, methylene is formed in the hlB, state.' Reactions of methylene are discussed in Section 4. Photolysis of methane-water mixtures with a low-pressure lamp gives rise initially to formaldehyde, acetaldehyde, and methanol, but upon prolonged irradiation, ethylene glycol, ethanol, acetone, isopropyl alcohol, t-butyl alcohol, methyl ethyl ketone, isobutyl alcohol, t-amyl alcohol, and neopentyl alcohol are formed.8 Interestingly, the addition of Na did not affect the product distribution, a fact of some importance in understanding the chemical evolution of the primit ive atmosphere. Conflicting reports as to the origin of the methyl radical in the 123.6nm photolysis of propane may have been resolved by a study using chemical trapping techniques in the photolysis of [l ,l,l-2H3]propane.e The isotopic distribution of products was incompatible with reaction (6) being the source of CH3; reactions CH,*

+ hv GHg + hv GH,

C.,H,*

____+

___+

___+

+ H* CH3* + CaH6* H + GH?** H* + CH3* + GH., CH3*

(6)

(7) (8)

(9)

(7)-(9) were proposed to be chiefly responsible. A theoretical study of the excited states and photochemistry of propane has appeared.1° I n the photolysis of cyclopropane at 147 and 163.4 nm, the relative importance of the primary processes (10)-(15) was evaluated as shown.f1 INDO calculations Cyclo-C,H,

* lo

+ hv

___+

147 nm

+ CH, C,H,* + CH,* CgH,

CH,=C=CHa CH,C=CH

67%

+ 2Ha

+ 2H*

163.4 nm 69% (10)

19%

18%

(11)

8%

7%

(12)

1%

1%

(13)

T. Watanabe and S. Nishikawa, Chem. Phys., 1975, 11, 49. G. Stella, J. Gelfand, and W. H. Smith, Chem. Phys. Leiters, 1976, 39, 146. J. Masanet and C. Vermeil, J. Chim. phys., 1975, 72, 820. J. P. Ferris and C. T. Chen, J. Amer. Chem. SOC.,1975,97,2962. J. Gawlowski, J. A. Herman, and P. Gagnon, Canad. J. Chem., 1975,53, 1348. P. M. Saatzer, R. D. Koob, and M. S. Gordon, J. Amer. Chem. SOC.,1975, 97, 5054. K. Shibuya, K. Obi, and I. Tanaka, Bull. Chem. SOC.Japan, 1975,48, 1974.

107

Gas-phase Photoprocesses

+ CH,. + H* C,H2 + CH,

C,H2

A

---+

3%

4%

(14)

1%

1%

(15)

have predicted that the photodecomposition of CH2F, will primarily consist of C-H cleavages at threshold excitation energies, but that at higher energies C-F bond cleavage will become possible.12 This unsurprising view has yet to be tested by experimental work. Because of the strength of the C-F bond, and the low spin-orbit coupling constant for fluorine, fluorinated compounds will not be considered here to be typical halogenated compounds, and are thus not specifically grouped in Section 7 along with chlorine-, bromine-, and iodinesubstituted molecular species. Photolysis of ethylene at 185 nm yields hex-1-ene and butane in a ratio which lies between 0.6 and 1.4 at 298 K, dropping to 0.25 at 153 K.13 Reactions (I 6)-( 18) accord with incident intensity and pressure effects. In the sensitized C2H** + (CH2)4

-

(234

+ C2H4

(CH,),

(16)

(CH2)6

(17)

hex-l-ene (1 8) (CH,), ----+ photolysis of vinyl fluoride, the overall reaction was found to be (19), with quantum yields extrapolated to zero pressure of 1.0 for Hg(V1) and Cd(3P,) CFH=CH,

C2H2

+ HF

(1 9)

sensitizers at 27 and 275 "C, respectively, and 0.31 for benzene (3B1u)as sensitizer.14 Two intermediates were implicated however, namely triplet vinyl fluoride and triplet 2-fluoroethylidene. From first-order rate constants for the reaction of 3.8 x lolo, 8.3 x lo8, and 4.6 x lo8s-l for Hg, Cd, and benzene sensitization respectively, the Arrhenius parameters for reactions (20) and (21) were evaluated $CHF=CH2 3:CHCH,F

%CHCH2F

(20)

+ HF

(21)

C2H2

as Azo = 9 x 1O1O s-l, Azl = 2.1 f 1.0 x 10l2s-l, EaO= 6.0 kcal mol-l, E2, = 22.4 f 1.7 kcal mol-I. By contrast, in the Hg(3P,)-sensitized photolysis of trifluoroethylene l5 the ethylidene intermediate is not implicated. The major process (22), the geminal elimination of HF, (22) has a quantum yield (at zero 3CHF=CFa 3CHF=CF2

:CHCF2 :CHF

+ HF

+ :CF2

(22) (23)

pressure) of 0.8. Reaction (23) is a minor observeable process, The CF2CH: species did not react with CO or 0,. H F elimination [as in (19)] is also the major fate of directly photoexcited vinyl fluoride and difluoroethylenes in argon matrices, although reaction (24) also occurs to a small extent.l8 H2C=CHF 18

13 14 15

+ hv

----+

+

C2H2F* H-

(24)

M. S. Gordon, Chem. Phys. Letters. 1976. 37, 593. M. Simon and R. A. Back, Canud. J. Chem., 1975,53, 1245. S. Tsunashima, H. E. Gunning, and 0. P. Strausz, J . Amer. Chem. SOC., 1976, 98, 1690. R. J. Nortstrom, H. E. Gunning, and 0. P. Strausz, J. Amer. Chem. SOC.,1976, 98, 1454. W. A. Guillory and G. H. Andrews, J. Chem. Phys., 1975,62,3208,4667.

108

Photochemistry

Hg(3P1)sensitization in l-methylcyclopentene l7 and penta-1,3-diene l8 has also been reported, as have the sensitized photolyses of hex-l-ene, ~is-oct-2-ene,~~ cisand trans-4,5-dimethylcyclohexenes,20 and cis-cyclononene.21 Each mechanism involves intramolecular hydrogen abstraction, and the major products from hex-l-ene are propene, trans-l,2-dimethylcyclobutane,and cis-l,2-dimethylcyclobutane with smaller amounts of methylcyclopentane, cyclo-hexane, and n-hexane. The major products in the direct photolysis of the 4,5-dimethylcyclohexenes were but-2-ene and buta-lY3-diene with smaller amounts of l-methylene-3,4-dimethylcyclopentane, whereas with HS(~P~)sensitization, dimers and other products of radical reactions were also identified. Photolysis of cis-cyclononene follows a similar course in that direct photolysis yields nona-l,8diene and vinylcycloheptane through internal conversion from the S, state, whereas the variety of products seen on Hg(3P1) and benzene (3B,,) sensitized photolysis can be accounted for by intramolecular hydrogen abstraction. In the flash photolysis of cycloheptatriene at 210nm benzyl radicals are ~ ~ to the thermal isomerization formed with a quantum yield of ~ 0 . 6in, contrast where benzyl is not formed. The molar decadic extinction coefficient of benzyl was determined as 6 x lo31 mol-1 cm-l at 253 nm. The effects of buffer gas on the optically pumped CH3Fi.r. laser,23the Dopplerfree two-photon spectroscopy of the v3 bands of CH3F (and v, bands of NH3),24 and other aspects of two-photon spectroscopy 26 have been reported. Highly structured emission in the 260-340 nni region has been observed in the reaction of 0, with C2F4, and attributed to an excited state of CF2.26 The emission is quenched by O,, but this is not due to the direct quenching of the emitting species, involving instead removal of a precursor to the CF,* species.

3 Aromatic Molecules A review has appeared of excimer (and exciplex) formation which discusses this phenomenon in aromatic molecules in Non-radiative transitions and luminescence in aromatic hydrocarbons have also been the subject of a brief report.28 The photophysics of benzene continues to exercise theoretical and experimental talents. Metz 29a has pointed out that earlier attempts 29b at solving the theoretical problem of describing the non-radiative decay of excited lBzu singlet vibronic levels of benzene in terms of undisplaced simple harmonic 17

20 21

a2 2s

24 25

26

28

G. R. De Mark, N . Anthenius, and J. Olbregts, J. Photochem., 1975,4, 299. G . R. De Mark and M. C. Fontaine, Reaction Kinetics Catalysis Letters, 1975, 3, 17. Y . Inoue, S. Takamuku, and H. Sakurai, J.C.S. Chem. Comm., 1975,896. Y.Inoue, S. Takamuku, and H. Sakurai, Bull. Chem. Sac. Japan, 1975,48,3101. Y. Inoue, S. Takamuku, and H. Sakurai, Bull. Chem. SOC. Japan, 1976,49, 1147. S. Luu, K. Glaenzer, and J. Troe, Ber. Bunsengesellshaftphys. Chem., 1975,79, 855. T. Y . Chang and C. Lin, J. Opt. SOC.Amer., 1976,66,362. W. K. Bischel, P. J. Kelly, and C. K. Rhodes, Phys. Rev. (A), 1976, 13, 1817, 1829. A. Ben-Reuven, J. Jortner, L. Klein, and S. Mukamel, Phys. Rev. (A), 1976, 13, 1402; R. L. Swofford and W. M. McClain, Chem. Phys. Letters, 1975, 34, 455. R. S. Sheinson, F. S. Toby, and S. Toby, J. Amer. Chem. SOC., 1975, 97, 6593. J. B. Birks, Reports Progr. Phys., 1975,38,903. R. N. Nurmukhametov and V. G. Plotnikov, Izoest. Akad, Nauk. S.S.S.R., Ser. j k , 1975,39, 2259.

29

(a) F. Metz, Chem. Phys., 1975, 9, 121; (b) See, for example, K. F. Freed, J. Luminescence, 1976, 12, 339.

109

Gas-phase Photoprocesses

oscillators, subsequently modified to include displaced oscillators with frequency changes in going from the S1to levels, used a generating function of vibronically excited states which either does not give a closed analytical expression, or relies for solution upon factorizing out the optically excited mode. This restricts comparison to relative rate constants rather than absolute determinations. Even in the latter case many vibronic overlap integrals have to be calculated using the saddle-point approximation several times for each single value of a rate constant, which is a very tedious procedure. Metz has now produced a treatment applicable to displaced and distorted harmonic oscillators which give a closed-form expression for the generating function, and this permits easy evaluation of rate constant within the saddle-point approximation. The treatment allows the evaluation of rate constants within the weakly distorted and strongly distorted oscillator approximations, and results show that adopting the former model results in calculated absolute rate constants being too small by more than six orders of magnitude, with the deuterium isotope effect too large by a factor of 25 compared with experiment. The strongly distorted model is thus more appropriate. If it is accepted that S1+-T1intersystem crossing is the dominant decay channel in the lower singlet vibronic levels of isolated benzene (perhaps a contentious assertion), then it must be concluded that there are some modes in benzene with strong distortions between S, and Tl states in the form of frequency changes, anharmonicities, or mode-mixing. It may be however that internal conversion from S1could provide an alternative non-radiative decay channel which has not yet received the attention that intersystem crossing has to date. This point is taken up in a recent paper by Formosinho and Da Sil~a,~O who apply a somewhat more empirical tunnelling theory to the benzene problem. In this, the rate of conversion, k, from the potential surface of one electronic state to another induced by a single-bond stretching vibration of frequency v and reduced mass p is given by equation (25), where EIp is the excess vibrational energy

of the initial electronic state, D is the energy above the initial state at which crossing of the potential surfaces occurs (or the energy of dissociation for transitions on the repulsive side of the potential surfaces when no crossing occurs), A, is the barrier width defined in Figure 1, and r] represents the relative number of hydrogen atoms in the molecule, given that the C-H stretching mode is the vibration of importance, or the relative number of carbon atoms for processes involving C-C bonds such as isomerization. For spin-forbidden processes (25) is premultiplied by an empirical parameter. Morse potentials for C-H and C-C stretching frequencies, and reasonable values of displacements and the parameter D were used in the calculation of values of rate constants for S, -+ So internal conversion and S1 intersystem crossing as a function of vibronic state excited, values being normalized to two known absolute values of these rate constants by appropriate choice of the spin-restriction factor and the degree of unimolecular vibrational redistribution assumed to occur prior to electronic relaxation. Although such calculations are clearly empirical in nature, ignoring so S . J. Formosinho and J. Dias da Silva, Mol. Photochem., 1975, 6, 409. --f

5

Photochemistry

110

-004

0

004

0 08

012

X,,(nm)

Figure 1 Potential energy curve of the CH vibrational stretching mode for So, T I , and S, in benzene and for Channel 3 (C,H, H), as a function of the normal co-ordinate

+

xCH

(Reproduced from Mol. Photochem., 1975, 6 , 409)

Gas-phase Pho topro cesses

111

for instance the role of promoting modes in the intersystem crossing process, the agreement of the calculated values with experiment is impressive over the five orders of magnitude change in total ICNR for Ev from 0 to 1000 cm-l (corresponding to a total excess energy in S1of 5000 cm-l). The treatment clearly demonstrates that the energy dependence of the rate constant for internal conversion is stronger than that for intersystem crossing, and similar agreement between experimental and computed values of rate constants was obtained for [2H6]benzene. The theory was also applied to the Channel I11 decay process in benzene, and the conclusion was reached that at high energies, predissociation through (26) and fulvene formation accounted for the new non-radiative decay

GH,

+ h~

____+

C6H5*

+ Ha

(26)

channel. The absence of reaction products at excess energies corresponding to the onset of Channel 111 was explained on the basis of there being a shallow potential minimum in the curve crossing the S1potential surface along the C-H stretch normal co-ordinate (see Figure 1). The exact nature of this state is not identified. Two recent studies 31 on the two-photon excitation spectrum of benzene vapour have shown that the pronounced structure in the region of the 14; transition envelope is not due to separate electronic transitions, but is due to resolved rotational structure which appears because of polarization effects on rotational selection rules for the two-photon process. Lombardi et al. reassigned the vle(el,) and v15(b2,) vibrational frequencies in the upper state to 1330 and 1143 cm-l in CsH6 and 1209 and 81 1 cm-l in C6DB,respectively. Intramolecular and collisional processes which change the nature of an emitting state will in general give rise to non-exponential fluorescence decay characteristics, and monitoring of such total fluorescence decay has been used to study, for example, exciplex formation and vibrational relaxation in benzenoid hydrocarbons. Such time-resolved methods would be immensely more powerful if simultaneous spectral and time resolution were possible so that fully timeresolved emission spectra were available. Very recently, an apparatus has been described with pulsed excitation at 257.25 nni (from a mode-locked, cavitydumped, frequency-doubled, argon-ion laser) which employs pulse durations as short as 500 ps, at repetition rates of up to 5 MHz with conventional singlephoton counting detection and a unique gating system for the recording of time (1 ns)- and wavelength (1 &-resolved spectra from benzenoid hydrocarbons.32a A preliminary report on the collisional relaxation of 3 Torr benzene excited in the 6O, It, 14: transition shows that time-resolved spectra obtained with the observation gate coincident with the pump pulse are very similar to the isolated molecule total fluorescence spectrum obtained under conditions of continuous illumination, and with the gate set some 60 ns after excitation, the fluorescence spectrum is identical with that of the Boltzmann distribution of upper levels.32a Intermediate spectra were recorded, which permit a three-dimensional surface for the 31

88

(a) L. Wunsch, H. J. Neusser, and E. W. Schlag, Chem. Phys. Letters, 1976, 38, 216; (b) J. R. Lombardi, D. M. Friedrich, and W. M. McClain, ibid., p. 213. (a) M. D. Swords and D. Phillips, Chem. Phys. Letters (in press); (b) G. R. Fleming, 0. L. J. Gijzeman, K. F. Freed, and S . H. Lin, J.C.S. Faraday ZZ, 1975,71,773; (c) R. P. Steer, P. M. Crosby, D. Phillips, M. D. Swords, and K. Salisbury, Chem. Phys. Letters (in press).

112

Photochemistry

spectral and time resolution of the fluorescence to be built up. This can then be simulated using appropriate models for the collisional vibrational relaxation This powerful technique has also been used to investigate isolated styrenes and substituted styrenes excited into the S2 state in the vapour phase.32c Conventional techniques showed that there was a dual exponential decay of the total fluorescence, compared with a single exponential decay for excitation into

Figure 2

(Reproduced from Clzem. Phys. Letters, 1976, 43, 461.) the S1level. Time-resolved spectra showed the presence of two different emitting states (see Figure 2) and it was clear that the short-lived component of the fluorescencewas emission from high vibrational levels of the S1state. The exact nature of the long-lived emitting state was not revealed, but the brief report indicates the potential uses of the technique for the study of isolated molecules. There have been some trivial corrections reported33a to an earlier paper concerned with the mechanism of electronic energy transfer in the vapour phase which included studies on benzenoid hydrocarbons; a theoretical treatment of energy-transfer processes has also appeared.33b A paper reporting the quenching of excited singlet states of substituted benzenes by heterocyclic molecules, alluded to in Volume 7, has now been published.34 Physical and chemical quenching processes in the photoexcitation of benzene-Me,SnH mixtures have been at 210 nm) has been at 265 nm, 2 x Weak fluorescence (@f = 4.4 x observed from 20 Torr chlorobenzene in the presence of 700 Torr propane.3* From a consideration of pressure and wavelength effects upon the values of @f it was concluded that a new non-radiative decay channel became of importance at wavelengths shorter than 240 nm. 33 34

s5

(a) G. L. Loper and E. K. C. Lee, J . Chem. Phys., 1975,63,5509; (b) S . H. Lin and H. Eyring, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 4205. M. E. Sime, D. Phillips, and Kh. Al-Ani, Mol. Photochem., 1976, 7,149. A. Delaby, D. Rondelez, and S. Boue, J . Photochem., 1975, 4, 399. T. Ichimura, T. Hikida, and Y. Mori, J. Chem. Phys., 1975, 63, 1445.

113 A stochastic model developed earlier for describing vibrational relaxation in excited states has been applied3' to the problem of the formation and decay of the triplet state of naphthalene vapour in which radiative and non-radiative decay are in competition with vibrational relaxation in both singlet and triplet manifolds. Qualitative agreement with experiment was found. A Green's function method of describing non-radiative decay of the S2state of naphthalene in the solid state has also been reported.38 A thorough study of the singlet vibronic level of fluorescence of naphthalene and [2H,]naphthalene, giving spectral and kinetic data, has appeared.39 It was shown that the only non-radiative decay process occurring from the S1state of isolated naphthalene was intersystem crossing, which could occur through any of five mechanisms. Careful quantum yield measurements on singlet vibronic levels of naphthalene gave values of relative rate constants for ISC to fluorescence. For excitation into S2 and S, levels, an electronic relaxation process other than intersystem crossing becomes of importance. A triplet lifetime (the longest yet reported) of 4.2 ms for naphthalene for the 0: band of the phosphorescence was obtained, and it was speculated that strong self-quenching may determine this value. Further high-resolution work has been carried out on the electronic relaxation of n a ~ h t h a l e n e .This ~ ~ will be discussed in detail in the next Volume. In larger polyatomic molecules, the possibility of intramolecular vibrational redistribution occurring prior to or in competition with electronic relaxation is usually considered, and a recent paper has considered this phenomenon using two models.41 It was concluded that in all the cases of redistribution thus far documented (in benzene, naphthalene, aniline, 3,4-benzpyrene, pyrazine, biacetyl, benzophone, quinoxaline, Rhodamine-B, and pyrene), the rate constant for the process is usually small in comparison to rate constants for electronic relaxation, being in the range 5 x lo7 to 5 x lo4s-l, even for excess energies as high as 5000-10000cm-1. In the case of pyrene, however, a recent study42has concluded that the two-component fluorescence decay observed upon excitation of the S1state is due to levels arising from fast vibrational redistribution, giving a randomly distributed population of levels in S1,similar to that produced by internal conversion following excitation to the S , and S, levels, in addition to the initially excited levels, and that the total lifetime is dictated by the redistribution process. However, the effects of sequence congestion cast doubts on such interpretations. Perhaps the use of time-resolved emission spectroscopic techniques may ultimately give the evidence required for redistribution, particularly if used with molecular beams in which the complicating effects of sequence congestion can be eliminated. A theory for time-resolved fluorescence spectroscopy based upon an earlier stochastic model for vibrational relaxation has also been applied to ~ y r e n e . ~ ~ ~ In a study of the excitation of the S2 level of pyrene at 170 "C (only 3900 cm-l Gas-phase Photoprocesses

38

40

41 42

K. H. Fung and K. F. Freed, Chem. Phys., 1976,14, 30. H.-K. Hong, Chem. Phys., 1975, 9, 1. M. Stockburger, H. Gattermann, and W. Klusmann, J . Chem. Phys., 1975, 63, 4519, 4529; H. Gattermann and M. Stockburger, ibid., p. 4541. W. E. Howard and E. W. Schlag, 2. Naturforsch., 1976, 31a, 399. C. Tric, Chem. Phys., 1976, 14, 189. C. J. Werkhoven, T. Deinum, J. Langelaar, R. P. H. Rettschnick, and J. D. W. van Woorst, Chem. Phys. Letters, 1975,32, 328.

Photochemistry

114

above Sl), it was concluded that the dual S2and S, fluorescences (cf. ref. 42) were due to S, levels which are actually in equilibrium with S, levels, since the lifetimes were almost the same (the differences could be accounted for by the effects of sequence congestion), and the efficiency of collisional quenching by 0,of the two fluorescences were identical, and almost of unit value. The strong intermediate coupling between S, and S, gives compound states with an anomalously long S2 lifetime. Vibrational relaxation was assumed to be of the stepladder type, with ca. 1000 cm-l excess energy being removed per collision with cyclohexane. Very similar results have been reported for 3,4-benzopyrene vapour in which AEs,-s, 2000cm-l, i.e. the rate of reversible internal conversion between S, and S2exceeds the rate of radiative decay of either, and 0, quenching is parallel in S, and S1manifolds.44 Triplet absorption in the vapours of complex organic compounds has been reported by the same It has been suggested that the observation of S, fluorescence should be general, rather than exceptional, in all molecules for which the S,-S, energy gap is relatively small.46 A method of observation of S, components (from solid samples) based upon polarization measurements was suggested. The quenching of fluorescence of /3-naphthylamine, anthracene, phenanthrene, and 3,6-diamino-N-methyl phthalimide vapours by absorption of a second photon followed by non-radiative decay of the higher state produced has been r e p ~ r t e d . ~ ' Most photophysical measurements on isolated polyatomic molecules are plagued by the fact that at ambient or elevated temperatures, no matter how narrow the spectral bandwidth of the excitation source, simultaneous excitations of the spectral feature under study and hot v-v sequence bands are invariably experienced. The decay characteristics of the pure state are thus, with very few exceptions, contaminated by the effects of the decay of states produced in these sequence transitions. Since these are hot bands, their removal requires low temperatures, which in a conventional gaseous system are impossible. In molecular beams, however, polyatomic species with very low vibrational and rotational temperatures may be produced, and results from the use of such techniques could clarify enormously the confused situation in many species. A brief report of just such an experiment has appeared in which an effusive beam of pentacene,4* colinear with a 300 ns pulsed flash-lamp-driven dye laser tuned to the 0-1 transition of the So --f S1system of the aromatic molecule, is excited at right angles by a probe laser (N, driven tunable dye) which produces transient fluorescence, viewed by a photomultiplier. By scanning the probe laser through the 600-665 nm region, a transient excitation spectrum is produced. This transient is ascribed to a hot ground-state pentacene molecule produced via internal conversion from S,. The experiment clearly demonstrates the feasibility of such studies for elucidation of electronic relaxation rates in complex polyatomic molecules, and further developments are eagerly awaited. (See also papers in later sections on I,/He and NO, using similar techniques.) N

48 44 45

K. Chihara and H. Baba, Bull. Chem. SOC.Japan, 1975, 48, 3093. V. A. Tolkachev and V. A. Tugbaev, Optika i Spektroskopiya, 1975,38,897. N . A. Borisevich, L. M. Bolotko, and V. A. Tolkachev, Doklady, Akad. Nauk S.S.S.R.,1975, 222, 1361.

4s

47 48

L. Margulies and A. Yogev, Chem. Phys. Letters, 1976,37,291. V. L. Bogdanov, V. P. Klochkov, and B. S. Neporent, Optika i Spektroskopiya, 1975,38,888. R. K. Sander, B. Soep, and R. N. Zare, J. Chem. Phys., 1976, 64, 1242.

Gas-phase Photoprocesses 115 The excess energy dependence of the fluorescence decay time and quantum yield of dilute vapours of tetracene and pentacene has been interpreted in terms of intramolecular vibrational redistribution occurring on a time-scale which is fast compared with electronic relaxation, provided the excess energy is For small energy gaps, however, studies on fluorene and p-naphthylamine indicate that vibrational redistribution will be slow in comparison with electronic relaxation For large energy gaps in the latter study it was shown that S1-+ Sointernal conversion was faster in a deuteriated molecule than in the protonated, contrary to what might be expected. Photodissociation of halogen-substituted toluene cations and benzyl halide cations proceeds as in reaction (27) for o-, m-,and p-fluorotoluenes and benzyl C,H,F*+

hv ____+

C7H,X*+

hv

>

+ H* C7H,+ + X* (X = C1, Br, or I) CTHBF+

(27) (28)

fluoride cations, and reaction (28) for o-, m-,and p-halogenotoluenes, benzyl chforide, and benzyl bromide cations.60 Evidence is presented that in the last two compounds the initially formed parent ion rearranges (to tropylium), but that this does not occur with the other ions. The photodissociation quantum yield for the benzoyl cation, protonated benzene, and protonated mesitylene has been shown to be close to unity in all cases.61 For benzene radical-cations there is evidence for the two-photon process (29),61 since dissociation occurs at wave-

lengths below the threshold. The intermediate may be the BElustate of the radical cation. The radiative lifetimes of excited benzyl, deuteriated benzyl, and methylsubstituted benzyl radicals (in EPA) have been measured as ,., s, showing that the first electronic transition in these cases is forbidden.62 However, an earlier report 22 gave the extinction coefficient for the benzyl radical at 253 nm as 6 x lo3I mol-1 s-l, which is large for a forbidden transition, but excitation here may not be to the fluorescent state. Emission spectra of the radical cations of hexa-, penta-, tetra-, and tri-fluorobenzenes produced on electron impact have been 4 Carbonyl and Oxygen-containing Compounds As before, CO, and CO are considered in the atmospheric section. An electronically excited reaction (30) has been reported.64 (CO+)* a (a) S. Okajima and E. C. Lim,

61

6a

64

+ co ____,

GO++ co

(30)

Chem. Phys. Letters, 1976,37,403; (b) C.4. Huang, J. C. Hsieh,

and E. C. Lim,ibid., p. 349. (a) R. C. Dunbar, Analyt. Chem., 1976, 48, 723; (b) E. W. Fu, P. P. Dymerski, and R. C. Dunbar, J. Amer. Chem. SOC.,1976,98, 337. (a) B. S. Freiser and J. L. Beauchamp, Chem. Phys. Letters, 1975,35, 35; (b) B. S. Freiser and J. L. Beauchamp, J. Amer. Chem. SOC.,1976,98, 3136. T. Okamura and I. Tanaka, J. Phys. Chem., 1975,79,2728. M. Allan and J. P. Maier, Chem. Phys. Letters, 1975, 34,442. M. T. Bowers, M. Chan, and P. R. Kemper, J. Chem. Phys., 1975,63, 3656.

116

Photochemistry

The vacuum U.V. spectrum of COSe has been and quantum yields for the production of Se(lS) from the photolysis of this compound in the 110200 nm region reported.66 The electronic transition is assigned as lX+ t lX+, and at 164-180 nm, (D(Se) is greater than 0.75. Major emission from this system is identified as &3Zu-) 3(3Eg-) fluorescence from reaction (31), for --f

-

Se(%) + OCSe Sea* + CO (31) which the total rate constant was measured as 1.6 k 0.2 x 10-lo cm3molecule-l s-l. Only 17% of the Se, was formed in the state, however. The C-Se bond dissociation energy was evaluated as 2.69 k 0.05 eV. Rate constants for quenching of Se(lS) by He, Ar, Kr, Xe, N,, H,, O,, CO, NO, Cl,, COSe, C 0 2 , N,O, CHI, C2H4, C2H2, NHs, and SF6 varied from 1.6 x 10-l6 cm3molecule-l s-l for the monatomic gases and N, up to the value quoted above for COSe itself. These results were compared with similar values for quenching of S(3lSO)and 0(2'S,,) (see Sections 6 and 11). Excitation of COS at 63.7-80.1 nm produces emission from the OCS+{x211(O,O,O) --f J?II(v1,0,v3)) No fluorescence from other than the zero-point level of COS+ was observed, indicating that higher levels predissociate, as do the B2C+ and states, as these are not seen in emission. When COS and CS, are excited at 68.6-123.9 nm, CS(#n f l Z + ) emission is observed, arising from reaction (32) 68 or (33). Although (32) is spin-forbidden, it nevertheless

c2E+

-

--f

cos + hv CS,

+ hv

cs(A111)+ o(3~)

(32)

CS(A"lII) + S(3P)

(33)

appears to be the major dissociation process for COS, no evidence being given for the spin-allowed O(l0) production. The quenching of the fluorescence of glyoxal and CS, by application of a magnetic field has been reported exten~ively.~~ Stern-Volmer self-quenching slopes change by a factor of two upon application of fields up to 15 kG, implying that either the bimolecular quenching rate constant or lifetime of the emitter is field dependent.sg Lifetime studies confirmed that the lifetime is a sensitive function of field, due to enhancement of non-radiative decay of the glyoxal or CS, by the magnetic field. The non-radiative process is intersystem crossing to the triplet manifold as confirmed by phosphorescence measurements on glyoxal. Lifetime values showed that the second-order rate constant for self-quenching was constant at 5.6 x 10-lo cm3molecule-l s-l and independent of applied field. Glyoxal is rapidly assuming the status alongside benzene of being one of the most important molecules for the detailed study of photophysical processes. Freed 60a has presented a theory for collision-induced intersystem crossing in b6

m b7 b8

eo

S. Cradock, R. J. Donovan, W. Duncan, and H. M. Gillespie, Chem. Phys. Letters, 1975, 31, 344. G. Black, R. L. Sharpless, and T. G. Slanger, J. Chem. Phys., 1976, 64, 3985, 3993. D. L. Judge and L. C. Lee, Internat. J. Mass Spectrometry Ion Phys., 1975, 17, 329. L. C. Lee and D. L. Judge, J . Chem. Phys., 1975,63, 2782. (a) A. Matsuzaki and S. Nagakura, J . Luminescence, 1976, 12, 787; (b) A. Matsuzaki and S. Nagakura, Bull. Chem. SOC.Japan, 1976, 49, 359; (c) A. Matsuzaki and S. Nagakura, Chem. Phys. Letters, 1976,37, 204. (a)K. F. Freed, Chem. Phys. Letters, 1976, 37, 47; (b) W. M. Gelbart and K. F. Freed, ibid., 1973, 18, 470.

117

Gas-phase Photoprocesses

glyoxal which is capable of explaining much of the earlier data of Lineberger et aL61 This may be summarized as (i) at zero pressure, S1+ TI ISC is absent; (ii) cross-sections for collision-induced ISC are large, and correlate well with those expected on the basis of complex formation, except for 02,CH,Cl, and CH,CN, and glyoxal itself; (iii) cross-sections increase linearly with excess energy; (iv) deuteriation does not change ISC rates from single vibronic levels in glyoxal ; and (v) cross-sections for rotational changes are extremely large, 150 A2. Freed's theory, based on an earlier note,6obconsiders that collisional broadening of vibronic levels is a possible mechanism for the conversion of small and intermediate case systems to the statistical limit. A further study on the fluorescence decay behaviour of glyoxal has confirmed the presence of two decay components.62 At low excitation energies (424.6 nm, 2 mTorr pressure) only a single exponential long-lived ( 900 ns) decay was observed, whereas at higher energies (e.g. 397.5 nm) a much faster exponential decay ( 10 ns) was observed in addition to the slow component. It was concluded that strong singlet-triplet vibronic mixing occurs in glyoxal only at higher excitation energies owing to the increasing triplet level density, and numerical values of parameters quantifying this coupling were obtained from fits to experimental data. The authors also point out the possibility of observing oscillatory decay behaviour in glyoxal at intermediate excitation energies. Fluorescence and phosphorescence excitation spectra at 600 mTorr pressure of glyoxal with and without 100 Torr He up to excess energies of 6000 cm-l in S,(lA") have been compared with values of quantum yield of CO production over the same wavelength region.63 The data suggest that the excited singlet state is the photochemically active one, but that the collision-induced ISC discussed above governs the photochemistry by competition. A brief note describing the application of time-resolved fluorescence spectroscopy (see also ref. 32) to glyoxal shows very clearly the different nature of the emitting states responsible for the slow and fast decays seen when total fluorescence is m ~ n i t o r e d .Thus ~ ~ when only those emitted photons coincident with the pump pulse are admitted to the detection system through a 50 ns wide gate, a highly structured fluorescence spectrum is observed. When the position of the gate is altered to much larger delays after the excitation process, e.g. 1000 ns, the fluorescence spectrum is radically different. Double-exponential decay in methylglyoxal vapour has also been observed, and this phenomenon has been further studied 66 Analysis of the data has been based upon the concept of a kinetic reversible intersystem crossing (Scheme l), whereas that of van der Werf et aLp2for glyoxal uses an alternative quantum mechanical coupling approach, although these may be taken to be eq~ivalent.~' In Scheme 1 initially prepared levels of the S,(lA") state of methylglyoxal may decay radiatively ( k f ) , internally convert (/&), or cross to triplet N

N

R. A. Beyer and W. C. Lineberger, J. Chem. Phys., 1975, 62,4024; R. A. Beyer, P. F. Zittel, and W. C. Lineberger, ibid., p. 4016. R. Van der Werf, E. Schutten, and J. Kommandeur, Chem. Phys., 1975, 11, 281. e3 D. Kumar and J. R. Huber, Chem. Phys. Letters, 1976, 38, 537. 8' E. Photos and G. H. Atkinson, Chem. Phys. Letters, 1975, 36, 34. 6s R. L. Opila, R. A. Covaleskie, and J. T. Yardley, J. Chem. Phys., 1975, 63, 593. 66 R. A. Covaleskie and J. T. Yardley, Chem. Phys., 1976, 13, 441 ; 1975, 9, 275. 67 F. Lahmani, A. Tramer, and C. Tric, J. Chem. Phys., 1974, 60,4431.

118

Photochemistry

levels (kist). The triplet levels may decay unimolecularly with rate constant kx, bimolecularly ( k p p ,where p is gas pressure), or reverse the ISC process (kr). For steady-state excitation at 435.8 nm, plots of total fluorescence intensity If/p (=Of)against p do not show normal Stern-Volmer behaviour. Analysis in terms of Scheme 1 reveals that equation (34) should hold, and thus plots of

If/p against l / p should yield straight lines. This behaviour was observed, giving a value for kJk, of 0.063 k 0.005 Torr-l, in satisfactory agreement with values from decay work of 0.048 Torr-1.66 The model seems to be valid, and the authors make the point that the behaviour of benzene at low pressures is very similarY6* implying that a similar model may be applied to the aromatic species. This is an important point and is worthy of further investigation. Amplification of the static study above to include decay measurements 66 has stressed the similarities of the behaviour of glyoxal, methylglyoxal, and biacetyl when analysed in terms of Scheme 1 , which shows the excitation and decay paths of methylglyoxal. The data are summarized in Figure 3, from which it can be seen that the rates of the fast decay process (as a function of excess energy) lie on the same line for all three molecules, and this can be interpreted as the S, +- Tl or Tz ISC process (ki). The slow decay rate can be decomposed into two contributions k,(= kIc + kf) and k,( = kx k p p ) . For low-energy excitation the decay of glyoxal is dominated by k,. Above 3000 cm-l the decay of biacetyl and glyoxal is dictated by kx but in methylglyoxal k, is still the dominant contribution. The processes on this analysis are thus all similar, with behaviour being determined by density of states functions. It should be noted that Van der Werf et aLB2have concluded that although phosphorescence is collision-induced in both glyoxal and biacetyl, the mechanism of phosphorescence induction in these molecules cannot be the same. In biacetyl, because of the high density of triplet levels the molecular eigenstates are almost pure triplet, and collisions transfer the molecule to a lowlying and thus long-lived triplet state, of high phosphorescent yield. In glyoxal the situation must be different since the triplet level density is low, and in this case collisional broadening of the zero order levels,60clfollowed by vibrational relaxation in the triplet manifold, may provide the ISC mechanism. Other simple aldehydes have been studied. The U.V. absorption spectrum of 2-furaldehyde has been shown to have two bands between 26 000 and 34 000 and 36 000 and 41 000 cm-l, respectively due to x l A ’ +-JIA’(nnS) and x l A ’ + BIA’(nn*)transitions.60 Studies on a series of ‘isolated’ linear aldehydes give

+

A. E. Douglas and C. W. Mathews, J. Chem. Phys., 1968,48, 4788. R. Zwarich and I. Rabinowitz, J. Chem. Phys., 1975, 63, 4565.

119

Gas-phase Photoprocesses

O

4

I

(a 1

,

I

I

1

I

I

1I

I

V I

2 0

0

"'L I 0 0

0

-

0

0

0

'I0

ooo

loo0

2000

(Ib )

i

1

3000

4000

5000

Excess Energy in Sl (crn-ll

Figure 3 (a) Collated data for decay rate constants for glyoxal, biacetyl, and methyZand 0 :glyoxal (diferent glyoxal as a function of excess vibrational energy. 0 , authors); and 0:methylglyoxal; A , V , V,and Q : biacetyl (different authors), (b) Semilog plots of kilk, (see Scheme 1); 0 : glyoxal; 0 : methylglyoxal; V and V: biacetyl (diferent authors) (Reproduced by permission from Chem. Phys., 1976,13, 441)

a,

Table 1 Rate constants for decay of isolated Compound

kr ( l a s-l)

HzCO Acetone CH3CH0 propanal n-butanal n-pentenal n-hexenal 5-hexenal

5 + 1 8i-2 5 + 1 1.7 f 0.4 1.7 4 0.4 1.2 f 0.3 1.8 f 0.4 1.8 5 0.4

km (1Oe s-l) 0.13 k 0.01 3.8 & 0.5 3.6 k 0.4 2.2 rf: 0.3 2.0 k 0.3 2.7 k 0.4 4.2 2 1.0 6_+2

states of linear aldehydes kII (108 s-') kist (1Oe 9 - 3 ? 3.8 5 0.5 2.8 2 0.4 < 2.2 < 2.0 2.1 < 2.6 ?

-

-

-0

0.6 1.6

3.5

120

Photochemistry

results summarized in part in Table 1.70 Notable features are: (i) the radiative lifetime T~ is within a factor of two of that calculated using the Strickler-Berg formula for the C3, Cp,C5, and c6 aldehydes, but T(SB) > 7, by a factor of six for acetaldehyde; (ii) large asymmetric aldehydes behave differently from symmetric methyl-substituted acetones and formaldehyde, presumably because of differing equilibrium geometries in the Inn* state, the larger aldehydes being more planar; (iii) The magnitude of kist of 2-3 x lo8 s-l for asymmetric aldehydes is comparable to that for ketones; (iv) formaldehyde behaves uniquely in that kr and k , are different from other members; (v) acetaldehyde is very similar to acetone; (vi) the total non-radiative decay in asymmetric aldehydes is made up of contributions from Sl Tl intersystem crossing, and Type I1 photochemistry. The latter process becomes of increasing importance at higher excitation energies in aldehydes with a y-hydrogen atom. Some theoretical considerations of lower excited states of tram-polyene carbaldehydes, including acrolein, have been recorded,71 and autoxidation of acetaldehyde (in the liquid phase) has been extensively reported.72 The photosensitization of polymerization on a surface of penta-1,3-dienes in the gas phase using benzaldehyde as sensitizer is a recent example of heterogeneous photo~atalysis.'~ The photochemistry of acetone has been studied for the past fifty years or so, and it is a great pleasure to note that one of the pioneers in this field is still producing new results on the system.'* The main conclusion in this recent work is that the recombination of acetyl radicals (35) is a wall reaction. -f

CH3C0

+ CH3C0

-

CH3C-CCH3

II

II

(35)

0 0

The interactions of the triplet states of acetone, pentan-3-one, and biacetyl with dimethyl-, trimethyl-, diethyl-, and triethyl-amines have been compared in the vapour phase and in It was found that the reactivity is less in the vapour phase and largely involves radical abstraction rather than charge transfer. Indications of the latter process in solution are provided by linear plots of log k~ (the Stern-Volmer quenching parameter) against ionization potential of amine, but these are obtained only if steric effects are also taken into account. Photocleavage of triplet butan-2-0ne,~~ photochemical processes involving the Inn* state of aliphatic ketones with y-hydrogen and the quenching of Norrish Type I1 reactions of butyrophenone by thiophen compounds 78 have been studied. 70 71 72

73

74

75 76 77

78

D. A. Hansen and E. K. C. Lee, J. Chem. Phys., 1975,63, 3272. I. KOZO,Bull. Chem. SOC.Japan., 1975, 48, 779. N. A. Clinton, R. A. Kenley, and T. G . Taylor, J. Amer. Chem. SOC.,1975, 97, 3746, 3752, 3757. G. R. De Mark, J. R. Fox, M. Termonia, and B. Tshibangila, European Polymer J., 1976, 12, 119. S. Y.Ho, R. A. Gorse, and W. A. Noyes, jun., J. Phys. Chem., 1975, 79, 1632. E. A. Abuin, M. V. Encina, E. A. Lissi, and J. C. Scaiano, J.C.S. Farudoy I, 1975, 71, 1221. E. A. Abuin and E. A. Lissi, J. Photochem., 1976, 5, 65. M. V. Encina and E. A. Lissi, J. Photochem., 1975, 4, 321. V. Avila, S. E. Braslavsky, and J. C. Scaiano, J . Photochem., 1975, 4, 375.

Gas-phase Pho toprocesses

121

The electronic absorption spectra of many alkyl peroxyl radicals produced by the mercury-photosensitized reactions of ketones, azo-compounds, biacetyl, and hydrocarbons in the presence of oxygen have been re~orded.'~ A recent study on the photophysics of fluorinated acetones in the vapour phase has suggested that in addition to the role of geometry changes between inn* state and ground state, changes in the frequency of out-of-plane bending vibration may be of importance in determining the magnitude of kr, the radiative rate constant for singlet state decay,8owhich was found to have a greater than linear dependence on the frequency of this promoting vibration for the radiative transition. Of perhaps more interest, it was shown that the 'excess' rate of nonradiative decay over that for the zero-point level of the inn* state in l,l,l-trifluoroacetone correlated very well with the simple calculations of Gillespie and Lim81 of the rate for S, + So internal conversion, indicating that at excess energies greater than 4000cm-l, the internal conversion may be the dominant non-radiative decay path. A similar conclusion has been reached for the case of hexafluoroacetone in a study in which quantum yields of decomposition were measured as a function of pressure at 24 "C and X = 313 nm.82 Best fits to the data were obtained with a model in which internal conversion to So from the ' Inn* (S,) state increases in importance with respect to intersystem crossing to 7 as the excitation energy is increased. A brief study on the phosphorescence decay of l,l,l-trifluoroacetone vapour has yielded results at variance with earlier Possible reasons, including trace impurity quenching, are discussed. Reactions of trifluoromethyl and methyl radicals generated by ketone photolysis are discussed on p. 123. Cyclopropanone vapour has been shown to have unit quantum yield of photodecomposition from 291 to 365 nm, with a fluorescence yield at 365 nm of 6.8 x and quantum yield of ethylene formation of 0.60 at this wavelength rising to unity at 291 nm.84 The suggested mechanism is shown in reactions (3 6)-(4 1).

lD-0

-

.yo (predissociation)

(37)

Internal conversion from the S, state of other cyclic ketones, including cyclobutanone, oxetan-3-one, and perfluorocyclobutanone, is followed by decomposition of the hot So molecules, and RRKM calculations have been carried 79

8a

B3 8p

H. E. Hunziker and H. R. Wendt, J. Chem. Phys., 1976,64,3488. J. Metcalfe and D. Phillips, J.C.S. Faraduy 11, 1976, 72, 1574. G. D. Gillespie and E. C. Lim, Chem. Phys. Letters, 1975, 34, 513. D. A. Knecht, Chem. Phys. Letters, 1975, 33, 325. S. W. Beavan, D. Phillips, and R. G. Brown, Chem. Phys. Letters, 1975, 36, 542. H. J. Rodriguez, J.-C.Chang, and T. M. Thomas, J. Amer. Chem. SOC.,1976,98,2027.

122

Photochemistry

.Yo+ c__+

[pol'

C2H4

[FO]'

+

CO

(39)

(internal conversion)

polymer

out on this thermal decomposition process.8s It was found that for a given excess of energy the perfluoro-compound reacts slower than cyclobutanone, whereas the oxetanone reacts faster. For the butanones, a step-ladder deactivation process is appropriate, and fits show that 3-6 kcal mol-l energy is removed per collision with propylene. The cyclobutanones and cyclopentanones have also been studied in matrices at 20 K, and the fact that strong phosphorescence is observed from cyclopentanone and perfluorocyclobutanone whereas none is seen in cyclobutanone itself argues that predissociation in the last ketone at high-lying levels is very rapid.8g The photochemistry of the anhydrides (R,CO)aO (R = CF, or C2F,) is simple in that the quantum yield of CO production (ca. 0.25) is independent of the presence of mercury and of pressure, and changes little over the wavelength region 190-250 nm.87 Thus the compounds have potential as vapour-phase primary actinometers at 254 nm, particularly since the molar decadic extinction coefficients are relatively high at these wavelengths, being 59.2 and 79.5 for the CF, and C,F, compounds respectively. Weak emission with total quantum yield 2 x and 2 x for CF3- and C,F,-substituted compounds respectively was seen, and in the absence of any sensitized emission from biacetyl or cis-trans isomerization of but-2-eneYa simple mechanism was proposed to account for the observations, neglecting the inefficient emission process. Associated with photoexcitation of ketonic species is the recent observation of chemiluminescence from simple ketones arising from thermal decomposition of dioxetan molecules. A review has been published,88and it has been shown that even the simplest dioxetan can give rise to chemiluniinescence (from excited formaldehyde in this case), although the dioxetan itself cannot be Reactions in Scheme 2 account for the observed emission. Thermal decomposition of other oxetans,BOn 91 including 3-ethyl-3-methyl- and 3,3-diethyl-oxetanesYg1 has been described. 88

s1

G. M. Breuer, R. S. Lewis, and E. K. C. Lee, J. Phys. Chem., 1975,79, 1985. L. T. Molina and E. K. C. Lee, J. Phys. Chem., 1976,80,244. G . A. Chamberlain and E. Whittle, J.C.S. Furuduy Z, 1975,71, 1978. C. S. Foote and T. R. Darling, Pure Appl. Chem., 1975,41,495. D. J. Bogan, R. S. Sheinson, and F. W. Williams, J . Amer. Chem. SOC.,1976, 98, 1034. (a) K. A. Holbrook and R. A. Scott, J.C.S. Furuduy I, 1975, 71, 1849; (b) N. J. Turro and W. H. Waddell, Tetrahedron Letters, 1975, 2069. A. D. Clements, H. M. Frey, and J. G. Frey, J.C.S. Faraday Z, 1975,71,2485.

123

Gas-phase Photoprocesses H&O*

+ H2C0 ----+H2C0

-I-hv

Scheme 2

Photoionization of vapours of acetic acid and alkyl-acetates has been described,g2 and the method of threshold photodetachment of electrons from phenoxides has been used to obtain the following values for electron affiniEA(o-CH3C6H40*)21 EA(C6HsO*)< 2.36 eV; EA(o-ClC,H,O*) d 2.58 0.08 eV; EA(C,H,S-) < 2.47 rt 0.06 eV. Free Radical Reactions.-Since studies on free radical reactions frequently use photochemical excitation of ketonic and related species as a means of production, some are considered below. Calculations on the excited state of ketenyo4 the 3A2and 3B2states of m e t h ~ l e n e , ~ ~ and the CH2(lA1) H2g6 and 3CH2 H2 and CH, O7 reactions have been performed. In the last study it was shown that for the reaction with methane there is a barrier height of some 7 kcal mol-l, consistent with BEBO calculationsYg* but not with earlier MIND0 calculations.gg A barrier of this height would exclude the reaction at room temperature. Excitation of keten-cis-but-Zene mixtures in the wavelength region 313-250 nm (lA" +- lAl transition in keten) and at 214 nm (lB2+ lAl) gives results which are consistent with the Z3B1 and @Al states of methylene being produced in constant ratio over the long-wavelength range, a perhaps surprising result.loO At 214nm, some CH2(b1B1) was also produced (as observed in the photolysis of methane at 123.6 and 104.8 nm7) and this contributes to formation of non-stereospecific addition products. The percentage of CH2(3B1)formed at 313 nm has been estimated at 30% from studies on keten-propane and keten-neopentane rnixtures,lo1of which some 85% is formed directly in the keten photodissociation, and the remainder by collisional deactivation of CH2(lA1). The decomposition of keten from the lA" state is thought to proceed via internal conversion to a hot ground state. Other studies lo2 suggest that at wavelengths longer than 313 nm, the yield of CH2(lA1)relative to that of the ground-state methylene is drastically reduced, measured ratios having the values [1.0] (313 nm), 0.16 (334 nm), and <0.0006 (366 nm). If correct, this implies that previous estimates of lCH2 production at 366 nm are in error, and would indicate that internal conversion of keten at long wavelengths of excitation becomes inefficient, perhaps intersystem crossing being favoured. Such notions are in line with current thinking about simple aliphatic ketones.

+

+

-

99

Y.Y. Villem and M. E. Akopyan, Zhur.fiz. Khim., 1976,50,674.

J. H. Richardson, L. M. Stephenson, and J. I. Brauman, J. Amer. Chem. Soc., 1975,97,2967. C. E. Dykstra and H. F. Schaefer, tert., J. Amer. Chem. SOC.,1976, 98, 2689. 96 J. F. Harrison and D. A. Wernette, J. Chem. Phys., 1975, 62, 2918. 96 C. W. Bauschlicher, jun., H. F. Schaefer tert., and C. F. Bender, J. Amer. Cfiem.SOC., 1976,98, 1653. 07 C. W. Bauschlicher, jun., C. F. Bender, and €1. F. Schaefer, tert., J . Amer. Chem Soc., 1976, 98, 3072. R. W. Carr, J. Phys. Chem., 1972,76, 1581. 99 N. Bodor, M. J. S. Dewar, and J. Wasson, J. Amer. Chem. SOC.,1972, 94, 9095. l o o V. P. Zabransky and R. W. Carr, jun., J. Amer. Chem. SOC.,1976,98, 1130. lol V. P. Zabransky and R. W. Carr, jun., J. Phys. Chem., 1975,79, 1618. l o = P. M.Kelley and W. L. Hase, Chem. Phys. Letters, 1975,35, 57. 93

94

124 Photochemistry The photolysis of keten at 325 nm has been used as a source of lCH2 for reaction with methylenecycl~propane.~~~ A complex mixture of products resulted, including ethylene, allene, methylenecyclobutane, methyl methylenecyclopropane, ethylidenecyclopropane, isoprene, and spiropentane, and RRKM theory was applied to the reactions of hot spiropentane. Results were most satisfactory for multistep deactivation of this species with 7.20 kcal mol-l removed per collision. Some interesting reactions of CF, and CF with NO have been studied using mass spectrometry and laser-induced emission.lo4 The species were produced by the flash photolysis of CF,Br, and CFBr,, and results challenge earlier interpretations of such systems. Reactions (42)-(51) are important. CF, emission from the O3 + C2F4reaction was mentioned earlier.2e CF2 CF2

+ NO

+ CF2 + M N + CF, N+NO

0

+ CF,

C F + NO

____+

+N +M FCN + F

(44)

NS+O

(45)

CF2O

(42)

GFk

(43)

+ 2F

(46)

FCN+O

(47)

+N

(48)

CO

FCO

O+CFN+CFFCO+M

CO+F

(49)

CN+F

(50)

CO+F+M

(51)

Two recent measurements of the rate constant for reaction (52)are in disagreement by a factor of two, the differences arising probably from the assumed values for the rate constant for the competing reaction (53). Both groups

+ CH,. CH3. + CH3. %H2 + 3CH2 3CH2

___+

C,H,*

(52)

C2H,

(53)

GH4

(54)

assumed k54 to have a value of 5.3 x 10-l' cms molecule-l s-l, but Pilling and Robertson lotitake k68 to be 4.2 x 10l1cms molecule-l s-l, giving a value for k52 whereas Laufer and Bass's lo6value of k,, of 9.5 x 10-l1cm3 of 5.0 x molecule-l s-l results in k62being measured as 1 .O k 0.1 x 10-locm3molecule-l S-l.

CD3 radicals produced by the flash photolysis of Hg(CD3)2were monitored using the B2A{ +- z 2 A / transition for which the oscillator strength was measured as 0.99 x 10-2.107The absolute rate constant for the recombination of CD3 ln3 ln4

ln6 lo6 lo'

H. M. Frey, G. E. Jackson, R. A. Smith, and R. Walsh, J.C.S. Furaday I, 1975,71, 1991. T. L. Burks and M. C. Lin, J. Chem. Phys., 1976, 64, 4235. M. J. Pilling and J. A. Robertson, Chem. Phys. Letters, 1975, 33, 336. A. H. Laufer and A. M. Bass, J. Phys. Chem., 1975,79, 1635. A. B. Callear and P. M. Metcalfe, Chem. Phys., 1976, 14, 275.

125

Gas-phase Photoprocesses

Table 2 Radical reaction rate constants at 25 "C

CD3 cZHK

+ CD, + cZH5

But* + But* But* But* But* + But*

+

a

-

Measured rate constant

cm3molecule-l s-1 4.9 k 0.4

Reaction GH6

____+

0.75 a 0.2 & 0.03 0.56 2 0.08 0.13 & 0.02b

C4H10

dimer disproportionation dimer

At 860 K, low pressure limit.

Ref. 107 108 109 109 110

Average value over temperature range 623-692

K.

radicals is given in Table 2, as are other rate constants for radical combinations.lo8-llo Absolute rate constants for the reaction of CH3 with NO and Oz have been given as 3.2 x cm, molecule-l s-l, respectively.111 and 1.7 x The reaction of l4CCH, with CF3 produced on photolysis of [l,3-14Cz]acetonehexafluoroacetone mixtures produces hot labelled 1,l,l-trifluoroethane, which may eliminate H F (55) or be stabilized collisionally (56).l12 Complicating reactions 14CH3CF3t 14CH3CF,t

+M

-

CH2=CF,

+ HF

14CH, CF3 + M

(55) (56)

include those between CF, and the olefin product of (55). The absolute rate constant for reaction (57) has been determined as log kK7= (7.95 - 2900/RT) ca1rn0l-l.l~~Similar reactions of, for example, CHFz radicals with olefins have *CF, + CH2=CH2 CF,

_I_+

+ NO

CF3CH2CH2*

(57)

CF,NO

(58)

been reported.l14 A velocity analysis of the CH, + Izand CH, + ICI reactions and the reaction of CH3 and CF3 with H2 and Dz116 have been studied from a theoretical viewpoint, and the rate constant for reaction (58) has been measured.l17 A flash photolysis study of the C2ClK O2 reaction118 and the addition of methoxyl radicals to olefins ll9 have been reported. Reactions of methoxyl radicals of atmospheric importance lzo# lZ1are discussed in Section 11. The electronic 70 and i.r.lZ2absorption spectra of organic peroxyl radicals in the vapour

+

lo8 log

K. Y. Choo, M. J. Perona, and L. W. Piszkiewicz, Internat. J. Chem. Kinetics, 1976, 8, 381. D. A. Parkes and C. P. Quinn, Chem. Phys. Letters, 1975, 33, 483. K. Y. Choo, P. C. Beadle, L. W. Piszkiewicz, and D. M. Golden, Internat. J. Chem. Kinetics, 1976, 8, 45.

A. H.Laufer and A. M. Bass, Internat. J. Chem. Kinetics, 1975,7, 639. R. R. Pettijohn, G. W. Mutch, and J. W. Root, J. Phys. Chem., 1975, 79, 2077. J. M. Tedder and J. C. Walton, Accounts Chem. Res., 1976,9, 183; H. C. Low,J. M. Tedder, and J. C. Walton, J.C.S. Faraday I, 1976, 72, 1300. 114 J. P. Sloan, J. M. Tedder, and J. C. Walton, J.C.S.Perkin ZI, 1975, 1841,1846; D. E. Copp and J. M. Tedder J.C.S. Faraday I, 1976,72, 1177. lls L. C. Brown, J. C. Whithead, and R. Grice, Mol. Phys., 1976,31, 1069. N. L. Arthur, K. F. Donchi, and J. A. McDonnell, J.C.S. Faraday I, 1975,71,2431,2442. 117 J. C. Amphlett and L. J. Macauley, Canad. J. Chem., 1976, 54, 1234. 11* Yu. A. Kirushin and V. A. Poluektov, Kinetika i Kataliz, 1975, 16, 972. 11* E. A. Lissi, G. Massiff, and A. Villa, Internat. J. Chem. Kinetics, 1975,7, 625. 120 W. A. Glasson, Environmental Sci. Technol., 1975, 9, 1048. lal J Weaver, J. Meagher, R. Shortridge, and J. Heicklen, J. Photochem., 1975, 4, 341. laa D. A. Parkes and R. J. Donovan, Chem. Phys. Letters, 1975, 36, 211. 111 lla

-

126

Photochemistry

R102*+ R2O2. products (59) phase have been recorded. The rate constant for reaction (59) has been measured as k,, = 2.4 x lo81 mol-1 s-l (R1= R2 = CH, or tva1zs-2,3-dimethyl-but-3-y1), and 6.2 x lo81 mol-1 s-l (R1= i-C,H,, R2 = CH3).123 5 Nitrogen-containing Compounds

As previously, atomic and molecular nitrogen and oxides of nitrogen are covered in Section 11. Luminescence from the CN(2Z+)radical produced in the vacuum U.V. photolysis of methylaminoacetonitrile and acetonitrile has been and the electronic transition moment of the red system of CN has been reported.126 The main reaction in flames at 1500 K between NH and NO is represented by (6O).lz8 The formation of NH(blZ+) in the photodissociation of NH, in the NO+NH

-

N2+O+H

(60)

vacuum U.V. has been r e p ~ r t e d . l ~ ' - lIn ~ ~the study by Zetsch and Stuhl, emission arising from the 6lZ+ + Z3Z- transition at 470.71 nm was monitored, and the rate constants for reactions (61) and (62) were measured as 4.1 x 10-13 and

+ NH, NH(&lX+)+ Ar

NH(&lC+)

____+

products

(61)

products

(62)

3.6 x cm3molecule-l s-l, respectively. Possible products in (61) are two NH2 radicals; physical quenching is also possible. The radiative lifetime of NH(blC+) was measured as 3 5 ms. This compares with ca. 17.8 ms for the u' = 0 level of the same species measured by Gelernt et aZ.lz9 Photolysis of NH, can also produce NH(a"lA) 130 and in the presence of fluoromethanes, this inserts into CH bonds, ultimately eliminating HF. Photolysis of NH, can produce NH, in the ground (2B,) or excited J 2 A 1 states. The fluorescence spectrum from the latter has been analysed and the radiative lifetime for various rotational levels of the 0,10,0 vibrational level of this state given as 350 ns.131 The measured lifetime of the 0,9,0state was 10 p, stated to be close to the radiative lifetime,132but not in agreement with the value given above for the 0,10,0 state. The rate constant for reaction (63) was given in N

+ NH,

-

products (63) this study as 1 .O x lo9cm3molecule-l s-l, which seems abnormally large. Relative deactivation rates for the 0,9,0level of NH2(2A1)by NH,, CO, H2,N2,CH4, Ar, and He were 1.0,0.47,0.46,0.40,0.30, 0.152, and 0.145, respecti~e1y.l~~ In a similar study, in which NH2(2A1)was produced by pulsed photolysis of ammonia at 170 nm (64), followed by laser excitation (65), the rate of disappearance of NH,(z2A,)

W. G. Alcock and B. Mile, Combustion and Flame, 1975, 24, 125. 1. P. Vinogradov and F. I. Vilesov, Optika i Spekroskopiya, 1976, 40, 31 1. l Z b D. C. Jain, J. Quant. Spectrosc. Radiative Transfer, 1975, 15, 571. 12e J. N. Mulvihill and L. F. Phillips, Chem. Phys. Letters, 1975, 35, 327. lZ7 C. Vermeil, J. Masanet, and A. Gilles, Internat. J. Radiation Phys. Chem., 1975, 7 , 275. lZ8 C. Zetsch and F. Stuhl, Chem. Phys. Letters, 1975,33, 375. laa B. Gelernt, S. V. Filseth, and T. Carrington, Chem. Phys. Letters, 1975,36,238. 130 P. R. Poole and G . C. Pimentel, J. Chem. Phys., 1975, 63, 1950. lS1 M. -011, J. Chern. Phys., 1975, 63, 319, 1803. lsa J. B. Halpern, G. Hancock, M. Lenzi, and K. H. Welge, J. Chem. Phys., 1975, 63, 4808. la3

Gas-phase Photoprocesses

+ hv

NH,

+~

NHa('91)

-

V L

127 NH2(aB,)

+ H*

NHZ('A1)

(64) (65)

NH2(,B1) + NO N, + H,O (66) ground-state NH, could be monitored, and a rate constant for (66) of 2.1 f 0.2 x 10-l1 cm3molecule-l s-l A similar monitoring technique has been used to observe N H , produced by the dissociation of NH, by an i.r. CO, laser. The amount of NH2(,A1) produced by this process was orders of magnitude less than that of the ground-state NH2(2B1).134Some details of the reactions of NH2(,Bl) with N O and 0,have been e1u~idated.l~~ In this system, NH,-NO or NH3-O2 mixtures were irradiated at 231.9 nm. For the former mixture, quantum yields of product N,, Ha, and N20 formation were 1.05 f 0.005, 0.33 k 0.03, and 0.09 f 0.02, respectively, for NH, pressure of 5-7 Torr, and somewhat smaller for a pressure of 11 Torr. The reaction mechanism given was (64), (66), H+NO+M H+HNO 2HN0

-

HNO+M

(67)

H,+NO

(68)

NpO

+ H,O

(69)

plus (67)-(69). The rate constant ratio (k&69)'/kss was measured as 1.6 2 0.6 x lo-,* cm9I2S-4. For NH3-O2 mixtures 0~~ was 0.23, independent of 0, pressure, with ON,Obetween 0.06 and 0.09 depending on 0,pressure. The main reactions were (70)-(74).lss NH2(,B1)

+ 0,+ M 2NH,O,

NH,O,

+ NHaO 2NH,O

NH,O,

+ HO,

____+

___I_,

+M 2NH,O + 0, N,O + 2H,O N, + 2H,O NH, + 20, NH,O,

(70) (71) (72)

(73)

(74)

The photoxidation of ammonia in the presence of N O and N02,13* the NH,-O,-NO reaction mechanism,13' HO, formation in shock-heated HN0,-NO, mixtures,138and the thermal decomposition of H N O lS9have all been reported. Photoacoustic detection spectroscopy and two-photon spectroscopy on the v, bands2* of ammonia were mentioned earlier. The rate constant of the ionic reaction (75) has been measured as 8.79 f 0.13 x 10-lo cms molecule-l s-l at N , H + + CO

-

HCO++

N2

(75)

297 K.140 This confirms earlier speculations concerning origins of N2H+ in interstellar space. G . Hancock, W. Lange, M. Lenzi, and K. H. Welge, Chem. Phys. Letters, 1975,33, 168. J. D. Campbell, G . Hancock, J. B. Halpern, and K. H. Welge, Optics Comm., 1976,17,38. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Phys. Chem., 1976, 80, 433. lZ6 R. A. Cox, R. G. Dement, and P. M. Holt, Chemospltere, 1975, 4, 201. lS7 R. K. Lyon, Internat. J . Chem. Kinetics, 1976, 8, 315. lS8 K. Glanzel and J. Troe, Ber. Bunsengesellschaft Phys. Chem., 1975, 79, 465. lsa A. B. Callear and R. W. e r r , J.C.S. Furaday IZ, 1975,71, 1603. l P 0 E. Herbst, D. K. Bohme, J. D. Payzant, and H. I. Schiff, Astrophys. J., 1975,201, 603.

lS3

lS4 lS6

128

Photochemistry

A study on the photochemical dissociation of nitrosyl chloride has been reported,141and a suggestion made that the species N,S is responsible for laser action in the chemical laser N2-CS, 143 Evidence has been given for predissociation in the c2Z+state of NS in that the intensities of the bands corresponding to u’ = 1, v’ = 2 relative to u’ = 0 are very weak.144 NSF is a molecule which resembles SO, in that it is a bent ground-state triatomic with 18 valence electrons, but a recent study shows that the excitedstate properties are very different.lq5 Whereas the first electronic state of SO, in the 26@-340 nm region is very strongly perturbed by ground-state levels, giving rise to dual fluorescence, NSF appears to be free of such effects. For low-lying vibronic levels the measured single-exponential fluorescence decay time is of the order of 10 ps, and this only shortens dramatically at an excess energy of some 4500 cm-l, when a unimolecular non-radiative decay process, probably photodissociation, becomes of importance. Photolysis of azo-compounds is often used as a convenient source of free radicals (see, for example, refs. 109, 121-123), but work is still reported concerning the primary photolysis mechanisms of such compounds. Thus photolysis at 366 nm of azoethane at very high pressures of added helium (60-140 atm) can result in quenching of the excited state, and the study concludes that RRKM assumptions are valid for this The electronic absorption spectra of diazo-n-propane and diazo-n-butane have two bands, that at 22 000 cm-l being an l A N + lA’ transition, and that at 44000cm-l being l A ’ t lA’.14‘ In the was unity, independent of wavelength and pressure, and any visible region adecornp since it was not detected. It luminescence must have a quantum yield of < has been estimated that in the photolysis of diazobutane in the first band, 60% of the available energy enters the initial product n-propylcarbene as internal energy.148 The photo-oxidation of [2H,]azomethane has been studied at 25 0C.149 Results are reported in Section 11. The number of very strongly luminescent polyatomic molecules is relatively small but, perhaps surprisingly, saturated amines are in this class. New results on the luminescence of the saturated amines NN’-dimethylpiperazine (l), NNN’N’-tetramethylethylenediamine (2), NNN’N’-tetramethylpro pane- 1,3diamine (3), and trimethylamine (4) have been A summary of the more interesting data is given in Table 3, from which it can be seen that the fluorescence maxima of (l), (2), and (3) are red-shifted with respect to (4). Compound (3) has two emission bands, that at 290 nm being ascribed to monomer emission, that at 365 nm to excimer emission. For compound (2), the results were explicable on the basis of Scheme 3, and rate constant values relating to this are given in Table 4. The last process, collision-induced radiative decay, is 141 142 143

14*

14b 146

lQ7 148 149

150

G. A. Isayan and Zh. M. Gasparyan, Kinetika i Kataliz, 1976, 17, 268. F. X. Powell, Chem. Phys. Letters, 1975, 33, 393. T. J. McGee and F. X. Powell, I.E.E.E. J. Quantum Electronics 1975,11,302. C.-L. Chiu and S. J. Silvers, J. Chem. Phys., 1975, 63, 1095. J. R. McDonald, Chem. Phys., 1976, 13, 339. S. Chervinsky and I. Oref, J . Phys. Chem., 1975, 79, 1050. M. J. Avila, J. M. Figuera, V. Menendez, and J. M. Perez, J.C.S. Faraday I, 1976,72,422. J. M. Figuera, J. M. Perez, and A. P. Wolf, J.C.S. Faraday I, 1975,71, 1905. J. Weaver, R. Shortridge, J. Meagher, and J. Heicklen, J. Photochem., 1975, 4, 109. A. M. Halpern and P. P. Chan, J . Amer. Chem. SOC.,1975,97,2971.

129

Gas-phase Photoprocesses

Table 3 Absorption and emission characteristics of saturated amines Compound (1) (2) (3) (4) a

Absorption Am,/nm emX/l mol-l cm-l 205 6800 195 7540 6470 202 198 3460

X,,/nm 313 304 365 287

Fluorescence rf/ns @f 770 71 42 45

0.23 0.23 0.11 1.0

k,/s-l 3.2 x lo5 3.2 x lo6 3.2 x lo5 2.1 x 107

Extrapolated to zero pressure.

unusual in large molecules but well known in atomic systems. The photochemistry of fluorinated amines and imines has been reported.161 Methyl di-imide on irradiation at 404.7 and 366 nm between 23-110 "C,1&-500 Torr Mono me r

Excimeric s t a t e

Scheme 3

undergoes a chain decomposition with overall # order kinetics and chain lengths Principal steps are of lo2 to lo6, depending upon the rate of light (76)-(78), and the primary quantum yield of photodecomposition, measured as 0.74, was probably unity.

Table 4 Rate constant values for processes in Scheme 3 Rate constant kr2

lsl lSa

Process Radiative decay of unrelaxed excimeric state Non-radiative decay of unrelaxed excimeric state Quenching of unrelaxed excimeric state Vibrational relaxation of unrelaxed state Radiative decay of relaxed excimeric state Non-radiative decay of relaxed state Quenching of relaxed state Collision-induced radiative decay

Value 3.5 x 106s-1 6.85 x 107 s-1 2.15 x 101llmol-ls-l 1.2 x 101llmol-ls-l 3.2 x lo6s-l 1.09 x 107 s-1 1.76 x 1O1O I mol-l s-l 4 x 1081mol-1s-1

K. E. Peterman and J. M. Shreeve, Znorg. Chem., 1975,14, 1223. S. K. Vidyarthi, C. Willis, and R. A. Back, J. Phys. Chem., 1976, 80, 559.

130

+ hv + CH3N2H

CHSNNH CH,

CHSN,

-

CH,

+ N2H + CH,N2

CH,

+

CHS

N2

Photochemistry (76) (77) (78)

Measured values of the radiative lifetime of 3,6-disubstituted derivatives of N-methylphthalimide (5) are in good agreement with calculated values (StricklerBerg relationship) except at 513 K, the lowest temperature used in the Experimental errors were blamed. The 'quenching' of luminescence in such compounds by absorption of a second photon was mentioned earlier.47 The l-nitropropane p ~ ~ n d ~ , ~ photochemistry of a-cyano-substituted n i t r ~ ~ ~ - ~ ~ m vapour,166and methyl nitrite 120 has been reported.

In the photolysis of the pyrazolines (6)-(8) in the Inn* state, for excitation near the onset of absorption, at 345 nm, the radiative and non-radiative lifetimes were measured as 380 and 529 ns, respectively, dropping to 85 and 91 ns, respectively, for excitation at 316 nm.lS6 It was concluded that dissociation in (6) and (7) is an excited singlet process resulting from rupture of a C-N bond: the resulting radical randomizes its energy amongst internal modes before subsequent dissociation. For compound (8) some 40% of the products appear to come from a hot ground-state molecule formed by internal conversion from S1,but the authors conclude that this is not a straightforward process, and may involve recyclization of the radical produced in C-N bond rupture in the parent species. Irradiation of pyridazine vapour at 365 nm and 110-160 "Cresults in elimination of nitrogen with quantum yield of 0.12, apparently by a singlet process.166 Photolysis of sym-tetrazine at 551.5 nm to give N2 and HCN can be used as a basis for isotope enrichment.167 An excellent thorough spectroscopic study on the lB,(Slnrr*) fluorescence from selected vibronic levels of pyrimidine vapour shows that extensive anharmonic coupling occurs in the excited state,15*and this may account for the two components in the total fluorescence decay of this species, one of duration 1 ns, the other of 10 ps.169 Isotope effects on vibronic coupling in pyrazine 160 have been discussed, and it was mentioned earlier 49b that in the case of @-naphthylamine,unimolecular vibrational relaxation appears to 163

lS4 lS6

T. V. Veselova, A. M. Makushenko, I. I. Reznikova, 0. V. Stolbova, and G. D. Chekhmataeva, Optika i Spektroskopiya, 1975,39, 870. B. G. Gowenlock, J. Pfab, and G. Kresze, Annalen, 1975, 1903. (a) K. A. Khan, Chem. Letters, 1975, 879; (b) F. H. Dorer and G. Pfeiffer, J. Amer. Chem.

97, 3579. J. R. Fraser, L. H. Low, and N. A. Weir, Canad. J. Chem., 1975,53, 1456. 16' R. R. Karl, jun. and K. K. Innes, Chem. Phys. Letters, 1975, 36, 275. 158 (a) A. E. W. Knight, C. M. Lawburgh, and C. S. Parmentor, J. Chem. Phys., 1975,63,4336. (6) A. E. W. Knight and C. S. Parmenter, J. Luminescence, 1976, 12, 151. lS9 K. Uchida, I. Yamazaki, and H. Baba, Chem. Phys. Letters, 1976, 38, 133. lE0 K. Kamogawa and M. Ito, J . Mol. Spectroscopy, 1976,60,277. SOC.,

166

Gas-phase Photoprocesses

131

be slow in comparison with electronic relaxation. The 'quenching' of luminescence in this compound by absorption of a second photon was also discussed earlier.47

6 Sulphur-containing Molecules Sulphur dioxide and H,S are discussed in Section 11. Radiative lifetimes for some 4p and 4d levels of singly-ionized sulphur have been reported.ls1 Rate coefficients for the deactivation of S(lS) by induced emission (79) produced in the photodissociation of OCS at 161 nm have been measured and range from for He to 1.1 k 0.05 x 10-l6 cm3molecule-l s-l for Xe.lSz 5.6 k 0.9 x

+M S(lS) + Xe SPS)

---+

+ M + hv S(3P) + M + hv SPD)

(79)

(80)

These values may be compared with the quenching data for O(1S)56,162and Se(1S).56 Xenon was also found to enhance reaction (80) with a rate coefficient of 5.5 k 1.0 x 10-19 cm3molecule-l s-l. In the photolysis of OCS at wavelengths longer than 105 nm, both S atom and 0 atom production are energetically feasible. However, the ratio of quantum yields @ s : @* was found experimentally to be b50.1a3 Some long-lived emission in the U.V. and vacuum U.V. seen in this study may be due to molecular fluorescence from OCS. The electronimpact excitation 16*and photoionization 57 of OCS have been reported, and the vibrational populations of CO produced in the reactions of O(3P)and O(l0) with OCS Emission from CS(zlTz) produced in the photolysis of OCS and CS, at 68.6-123.9 nm was discussed earlier,58 and 46.2-97.7 nm excitation (of CS,) has been shown to produce CS,+ in the B2Xcu+and states, which are seen in emission.166 The magnetic quenching of CS, fluoresc e n ~ eand , ~ ~a CO chemical laser from the photoinitiated CS,-0, system have been discussed. In a spectral study lee on the second excited singlet state of thiophosgene, emission was seen only from absorption processes which produced states 3"4" with rn or n = 0 or 1 (where v1 = C-S stretch; v, = C-C1 symmetric stretch; v 3 = CCl, scissor mode; v4 = out-of-plane bend; v5 = C-Cl asymmetric stretch; and v6 = CCl, rock); levels above these do not fluoresce. The most prominent emission bands were those with 1,3,4, with 1 < p < 10; 1 < q d 5 and 1 < r < 4. A review has been given of other sulphur compounds which emit relatively strongly from the second excited A redetermination of the quantum yield of decomposition of dimethyl sulphide, methyl ethyl sulphide, and diethyl sulphide gives values of 0.51, 0.46, and 0.49,

x2rI,

L. Maleki and C. E. Head, Phys. Rev. (A), 1975,12,2420. G. Black, R. L. Sharpless, and T. G . Slanger, J. Chem. Phys., 1975, 63,4546,4551. I e 3 R. B. Klemm, S. GIicker, and L. J. Stief, Chem. Phys. Letters, 1975,33, 512. 164 R. J. Van Brunt and M. J. Mumma, J. Chem. Phys., 1975, 63, 3210. le6 R. G. Shortridge and M. C. Lin, Chem. Phys. Letters, 1975, 35, 146. 166 L. C. Lee, D. L. Judge, and M. Ogawa, Canad. J. Phys., 1975,53, 1861. 16' (a) Y . N. Zhitnev, G . N. Kashnikov, B. M. Popov, M. P. Popovich, E. V. Skokan, V. V. Timofeev, and Y . V. Filippov, Zhur. fiz. Khim., 1976, 50, 278; (b) A. S. Bashkin, A. N. Oraevsky, V. N. Tomashov, and N. N. Yuryshev, Kvantovaia Electronika, 1976, 3, 362. lea T. Oka, A. R. Knight, and R. P. Steer, J . Chem. Phys., 1975, 63, 2414. 108 P. De Mayo, Accounts Chem. Res., 1976, 9, 52. Ie2

132

Photochemistry

respectively, compared with 0.38 for the Hg(63P1) photosensitized reaction of the diethyl Both direct and sensitized processes are assumed to occur through the triplet state with reaction (81) being four times as efficient as reaction (82). Photolysis of methanethiol and ethanethiol at 185 nm has been r e ~ 0 r t e d . l ~ ~ 3Et2S*

-

+ Et* C2H4 + EtSH

EtS*

(81)

(82)

In the direct photodecomposition of thietan and its derivatives, the biradicals produced (Scheme 4) may in the gas phase decompose or ring-close before Et

Et

+ R

hu

+ products

and

___+

R

R Scheme 4

complete equilibration of rotational isomers occurs to give observed products, whereas in solution or in glassy matrices these species are equilibrated or trapped.172 Triplet states in thiophen have been identified at 3.75 and 4.62 eV, compared with 3.99 and 5.22 eV for furan, and 4.21 eV for p y r r 0 1 e . ~ ~ ~

7 Halogen Atoms and Halogenated Compounds Photodissociation of halogenated compounds has become an area of increasing interest, partly owing to the fact that the production of electronically excited halogen atoms may form the basis of chemical i.r. lasers. The possible dangers from stratospheric Freon photodecomposition to produce chlorine atoms reactive towards ozone has also provided considerable impetus. The DF-COB and H2-F2energy-transfer chemical lasers may be initiated through reaction (83), NO+F2

-

FNO+F

(83)

for which the rate constant has been measured as 7.0 x 10-13 exp (- 1150/T).174 The rate constants for atom reactions with NO and various third bodies,175 theoretical studies on the reactions of 18Fand F(2P*)with H2,176and other electronically excited halogen atoms with H2, HD, and D2,177 have been reported. Kinetics of ground-state F(2P) atom reactions forming inorganic fluorides 178 and with S02179have been investigated. Lifetimes of 19F(23P2and 23P0) 170

171 373 17s 174

176 176 177 178 178

C. S. Smith and A. R. Knight, Canad. J. Chem., 1976, 54, 1290. D. Kamra and J. M. White, J. Photochem., 1975, 4, 361. D. R. Dice and R. P. Steer, Canad. J. Chem., 1975, 53, 1744. W. M. Flicker, 0. A. Mosher, and A. Kupperman, Chem. Phys. Letters, 1976, 38, 489. C. E. Kolb, J. Chem. Phys., 1976, 64, 3087. E. G. Skolnik, S. W. Veysey, M. G. Ahmed, and W. E. Jones, Canad. J. Chem., 1975,53,3188. (a) E. R. Grant and J. W. Root, J. Chem. Phys., 1976,64,417; (b) F. Rebentrost and W. A. Lester, jun., ibid., p. 4223; (c) F. Rebentrost and W. A. Lester, jun., ibid., p. 3879. I. H. Zimmerman and T. F. George, J.C.S. Furaday ZZ, 1975, 71, 2030. (a) E. H. Appelman and M. A. A. Clyne, J.C.S. Faraduy Z, 1975, 71, 2072; (b) P. P. Bemand and M. A. A. Clyne, J.C.S. Faraduy ZI, 1976, 72, 191. C. A. McDowell, F. G. Herring, and J. C. Tait, J. Chem. Phys., 1975, 63, 3278.

133

Gas-phase Photoprocesses

atoms,lso and gas-phase reactions of F- and OH- with alkyl formateslsl have been discussed. The deactivation of HF(V = 1) by 0, CI, and F atoms has been studied quantitatively.ls2 Rate constants for reactions of ground-state c h l o ~ i n e , ~Br(2P3),1879 ~ ~ - ~ ~ ls8 ~ Br*(2P&188vlagI(2P*),190and I*(2Ph)lg1-lg7are summarized in Table 5. These data are included here because of their immense

Table 5 Rate constants for reactions of halogen atoms, ambient temperature Substrate

c1z

Br2 Brz 12

IC1 CO, Ar (Ar third body) CO, He (He third body) Xe, Ar (Ar third body) HOF

Cl(2P) Cl(2P) Cl(2P) Cl(2P) Cl(2P)

HI HBr DI DBr HCl(u = 1)

Cl(2P)

DCl(u = 1)

HCl(u = 1) HCl(u = 2)

Ref. 178a 178a 178b 178a 178a 178a 178a 178a 178a

1.64 x 10-lo 7.4 x 10-12 0.89 x 10-lo 4.9 x 10-l2 9.0 f 1.7 x 10-l1 exp (- 5.8 f 0.5) kJ/RT 2.4 k 0.5 x 10-l' exp (-3.2 & 0.9) kJ/RT 1.9 0.2 x 10-10 2.94 f 0.49 x 10-l1 exp (-298 f 39/T)

183 183 183 183 184

2.8 k 0.52 x 10-13 1.8 f 0.33 x 10-l2

187 187

*

Cl(2P) Cl(2P) W2P#) Br(2Pa)

Rate constant cm3 molecule-l s-l 1.6 f 0.5 x 10-lo 3.1 5 0.9 x 10-lo 1.4 2 0.3 x 10-lo 4.3 f 1.1 x 10-lO 5 & 2 x 10-10 5 1 x 10-320 0.5 x 10-32a 3.4 2 x 10-33 a 2 x 10-10

184 185 185

J. R. Mowat, P. M. Griffin, H. H. Haselton, R. Laubert, D. J. Pegg, R. S. Peterson, I. A. Sellin, and R. S . Thoe, Phys. Rev. (A), 1975,11,2198. J. F. G . Faigle, P. C. Isolani, and J. M. Riveros, J. Amer. Chem. SOC.,1976, 98, 2049. (a) G. P. Quigley and G. J. Wolga, J. Chem. Phys., 1975,62, 4560; (b) G. P. Quigley and G. J. Wolga, ibid., 1975, 63, 5263. lB3 K. Bergmann and C. B. Moore, J. Chem. Phys., 1975, 63, 643. lE4(a) R. D. H. Brown, G. P. Glass, and I. W. M. Smith, J.C.S. Faraday IZ, 1975, 71, 1963; (b) 1. W. M. Smith, Accounts Chem. Res., 1976, 9, 161. lB6 P. P. Bemand and M. A. A. Clyne, J.C.S. Faruday ZI, 1975,71, 1132. lS6 M. J. Kurylo and W. Braun, Chem. Phys. Letters, 1976, 37, 232. lS7 S. R. Leone, R. G. Macdonald, and C. B. Moore, J. Chem. Phys., 1975, 63, 4735. lB8K. Bergmann, S. R. Leone, and C. B. Moore, J. Chem. Phys., 1975,63,4161. l g BF. J. Wodarczyk and P. B. Sackett, Chem. Phys., 1976, 12, 65. l o o H. W. Chang and G. Burns, J. Chem. Phys., 1975,62,2426; 1976,64, 349. lB1 I. Arnold, F. J. Cornes, and S . Pionteck, Chem. Phys., 1975, 9, 237. lg2 D. H. Burde and R. A. McFarlane, J. Chem. Phys., 1976, 64, 1850. lg3 V. I. Tal'roze, M. N. Larichev, I. 0. Leipunski, and I. I. Morozov, J. Chem. Phys., 1976, 64, 3 138. lB4 (a) T. Donohue and J. R. Wiesenfeld, J. Chem. Phys., 1975, 63, 3130; (b) T. Donohue and J. R. Wiesenfeld, Chem. Phys. Letters, 1975, 33, 176; (c) T. Donohue and J. R. Wiesenfeld, J. Phys. Chem., 1976, 80, 437. lg6 R. J. Donovan and C. Fotakis, J. Chem. Phys., 1974, 61, 2159. lg6 R. J. Donovan, F. G. M. Hathorn, and D. Husain, Trans. Faraday SOC.,1968,64,3192. lB7 P. Cadman, J. C. Polanyi, and I. W. M. Smith, J. Chim. phys., 1967, 64, 111.

lB0

Photochemistry

(cont.) Rate constant

cm3 molecule-l s-l

Substrate

HI HI HF

1.0 f 0.3 x 10-l1 -0.25 X 3.44 x 10-11

- 2.798 log T/300 I(2P&(DCI third body) log k = 10.193 (at 300 K) I(2Pi)(HBr third body) log k = 10.274 - 2.956 log 77300 I(2Pj)(SO, third body) log k = 10.272 - 2.574 log T/300 I(2Pi)(HI third body) log k = 10.47 - 3.5 log 27300 3.0 f 0.1 x 10-l’ I2 3.6 k 0.3 x I2 2.5 f 0.3 x 10-l1 0 2 1.9 k 1 x 10-11 0 2 2.5 f 0.3 x H2O 4.3 _+ 0.6 x 10-14 D2O 1.0 0.2 x 10-11 H20, 1.3 f 0.2 x 10-ls COZ CH31 6.2 f 1.4 x CHJ 5.7 f 0.6 x 10-13 CH31 2.6 f 0.6 x 10-13 6.1 _t 0.3 x 10-13 C2Hd 1.9 2 0.2 x 10-13 C2HJ n-propyl iodide 8.0 f 0.7 x n-propyl iodide 2.0 ~f:0.2 x 10-13 isopropyl iodide 6.3 1.9 x 10-13 2.0 f 0.2 x 10-13 isopropyl iodide 9.3 5 0.3 x 10-13 n-butyl iodide 2.9 _t 0.2 x 10-13 n-butyl iodide s-butyl iodide 1.2 f 0.2 x 10-12 isobutyl iodide 1.11 f 0.07 x 10-l2 2.9 0.2 x 10-13 isobutyl iodide ~ 5 . 2x 10-13 t-butyl iodide t-butyl iodide 3.8 rf: 1.3 x 10-13 HI 1.5 rt: 0.2 x 10-13 HI 1.5 rf: 0.4 x 10-13 1.8 f 0.4 x CDJ CD31 4.6 rf: 0.8 x 10-16 CF31 3.5 2 0.6 x 10-ls a In units of ,me molecule-2 s-1. Average value. In units of 1 mol-’s-l. (HCI third body)

log k = 10.287

*

Ref. 188 188 189 190 190 190 190

190 191 192 192 193 192 192 192 192 1946 194c 195 194c 196 194c 196 194c 196 194c 196 194c 194c 196 194c 196 194c 197 194c 195 1946

importance in the area of chemical lasers. Several of the papers mentioned have studied the interesting electronic to vibrational energy transfer process (84), where X and Y are halogen atoms,1*2a~ IBg and other papers on this subject have appeared recently.1g8~lQg I n the case of X = Br, Y = F, the reverse of reaction X(2P6) + HY(v = 0) Ig8 lg9

__I_,

X(2P,)

+ HY(Y = 1)

T. G. Slanger, G. Black, and J. Fournier, J. Phutuchem., 1975, 4, 329. A. B. Petersen, C. Wittig, and S. R. Leone, Appl. Phys. Letters, 1975, 27, 305.

(84)

135

Gas-phase Photoprocesses

(84) has been investigated.lE2"Deactivation processes of vibrationally excited HY molecules include simple inelastic collisions (85) and 'reactive' collisions (86).

+ HY(u = 1) X + HY(u = 1)

X

-

___+

+ HY(v = 0) HX(o = 0) + Y

X

(85) (86)

When X = Y = C1, the total rate constant in Table 5 is for (85) and (86), and the result, while agreeing with recent measurements,200is in strong disagreement with a result obtained earlier by the same authors.201 When X = Br and Y = C1, (85) predominates and is accounted for mainly by vibrational to translational, rotational energy conversion.1E7For t, = 2, however, the reactive pathway predominates and may be the basis of an efficient isotope separation process for CI The rate constant quoted in Table 5 for Br(2P#)and HI is for the reaction (87), whereas that for the analogous quenching of Br*(2P*)includes electronic quenching and electronic to vibrational energy-transfer pathways also. The

+ HI

Br(2P&

-

HBr

+ I(2Pg)

(87)

recombination of iodine atoms in the temperature range studied lQoclearly occurs through radical-molecule complex formation [reactions (88) and (89)].

+M OM)* + I

I(2Pi)

-

OM)* I2 + M

The values of quenching o f I*(lP*) by a variety of substrates obtained by Donohue and WiesenfeldlB4are notably larger than those found by Donovan et aZ.,1959196though no explanation for this is currently forthcoming. HCl chemical lasers based upon C1 atom reactions with hydrides of Groups IV, V , and VI,202and deactivation of HCl(u = l), by a variety of species including HF, HBr, H2, D2, N2, and C2 203 have been reported, and various aspects of The possibility the iodine I(2Pi -+ 3P+)photodissociation laser of a molecuIar I 2 laser in the 342 nm the XeBr exciplex and XeBr pumping of the I 2 laser 215 have been studied. Rate constants for the deactivation of Br2(B3IIOI+)by He, Ne, Ar, Kr, N2, 02, COz, and Br, have been given as 2.9, 3.0, 4.3, 4.0, 5.6, 4.2, 6.6 and 4.2 x 10-lo 202p

R. G. Macdonald, C. B. Moore, I. W. M. Smith, and F. J. Wodarczyk, J. Chem. Phys., 1975, 62, 2934. aol 202 203

aor *05

ao6

207 208 208

210

211

213

ala 21s

B. A. Ridley and I. W. M. Smith, Chem. Phys. Letters, 1971,9,457. R. D. Coombe, A. T. Pritt, jun., and D. Pilipovich, Chem. Phys. Letters, 1975, 35, 345, 349. J. F. Bott and N. Cohen, J. Chem. Phys., 1975, 63, 1518. V. A. Katulin, V. Yu. Nosach, and A. L. Petrov, Kvantovaia Elektronika, 1976, 3, 386. R. J. Pirkle, jun., C. C. Davis, and R. A. McFarlane, Chem. Phys. Letters, 1975, 36, 305. F. T. Aldridge, I.E.E.E. J. Quantum Electronics, 1975, 11, 215. V. Yu. Zalessky and S. S. Polikarpov, Kvantovaia Elektronika, 1975,2, 1536. I. M. Belousova, N. G. Gorshkov, and 0. B. Danilov, Zhur. fiz. Khim., 1975, 49, 107. R. J. Pirkle, C. C. Davis, and R. A. McFarlane, J. Appl. Phys., 1975, 46, 4083. A. K. Hays, J. M. Hoffman, and G. C. Tisone, Chem. Phys. Letters, 1976, 39, 353. J. J. Ewing and C. A. Brau, Appl. Phys. Letters, 1975, 27, 557. R. S. Bradford, jun., E. R. Ault, and M. L. Bhaumik, Appl. Phys. Letters, 1975, 27, 546. M. V. McCusker, R. M. Hill, and D. L. Huestis, Appl. Phys. Letters, 1975, 27, 363. S. K. Searles, Appl. Phys. Letters, 1976, 28, 602. J. C. Swingle, C. E. Turner, jun., J. R. Murray, E. V. George, and W. F. Krupke, Appl. Phys. Letters, 1976, 28, 387.

136

Photochemistry

5

0.3 ppm IODINE IN H E L l U M

P, = 8 4

atm

D = 2 5 u

V, = 16,895.0 cm

I2

10-0 BAND

I2

0

12-1 BAND

L -4 20

-210

v,

210

420

630

FREQUENCY ( GHz ) Figure 4 Fluorescence excitation spectrum of a supersonic jet of I, in He in the spectral region near the 10-0 vibronic band of the B +- 2 transition of I,. Laser bandwidth 1 GHz FWHM (Reproduced by permission from J. Chem. Phys., 1976,64, 3266)

137 cm3molecule-l s-l, respectively.21e Predissociation induced in this system by the argon ion laser has been proposed as a means of Br isotope Fluorescence quenching cross-sections for the B3IIOw+state of I, 218 and predissociation in this species 21g have been studied. The measured fluorescence decay time of selected levels of the 12(3110,,+) state show a clear inverse dependence on J ( J + l), showing that predissociation occurs,21gb as predicted from theory,21ga The fluorescence of I, excited by a single mode tunable dye laser has been further studied.220 As stressed earlier, the complexity of emission spectra of polyatomic and even small molecules often arises from the excitation of overlapping hot bands in the absorption act. This difficulty has been removed in a very elegant fashion in a recent study on I, by the use of a rotationally cooled beam of the molecular system, with rotational temperatures as low as 0.4 K, vibrational temperature 50 The resulting fluorescencespectrum is free of complications, as shown in Figure 4,and in the presence of He permits the observation of bands A and C due to HeI,, and B due to He,!,. The excitation spectrum of the fluorescence of the HeI, complex in the 2 -+ B system shows that predissociation occurs, with a rate constant of 5 x 1O1O s-l for the vl’ = 27 vibrational level, falling to < 5 x lo9 s-l for vl’ < 7. This exciting work clearly demonstrates the enormous advantages of molecular beams for photophysical studies, and further studies, particularly those using time-resolved methods, are eagerly awaited. Photofragment spectroscopy is another recently developed powerful spectroscopic tool, and has been applied lately to studies on Br2,,,, HI,,,* ICN,225 HCN,226and BrCN.226The first study confirms that the first absorption band in Br, is the overlapping of transitions to the XlU(3rI),80U+(311), and lU(lII) states.,,, The new technique of double-absorption photofragment spectroscopy was applied to In the photolysis of HI and DI at 266.2 nm (fourth harmonic of the Nd3+ laser) 36 k 5% of the product iodine atoms were produced in the The transition responsible is excited state for HI, 26 k 3% for DI.224 polarized parallel, as found also for ICN, and in HI corresponds to overlapping 3111,31-10+ and lTZ contributions. The parallel assignment in ICN is in disagreement with previous assignments. Both ground-state CN and CN(J211) were formed (the latter to an extent of 60%) and explanations offered included the following: (i) the simultaneous excitation of two different upper states; and (ii) the initial exclusive production of CN(A211) followed by quenching of a fraction of this by a ‘half-collision’ with the recoiling iodine atom. Gas-phase Photoprocesses

218

E. D. Bugrim, S. N. Makarenko, and I. L. Tsikora, Optiku i Spektroskopiyu, 1975, 93, 27.

217

218

21* 220

R. M. Lum and K. B. McAfee, jun., J. Chem. Phys., 1975, 63, 5029. M. H. Ornstein and V. E. Derr, J. Opt. Sac. Amer., 1976, 66, 233. (a) J. Vigue, M. Broyer, and J . 4 . Lehmann, J. Chem. Phys., 1975, 62,4941; (b) M. Broyer, J. Vigue, and J.-C. Lehmann, ibid., 1975, 63, 5428. H. Kato, S. R. Jeyes, A. J. McCaffery, and M. D. Rowe, Chem. Phys. Letters, 1976, 39, 573.

232 223

224 2 2

226

R. E. Smalley, D. H. Levy, and L. Wharton, J. Chem. Phys., 1976, 64, 3266. R. J. Oldman, R. K. Sander, and K. R. Wilson, J. Chem. Phys., 1975, 63, 4252. R. K. Sander and K. R. Wilson, J . Chem. Phys., 1975, 63, 4242. R. D. Clear, S. J. Riley, and K. R. Wilson, J . Chem. Phys., 1975, 63, 1340. ~J. H. Ling and K. R. Wilson, J. Chem. Phys., 1975, 63, 101. G. A. Chamberlain and J. P. Simons, J.C.S. Furuduy 11, 1975, 71, 2043.

Photochemistry

138

The vacuum U.V. photodissociation of HCN and BrCN produced CN(B’%+), and it was shown that the primary excited states predissociating were all nonlinear, with A’ symmetry.220Predissociation lifetimes were 0.6 ps, with states in which the C-Br stretch was excited being shorter-lived than those in which the C-N mode was excited. Theories of photodissociation have been applied to the cases of HCN and ICN,227and other models applied to C-CI scission in chloroacetylene.228 Other chlorine-atom reactions of interest include hydrogen abstraction from h a l ~ e t h a n e s hydrocarbons ,~~~ and and initiation of oxidation The thermal 233 and in 1,1,1,2-tetrachloroethane231 and di~hloroethylene.~~~ photochemical 141dissociations of nitrosyl chloride have been investigated. The important atmospheric reactions (90) and (91) have measured rate constants

c10 + c10

c10 + 0

+ 0, ___, c1 +

____+

c1,

0 2

(90)

(9 1)

of k,, = 1.4 f 0.7 x cm3molecule-l s-l at 1250 K and kgl = 7 & 1.5 x cm3molecule-l s-l 234 at 1250 K. The kinetics of the photochemical decomposition of ClO, have been investigated.235Other papers of interest are on the following subjects: photoionization processes in methyl halides,23sphotochemical reactions of CFJ with benzene and halogenated benzenes,237the photodissociation of molecular beams of methylene di-iodide and i o d o f ~ r m , ~ ~ ~ the photolysis of fluoroiodo-, fluorobromo-, and fluor~chloro-alkanes,~~~~ 114* 23B including CFCl, and CF2Cla,239and trans-1,2-dichloroethylene,240reactions between methyl radicals and 12, IC1,116 photoemission from HCl+, CH, CH+, CCl, H, and C1+ excited by electron impact on HCI and chlorinated methanes,,*l heavy-atom kinetic isotope effects (or rather the lack of them) in the thermal decomposition of ethyl the reaction of recoil soBr atoms with cyclopropane and bromocyclopropane,243and a potentially highly efficient C1, 227

ep8

(a)Y. B. Band and K. F. Freed, J. Chem. Phys., 1975,63,3382; (b)Y . B. Band and K. F. Freed, J. Chem. Phys., 1975, 63, 4479; (c) M. J. Berry, Ann. Rev. Phys. Chem., 1975, 26, 259. D. Florida and S. A. Rice, Chem. Phys. Letters, 1975, 33, 207. C. J. Martens, M. Godfroid, J. Delvaux, and J. Verbeyst, Znternat. J. Chem. Kinetics, 1976,8, 153.

230

231 232

233 2y4

236

238 237

238 239 240

P. Cadman, A. W. Kirk, and A. F. Trotman-Dickenson,J.C.S. Faraday I, 1976,72,996,1027. D. Gillotay and J. Olbregts, Znternat. J. Chem. Kinetics, 1976, 8, 11. E. Sanhueza and J. Heicklen, J. Photochem. 1975, 4, 17. M. Quack and J. Troe, Ber. Bunsengesellschaft Phys. Chem., 1975, 79, 469. C. Park, J. Phys. Chem., 1976,80, 565. V. I. Gritsan and V. N. Panfilov, Kinetika i Kataliz, 1975, 16, 312. V. S. Ivanov and F. I. Vilesov, Optika i Spektroskopiya, 1975,39, 857. J. M. Birchall, G. P. Irvin, and R. A. Boyson, J.C.S. Perkin ZZ, 1975,435. M. Kawasaki, S. J. Lee, and R. Bersohn, J. Clzem. Phys., 1975, 63, 809. R. E. Rebbert and P. J. Ausloos, J. Photochem., 1975, 4,419. R. Ausabel and M. H. J. Wijnen, J. Photochem., 1975, 4, 241 ; Znternat. J. Chem. Kinetics, 1975, 6, 739.

241 242 243

244

M. Toyoda, T. Ogawa, and N. Ishibashi, Bull. Chem. Soc. Japan, 1976,49, 384. J. R. Christie, W. D. Johnson, A. G. London, A. MacColl, and M. N. Mruzek, J.C.S. Faraday Z, 1975,71, 1937; A. MacColl, Ann. Reports ( A ) , 1974, 71, 77. M. Saeki and E. Tachikawa, J.C.S. Faraday I, 1975, 71, 2121. C. H. Chen and M. G . Payne, Appl. Phys. Letters, 1976, 28, 219.

Gas-phase Photoprocesses

139

Photo-oxidation studies include the subjects of reactions of O(l0) with c h l o r ~ f l u ~ r ~ m e t h a n246e sphoto-oxidative ,~~~~ decomposition of vinyl formation of phosgene and trichloroacetyl chloride in the direct photo-oxidation of perchloroethylene in airyz4*"the Hg(3Pl)-sensitized photo-oxidation of tri~ h l o r e t h y l e n eand , ~ ~ gas-phase ~~ ozonolysis of cis- and trans-dichlor~ethylene.~~~ 8 Metal Atom Reactions Halogen atom and sulphur atom reactions have been discussed in the preceding sections, and rare-gas, hydrogen, oxygen, and nitrogen atoms are discussed in Section 11. Attention in this section is focused upon metal atom excited-state processes. Excimer and exciplex production in such systems have been reviewedYz7several papers have dealt with the problem of resonance radiation i r n p r i ~ o n m e n t , ~and ~ ~ -atomic ~ ~ ~ and molecular photoassociations have been described.264

Mercury.-Oscillator strengths for the Hg' lP1c ?Yo transition,26s an experimental study of the Hg 404.7 (63P0+ 73&) absorption profile,2S6radiative lifetimes of various mercury atom excited level^,^^^^ 258 visible continua in the Cd-Hg the formation of supersonic beams of Hg(3P0),280 electric field effects on mercury resonance fluorescence,261the effects of 253.7 nm radiation on mercury vapour,262and the excitation mechanism in helium-mercury lasers 263 have been reported. Excitation of the Hg(3P') state by electronically excited CO, N2,and Kr has been In a crossed-beam experiment Hg(63p0,2)interacted with velocity-selected LiI, NaI, and NaBr, and resulting fluorescence from Li(22P) or Na(3V) was monitored. The cross-sections for production of excited alkali atoms decreased rapidly with increasing collision Similar studies were carried out with thallium-mercury mixtures, thallium fluorescence being

257

R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Photochem., 1975, 4, 381. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J. Photochem., 1975, 4, 203. T. Kagiya, K. Takemoto, and Y . Uyama, Nippon Kagaku Kaishi, 1975, 1922. (a) H. F. Andersson and J. A. Dahlberg, Acta Chem. Scand. (A), 1975, 29, 473; (6) E. Sanhueza and J. Heicklen, J. Photochem., 1975,4, 161. C. W. Blume, I. C. Hisatsune, and J. Heicklen, Internat. J. Chem. Kinetics, 1976, 8, 235. E. L. Lewis and C. S . Wheeler, J. Phys. ( B ) , 1976,9, 577. L. F. Phillips, J. Photochem., 1975, 4, 407. M. J. Boxall, C. J. Chapman, and R. P. Wayne, J. Photochem., 1975, 4,281. P. L. Knight and P. W. Milonni, Phys. Letters (A), 1976,56,275. B. Stevens, Ann. Reports (A), 1974, 71, 29. R. Abjean and A. Johann-Gilles, J. Quantum Spectroscopy Radiative Transfer, 1976, 16, 369. Z. Ben-Lakhdar-Akrout, J. Butaux, and R. Lennuier, J. de Phys., 1975,36,625. A. L. Osherovich, E. N. Borisov, M. L.Burshtein, and Ya. F. Verolainen, Optika i Spektro-

268

T. Anderson, 0.Poulsen, and P. S . Ramanujam, J. Quantum Spectroscopy Radiative Transfer,

246

2 4 ~

247

248 a48 2Lo

261 262 z6s

264

2so

skopiya, 1975, 39, 820. 1976, 16, 521.

2Gs 260

M. W. McGeoch, G. R. Fournier, and P. Ewart, J. Phys. (B), 1976,9, L121. J. A. Haberman, B. E. Wilcomb, F. J. Van Itallie, and R. B. Bernstein, J . Chem. Phys., 1975, 63,2772; 62,4466.

261

264

266

W. J. Sandle, M. C. Standage, and D. M.Warrington, J. Phys. (B), 1975,8, 1203. G. R. De Mark, Analytical Letters, 1976, 9, 161. I. M. Littlewood, J. A. Piper, and C. E. Webb, Optics Comm., 1976, 16, 45. W. Lee and R. M. Martin, J. Chem. Phys., 1976, 64, 678. L. C.-H. Loh and R. R. H e m , Chem. Phys. Letters, 1976,38,263.

140

+ Tl(6,Pt) T1(72Sb)

___+

Photochemistry

+ T1(72S4) + hv,;,nm

Hg(61So)

(92)

Tl(6'Pt)

(93)

monitored [reactions (92) and (93)].266,267 Emission from T1(62Di,p) has been seen in thallium-fluorine flames, these excited states arising from energy-transfer processes involving the T1(62Pt) state.2ss The cross-section for production of Zn(43P) in the collision of Zn(4lSO) with Hg(63P) was found to be 5.9 x 10-2A2.268This was increased on addition of N,, presumably due to initial Hg(63P0)formation. The cross-section for quenching of Zn(43P1) was given as 0.19 The vibrational excitation of N2by Hg(63P1)to produce Hg(63P0)and N2(v = 1) has been further The lifetime of the N2(u = 1) state was determined as 0.85 k 0.2 s for 1 Torr N, with 9 Torr Ar. Collisions between rare-gas atoms and N2 and the 6s6d lD2 and 3D2 levels of Hg, populated with a CW dye laser tuned to the appropriate absorption from the 6s6p lP1state, have shown that when the difference between the levels excited and the receiving level of rare gas or N, is less than 3 cm-l, cross-sections increase with atomic mass.27oWhen the energy mismatch is greater than 30 cm-l, N2is the best deactivator. Mercury excimer formation has been 272 and two states observed lreactions (94)-(97)]. The rate constant for (96) was measured as 4.8 k 0.5 x

w2.

10-15 cm3molecule-1 s-l, and the rate coefficient for the collision-induced emission process (97) was given as 5.7 f 0.3 x 10-15 cm3molecule-l s-l.,', A study on Hg(63P0)exciplexes with ammonia and water has given results in good agreement with others in the case of NH3, less so in the case of H20.273The rate cm6molecule-2 s-l constant for reaction (98) was found to be 2.2 k 0.2 x NH3

+ Hg(63P0) A

(NH,Hg)*

(98)

when M = N,, and the lifetime of the exciplex was 1.7 k 0.1 x s. The corresponding lifetime for the H,O exciplex was 3.3 x s, with a pseudo second-order rate constant for its formation of 9.4 k 0.5 x cm3m~lecule-~ s-l (when M = 1 atm N,). HgH was seen in absorption and OH in emission and absorption in this study, both being produced by the non-radiative decay of the z66

L. C.-H. Loh, C. M. Sholeen, R. R. Herm, and D. D. Parrish, J . Chem. Phys., 1975,63,1980.

272

E. K. Kraulinya, S.. Y. Liepa, and A. Ya. Skudra, Optika i Spektroskopiya, 1976,40,766. (a)J. Maya and P. C. Nordine, J. Chem. Phys., 1976,64,84; (b)M. Czajkowski and L. Krause, Canad. J. Phys., 1976, 54, 603. H. Horiguchi and S. Tsuchiya, J.C.S. Faraday ZI, 1975, 71, 1164. D. Lecler and B. Laniepce, J. de Phys., 1976, 37, 55. I. N. Siara and L. Krause, Phys. Rev. (A), 1975, 11, 1810. K. G. Ong, C. G . Freeman, M. J. McEwan, and L. F. Phillips, J.C.S. Faraday ZI, 1976, 72,

27s

A. B. Harker and C. S. Burton, J. Chern. Phys., 1975, 63, 885.

267 268

2e9 270

271

183.

Gas-phase Photoprocesses

141

Hg,H20 exciplex. This work can be compared with a study in which the fractional vibrational, rotational, and translational excitation of OH produced in reaction (99) was measured as 0.11, 0.31, and 0.58, respectively.274These ratios Hg(3P0)

-

+ OH(X211, u = 0, K)

Hg(6'So)

+ OH(,Z+, v, K)

(99)

are close to statistical, implying the formation of a relatively long-lived complex between Hg(63P0)and OH(f2). The mercury-photosensitized reactions of H, with NO are assumed to occur through reactions (100)-(109).275

+ H, Hg(6,PI) + NO NO*("II) + H, Hg(6,P1)

H+NO+M HgH

+ NO

HNO+2NO 2HN0 OH+HNO NO,+HNO NO,

+ NO

+ H*

HgH

NO*(411)

(101)

+H

(102)

HNO+M

(103)

HNO

----+

(loo)

Hg(6'So)

+ HNO

(104)

N2+H+N03

(105)

+ NzO

(106)

HZO

H,O+NO

(107)

OH+2NO

(1 08)

2N02

(1 09)

Glyoxal has been synthesized in mercury-sensitized reactions of 0,-acetylene Mercury-photosensitized reactions of various olefins,l4~l5,17-21s 248b ketones and ~ z o - c o ~ ~ olog u and ~ ~ ethyl s , ~ sulphide ~ ~ 170 have been discussed elsewhere. Many other Hg-sensitized reactions are covered in other parts of this volume, and will not be documented here. The flash-photolysis of [2H6]dimethylmercury was discussed earlier.lo7 Cadmium, Zinc, and Magnesium.-Visible continua in the Cd-Hg and lifetime measurements in many levels of Cd and Zn 268* 277 have been reported. Cross-sections for quenching of Cd(3P0,1,2)by H, and D2 have been found to be much greater for the ,PIstate than for 3P0,zstates, absolute values for ,PIbeing 33 k 5 A2 for H2, 16 k 3 A2 for D2.278Similar cross-sections have been given for quenching of Cd(53&,~)by benzene, propylene, ethylene, H,, D2, NH3, N2, CO1, and CO,, and good agreement with previous results was Exciplex formation is implicated in the quenching of triplet Cd(53P0)by water, alcohols, and ethers,280but quantum yields of photosensitized luminescence are very much smaller than in the case of ammonia and amines281where values range from a74 276 276 277 278

279

281

A. C. Vikis, Chem. Phys. Letters, 1975, 33, 506. K. Tadasa, N. Imai, and T. Inaba, Bull. Chem. SOC.Japan, 1976, 49, 579. S. L. N. G. Krishnamachari and T. V. Venkitachalam, Mol. Photochem., 1976, 7, 75. M. Chantepie, J.-L. Cojan, and J. Landais, J . de Phys., 1975, 36, 1067. 0. Nedelec and J. Dufayard, J. de Phys., 1976, 37, 81. W. H. Breckenridge, T. W. Broadbent, and D. S. Moore, J. Phys. Chem., 1975,79, 1233. S. Yamamoto, K. Tanaka, and S. Sato, Bull. Chem. SOC.Japan, 1975, 48, 2172. (a) S. Yamamoto, S . Tsunashima, and S. Sato, Bull. Chem. SOC.Japan, 197548, 1172; (b) S. Yamamoto and S. Sato, ibid., p. 1382.

6

142

Photochemistry

0.67 for NH3 down to 0.04 for t-butylamine, luminescence being observed in the 432-454 nm region. The quantum yield for the Cd(3Po,,)-sensitizeddecomposition of cyclopentane through reactions (110) and (111) has been found to be unity.282 cyclo-C5H,o 3- Cd('PO,,) CdH

-

cyclo-C,H,= CdCS,)

+ CdH

+H

(1 10) (1 11)

Rate constants for the bimolecular deactivation of the 3P2and 3& excited levels of Mg are identical, varying from 1.13 k 0.15 x 10-lo for benzene to < 6.6 x 10-le cm3molecule-l s-l for He.2s3 The results thus show that equilibration amongst the sub-levels of magnesium is very fast. Alkali Metals and Alkaliiie Earths.-Polarization in transfer of energy in collisions of alkali-metal 286 collision broadening of resonance lines of Li and Na in helium,28sand i.r. photodetachment studies on Li- and P-287have been reported. There have been lifetime studies on the 3P level of Na(II),288Na(42P),289 S states of K, Rb, and C S , ~ ~ the O 42P4 state of K,291triplet levels of Sr,292,293 and the 72P*and 72Pt states of c a e ~ i u m296 . ~ Two-photon ~~~ processes in caesium,296-300 thallium,2es and rubidium 2 9 6 ~ and three-photon absorption in potassium 302 have been described. Alkali-atom fluorescencehas been seen in collisions between mercury atoms and LiI, NaI, NaBr,26sand amongst collisional processes studied are included electronic-to-electronic energy transfer in sodium-sodium 303 and sodium-potassium 304 mixtures, excimer formation in sodium-rare-gas and potassium-rare-gas sodium-rare-ga~,~~~ cae~ium-rare-gas,~~~ potasB. L. Kalra and A. R. Knight, Canad. J. Chem., 1976,54,77. R. P. Blickensderfer, W. H. Breckenridge, and D. S . Moore, J. Chem. Phys., 1975, 63, 3681. 284 E. I. Dashevskaya, Optika i Spektroskopiya, 1975, 39, 1022. 2 8 b E. I. Dashevskaya and E. E. Nikitin, Canad. J. Phys., 1976, 54, 709. 286 C. Bottcher, T. C. Cravens, and A. Dalgarno, Proc. Roy. SOC.1975, A346, 157. 287 D. Feldmann, Z. Phys. (A), 1976, 277, 19. 288 W. Schlagheck, Phys. Letters (A), 1975, 54, 181. R. Bersohn and H. Horwitz, J. Chem. Phys., 1975, 63, 33. B. R. Bulos, R. Gupta, and W. Hopper, J. Opt. SOC.Amer., 1976, 66, 426. zB1 D. Zimmermann, 2.Phys. (A), 1975, 275, 5. 29a M. D. Havey, L. C. Balling, and J. J. Wright, Phys. Rev. (A), 1976, 13, 1269. 208 F. M. Kelley and M. S . Mathur, Canad. J. Phys., 1976, 54, 800. 204 P. W. Pace and J. B. Atkinson, Canad. J. Phys., 1975,53,937. 296 J. F. Kielkopf, J. Chem. Phys., 1975, 62, 4809. E. H. A. Granneman, M. Klewer, K. J. Nygaard, and M. J. Van der Wiel, J. Phys. (B), 1976, 9, L87, 865. 2B7 S. M. Curry,C. B. Collins, M. Y.Mirza, D. Popescu, and I. Popescu, Optics Comm., 1976, 16, 251. 208 C. W. Wang and L. D. Davis, jun., Phys. Rev. Letters, 1975,35, 650; J. F. Ward and A. V. Smith, ibid., p. 653. E. H. A. Granneman and M. J. Van der Wiel, J. Phys. (B), 1975, 8, 1617. A. F. J. van Raan, G. Baum, and W. Raith, J. Phys. (B), 1976,9, L173. Y. Kato and B. P. Stoicheff, J. Opt. SOC.Amer., 1976, 66, 490. sox P. Bensoussan, Phys. Res. (A), 1975, 11, 1787. H. L. Chen and S. Fried, I.E.E.E. J. Quantum Electronics, 1975, 11, 669. E. K. Kraulinya, E. K. Kopeickina, and M. L. Janson, Chem. P ~ J Y Letters, S. 1976, 39, 565. A. C. Tam, G. Moe, B. R. Bulos, and W.Happer, Optics Comm., 1976,16, 376. 300 J. Pascale and R. E. Olson, J. Chem. Phys., 1976, 64, 3538. B. Niewitecka and L. Krause, Canad. J. Phys., 1976,54,748. 28z

283

Sn(51So)b

<0.05

-

Sn(53P,)

(0.01 1.2

Sn(53P3e

Sn(5ID.J'

-

<0.2

9.5

x 10'

x 10'

2.7

<0.1

x 103

5.7

x 103 260

1.5

0.32 (200

-

1.6 x 104

5.3 x 10'

1.4 870 x 104 560 t l

0 a2

230

0

<0.005

910

230

-

-

-

-

x 10'

x 103

9.2

x 108

16

-

-

<0.4

Ref. 320.

x 10'

x 10'

7.6

-

Ref. 319.

330

21

54

810

0.5

<0.2

-

1.1

0.1 1.1 x 104 930

24

7.9

1

d 0.24

Bi(,Dt)" 0.05

-

2.2 t l x 10' 1.6 t l x 103
4.3

47

4

0.8

(0.09

0.2 700

-

tlOO

e0.03

<0.06

0.29

-

c 0.2

0.2 23 100

Pb(ti1Ddb

<0.02

-

t0.01 0.15

t0.02

-

Pb(63Pl)d

c 0.06

tO.O1

-

Pb(61So)b Pb(6'Pi)'

<0.2 t0.17 5.5 115
-

<0.5

Complete data are not included here. Rate constants are in units of cm8 molecule-l s-l x 10-14 throughout. Ref. 317. f Ref. 321.

C,H,

CF4

CH4

CO,

NO

co

0 2

N2

D*

Substrate He Ar Xe H*

Table 6 Selected quenching rate constants for metal atoms,a 300 K

Ref. 318.

-

<0.01

<0.06

350

(0.05

-

2

E

Q

Photochemistry sium-rare-gas mixtures,308* 309 ~ o d i u m - N ~311 , ~and ~ ~sodium-iodine * 289 quenching processes. In the last study, the quenching of Na(42P) by I2 had the very large cross-section of 175 k 22 A2, compared with only 22 A2 for the Na(32P) state. Detection of 20Naatoms 312 and observation of 3F metastable states of neutral barium 313 have been reported. 144

Miscellaneous.-Radiative lifetimes of excited states of ionic and neutral beryll i ~ m ,u.v.-irradiation ~l~ studies on selenium v a ~ o u rradiative , ~ ~ ~ lifetime measurements in Si1r-Siv,31s and quenching studies on Bi(6p3, ”+), Bi(6p3, 20~),317 Sn(5p2, and Sn(5p2, 3P2),319 Sn(SISo,5lDZ)320 and Pb(6p2, 3P2)and Pb(3P1),318 Cu(3d94s2,2D+)321 have been reported. Some of the quenching data from these last studies are collected in Table 6. A copper(1r) laser at 250 nm 322 and a copper laser using CuI vapour 323 have been suggested. 9 Miscellaneous An optically pumped superfluorescent Na, molecular laser,324 two-photon ionization of Cs,, Rb2,and R ~ C Slifetimes , ~ ~ ~of Rb-He quasibound m01ecules,~~~ the photochemistry of weakly bound Li+ complexes,326tunable alkali halide lasers,327and the laser-induced photoluminescence of Biz 328 have been discussed. Radiative lifetimes of the LiH(xlE+) state in various vibration-rotation levels have been measured as v ‘ = 2, J’ = 3 (29.4 k 1.3 ns);329 v ’ = 5, J = 3 (30.5 k 1.3 ns);329 v’ = 5, J’ = 5 (32.6 5 4.2 1 x 3 ) ; ~ ~v ’~ = 5 , J’ = 10 (32.6 k 3.0 v’ = 5, J’ = 15 (29.0 k 3.2 v’ = 7, J’ = 12 (36.9 & 1.9 ns).329 Self-quenching rate constants for these levels were of the order of 8 x cm3molecule-l s-l (for the u’ = 5, J’ = 5 and the lifetime of the v ‘ = 8, J’ = 3 level of NaH(JIX+) was given as 22.7 k 1.6 ns.320 Laser fluorescence measurements on LiD(BIX+)331 and CsH 332 have been made. 3 08 300

310 311 312 313 314 315 316

317 318 319 320 321 321

323 324

326 328 327 328 329 33 0 331 332

B. Niewitecka and L. Krause, Canad. J. Phys., 1975,53, 1499. R. W. Anderson, T. P. Goddard, C . Parravano, and J. Warner, J. Chem. Phys., 1976, 64, 403 7. C. Bottcher, Chem. Phys. Letters, 1975, 35, 367. I. V. Hertel, H. Hofmann, and K. J . Post, Phys. Rev. Letters, 1976, 36, 861. F. C. M. Coden and H . L. Hagedoorn, J. Opt. SOC.Amer., 1975,65,952. S . G. Schmelling and G. 0. Brink, Phys. Rev. ( A ) , 1975,12,2498. 0. Poulsen, T. Andersen, and N. J . Skouboe, J. Phys. (B), 1975, 8, 1393. L. Kolditz, K. Hellwig, and K. H . Grupe, Z. phys. Cliem. (Leipzig), 1976, 257, 101. A. E. Livingston, J. A. Kernalian, D. J. G. Irwin, and E. H. Pinnington, J. Phys. (B), 1976,9, 389. M. J. Bevan and D. Husain, J. Phys. Chem., 1976, 80,217. D. W. Trainor and J. J. Ewing, J. Chem. Phys., 1976, 64, 222. P. D. Foo, J. R. Wiesenfeld, M. J. Yuen, and D. Husain, J. Phys. Chem., 1976, 80, 91. A. Brown and D. Husain, J.C.S. Faraday II, 1975, 71, 699. D. W. Trainor, J. Chem. Phys., 1976, 64, 4131. J. R. McNeil, G. J. Collins, K. B. Persson, and D. L. Franzen, Appl. Phys. Letters, 1976, 28, 207. I. Smilanski, L. A. Levin, and G. Erez, Z.E.E.E. J. Quantum Electronics, 1975, 11, 919. M. A. Henesian, R. L. Herbst, and R. L. Byer, J. Appl. Phys., 1976,47, 1515. F . A. Franz and C. Volk, Phys. Rev. Letters, 1975, 35, 1704. B. S. Freiser, R. H. Staley, and J . L. Beauchamp, Chem. Phys. Letters, 1976, 39, 49. J. Walker, Nature, 1975,256, 695. G. Gerber, K. Sakurai, and H. P. Broida, J. Chem. Phys., 1976,64, 3410, 3423. P. J. Dagdigian, J. Chem. Phys., 1976, 64, 2609. P. H. Wine and L. A. Melton, J. Chem. Phys., 1976,64,2692. G. Ennen and C. H. Ottinger, Chem. Phys. Letters, 1975, 36, 16. A. C. Tam and W. Happer, J. Chem. Phys., 1976, 64, 2456.

145

Gas-phase Photoprocesses

Molecular fluorescence in Ca(OH),, Sr(OH),, and BaC1,,333 and chemiluminescence has been observed in thallium-fluorine flames resulting in TIF(x31T -+ XlZ) and (B3n-+ Z1Z) emission,268in SiO and Ge0,334in A10,335in BCI,-H,S in Ba-N,O mixtures [reaction (1 12)],337-339and in reactions of Ba, Sm, and Eu with NzO, 03,02,F,, and NF3,338 in B-N,O and Ho-N,O mixtures [reactions analagous to (112)],340and in Ba-SO, 341 and Ba-0,.342 The importance of Ba(3D) as a carrier in the overall reaction (112) has been discounted.33n Reaction (113) is spin-forbidden, and is proposed to occur via complex formation.341 Ba

Ba(lS)

+ N,O

+ SO,(lA)

-----+

BaO

+ N,

BaO(lC+) + SO$C-)

(1 12) (113)

Excitation of single vibronic levels of chromyl chloride vapour (CrO,CI,) is not possible owing to the degeneracy of the v,' and vqN modes in the gas phase, which thus results in simultaneous excitation of sequence bands.343 All levels except the zero-point level (of lifetime 1.34 k 0.08 ps) have lifetimes shortened by unimolecular non-radiative decay, the rate of which is strongly dependent upon the partitioning of energy into various excited-state modes. The apparent bimolecular quenching rate was 5.9 k 0.2 x 10-lo cm3molecule-l s-l. Nonradiative decay in the vapour-phase excited state of the complex TbX, (X = 2,2,6,6-tetramethylheptane-3,5-di0ne),~~~ U.V. emission from tin in Me,Sn-H,-F, the flash photolysis of Et,Sn and tetravinyl tin,34sand the vacuum U.V. photolysis of C,Me4, Si,Me,, and Me3CSiMe3347 have been reported. Mercury-photosensitized photolysis of trichlorosilane produces (SiC13)4Si,348 and of monogermane-NO mixtures gives the products shown in reactions (114)-(119).349 Stimulated emission in dyes in the vapour phase has been o b ~ e r v e d351 .~~~~ GeH,

+ Hg(3P1) GeH,

333 334 335 336 337 338 338 340 341

34a 343 344 345

346

347 348 348 350

361

+H

----+

GeH, GeH,

+ H + Hg(lS0)

(1 14) (115)

H. G . C. Human and P. J. Th. Zeegers, Spectrochim Acta, 1975, 30B, 203. G. Hager, R. Harris, and S. G. Hadley, J . Chem. Phys., 1975, 63, 2810. S. Rosenwaks, R. E. Steele, and H. P. Broida, J. Chem. Phys., 1975,63, 1963. S. D. Rockwood, Chem. Phys., 1975,10,453. D. J. Wren and M. Menzinger, J. Chem. Phys., 1975, 63, 4557. D. J. Eckstrom, S. A. Edelstein, D. L. Huestis, B. E. Perry, and S. W. Benson, J . Chem. Phys., 1975, 63, 3828. B. G. Wicke, M. A. Revelli, and D. 0. Harris, J . Chem. Phys., 1975, 63, 3120. S. P. Tang, N. G . Untterback, and J. Friichtenicht, J. Chem. Phys., 1976, 64, 3833. R. Behrens, jun., A. Freedman, R. R. Herm, and T. P. Parr, J. Amer. Chem. SOC.,1976, 98, 294. M. A. Revelli, B. G. Wicke, and D. 0. Harris, Chem. Phys. Letters, 1976, 39, 454. J. R. McDonald, Chem. Phys., 1975, 9, 423. R. R. Jacobs, M. J. Weber, and R. K. Pearson, Chem. Phys. Letters, 1975, 34, 80. U. C. Sridharan and D. L. McFadden, J . Chem. Phys., 1975,63, 5061. M. Christianson, D. Price, and R. Whitehead, J. Organometallic Chem., 1975, 102, 273. P. Boudjouk and R. D. Koob, J . Amer. Chem. SOC.,1975, 97,6595. K. G. Sharp, P. A. Sutor, T. C. Farrar, and K. Ishibitsu, J . Amer. Chem. SOC.,1975,97,5612. R. Varma, K. R. Ramaprasad, and A. J. Signorelli, J . Inorg. Nuclear Chem., 1975, 37, 563. B. Steyer and F. P. Schafer, Appl. Phys., 1975,7, 113. N. A. Bonsevich, Spectroscopy Letters, 1975, 8, 607.

146 GeH,

+ NO

2GeH,ON GeH,ONNOGeH, GeH,O

+ GeH,

___+

Photochemistry GeH,ON

(116)

GeH,ONNOGeH,

(117)

2GeH,O

+ N,

(118)

GeH,OGeH,

(1 19)

10 Laser Isotope Separation The dramatic upsurge of interest in this area, for obvious commercial reasons, is evidenced by the large number of papers appearing on the subject during the past year, including two Many of these studies use very high powered i.r. lasers which selectively excite low-lying vibrational transitions in isotopic mixtures of simple molecules, and decompose these through multiphoton i.r. absorption by way of processes which are as yet not at all well understood. Papers on these subjects are simply mentioned here, and attention is focused on the few papers which report isotope separation through electronic excitation. Amongst subjects in the former category are included separation of D2,354 chlorine isotopes through HCl,365methylene CC14,357and CF2CICF,CI 358 i.r. laser irradiation, loB and llB separation using BC13,369-361 sulphur isotope separation using SF6,362i.r.-laser-driven specific reactions of the type shown in reaction (120),363an i.r.-laser-driven thermal explosion in C2H51,364 B(CH,),Br,

+ HBr

-

B(CH3),+lBrm+l

+ CH,

(120)

dissociation of NH, to ground-state fragments with a CO, and several others dealing with various aspects of i.r. laser photolysis and isotope enrichment.365-383 ssa 363 354

356

s67

868

36D

361 s62

V. S. Letokhov and C. B. Moore, Kvantovaia Elektronika, 1976, 3, 248. V. S. Letokhov and C. B. Moore, Kvantovaia Elektronika, 1976, 3, 485. J. B. Marling, Chem. Phys. Letters, 1975, 34, 84. D. Amoldi, K. Kaufmann, and J. Wolfrum, Phys. Rev. Letters, 1975, 34, 1597. A. Yogev and R. M.J. Benmair, J. Amer. Chem. SOC.,1975, 97, 4430. R. V. Ambartzumian, Yu. A. Gorokhov, V. S.Letokhov, G. N. Makarov, and A. A. Puretzki, Phys. Letters, (A), 1976, 56, 183. R. N. Zitter and D. F. Koster, J. Amer. Chem. Soc., 1976, 98, 1613. R. V. Ambartsumyan, Yu. A. Gorokhov, V. S. Letokhov, G. N. Makarov, E. A. Ryabov, and N. V. Chekalin, Kvantovaia Elektronika, 1975, 2, 2197. S. D. Rockwood and J. W. Hudson, Chem. Phys. Letters, 1975, 34, 542. V. N. Bourimov, V. S. Letokhov, and E. A. Ryabov, J. Photochem., 1976, 5, 49. R. V. Ambartsumyan, Yu. A. Gorokhov, V. S.Letokhov, and G. N. Makarov, JETP Letters, 1975, 21, 171.

363 384

3B5

367

368 368 370

371 37a

s7s 874

H. R. Bachmann, H. Noth, R. Rinck, and K. L. Kompa, Chem. Phys. Letters, 1975,33,261. J. C. Bellows and F. K. Fong,J . Chem. Phys., 1975, 63, 3035. I. Tanaka, Uyo Butsuri, 1975, 44, 1098. E. M. Belenov, V. A. Isakov, E. P. Markin, and V. I. Romanenko, Kvantovaia Elektronika, 1975,2, 1629; E. M. Belenov, V. A. Isakov, and V. I. Romanenko, ibid., 1633. R. V. Ambartsumyan, Yu. A. Gorokhov, V. S. Letokhov, G. N. Makarov, E. A. Ryabov, and N. V. Chekalin, Kvantovaia Elektronika, 1976, 3, 802. A. M. Ronn, Spectroscopy Letters, 1975,8, 302. F. Klein, F. M. Lussier, and J. I. Steinfeld, Spectroscopy Letters, 1975,8, 247. N. G. Basov, A. N. Oraevsky, and A. V. Pankratov, Kvantovaia Elektronika, 1976,3, 814. R. V. Ambartsumyan, N. V. Chekalin, V. S. Letokhov, and E. A. Ryabov, Chem. Phys. Letters, 1975, 36, 301. B. F. Gordiets, and Sh. S. Mamedov, Kvantovaia Elektronika, 1975,2, 1992. F. S. Klein and J. Ross, J. Chem. Phys., 1975, 63, 4556. V. M. Akulin, S. S. Alimniev, and N. V. Karlov, Pisma Zhur. eksp. i teor. Fiz., 1975, 22, 100.

147

Gas-phase Photoprocesses

The technique of selective electronic excitation using Doppler-free two-photon excitation processes for isotope separation has been commented 38s The transitions used in the selective excitation of 236Uto an ionized (and thus separable) level are shown in Figure 5.38e The 514.5 nm argon ion laser line has -L-IONIZATION POTENTIAL =

I

'\IEN 23433 J = 5CM-'

6.187c v

IONIZER

f-L

EXCITED STATE, T O E C A Y

I O - ~s

EXCITER

A= 4266.275 u 1 0 - lcm2 ~

Figure 5 Schematic level diagram for a typical two-photon selective laser ionization (Reproduced b y permission from I.E.E.E. J. Quantum Electronics, 1976, 12, 111)

been used selectively to photodissociate the Br2(B3IIOu+) and selective dissociation of ortho-I, 387-389 has also been achieved. Carbon isotope separation based upon predissociation of formaldehyde vapour selectively excited has been In this study a 1 : 10 mixture of 12CH20: 13CHz0 was enriched 80-fold in 12CH,0 in this manner, NO enhancing the enrichment rate. Both N and C isotopes can be enriched in the 10 mW CW 55.5 nm dye-laser photolysis (AA = 0.005nm) of ~yrn-tetrazine.~~~ Each of the HCN and N2products and starting materials can be altered in isotopic composition by change in wavelength in a single step by a factor of 72, with an enrichment rate of 17 moles separation 376 378 377 378

370

380

R. V. Ambartsumyan and Yu. A. Gorokhov, Pisma Zhur. eksp. i teor. Fiz., 1975,22,374. C. T. Lin, Spectroscopy Letters, 1975, 8, 901. U. Devi and M. Mohan Phys. Letters (A), 1975, 53A, 421. R. V. Ambartsumyan and N. V. Chekalin, Chem. Phys. Letters, 1975,36, 301. V. M. Akulin, S. S. Alimpier, and N. V. Karlov, Zhur. eksp. teor. Fiz., 1975,69, 836. K. Bergrnann, S. R. Leone, R. G. MacDonald, and C. B. Moore, Israel J. Chem., 1975, 14, 105.

381

3a3 3a3 384

3a6 386 887

3s8

J. Bron, Canad. J. Chem., 1975,53,3069. J. H.Birely and J. L. Lyman, J. Photochem., 1975,4, 269. S. Datta, R. W. Anderson, and R. N. Zare, J . Chem. Phys., 1975, 63, 5503. K. Shimoda, Appl. Phys., 1976, 9, 239. F. Shimiza and K. Namba, Phys. Letters (A), 1975,54, 179. G. S. Janes, I. Itzkan, C. T. Pike, R. H. Levy, and L. Levin, I.E.E.E. J. Quuntwn Electronics, 1976,12, 1 1 1. V. S. Letokhov and V. A. Semchishen, Doklady Akad. Nuuk S.S.S.R.,1975,222,1071. V. S. Letokhov and V. A. Semichishen, Spectroscopy Letters, 1975,8,263. J. C . Lehmann, Kvuntovaia Elektroniku, 1976,3, 811. J. H. Clark, Y. Haas, P. L. Houston, and C. €3. Moore, Chem. Phys. Letters, 1975,35, 82.

Photochemistry

148

per kWh laser energy possible. Further prospects for the use of dye-lasers in this field have received comment,301and isotope separation involving photoinduced changes in the electrical and magnetic properties of molecules and atoms has been 11 Atmospheric Photochemistry

Lack of space has necessitated the contraction of this important section to discussion of very few photochemically biased papers with cryptic comment on many others. Papers of more general interest are listed first, followed by discussion of the photochemistry and physics of atmospheric constituents. Extraterrestrial Phenomena.-Recent publications have discussed the photoelectric heating of interstellar gas,393the ionizing flux of cosmic background radiation,394 and its effect on the abundance of highly ionized interstellar the radiative lifetimes of states involved in transitions in sulphur and silicon observed in interstellar space,396and interstellar SO, and sulphur reactions.397 Na' and Ca11,398 HDC0,399photochemical formation of cometary free radicals,400the solar ~ p e c t r u m , ~excited-state ~ ~ - ~ ~ ~ reactions in planetary the photochemistry of hydrocarbons 408 and other organic compounds *09 in Jupiter, electronic absorption in atmospheric H, in Jupiter and and i.r. emission 411 and other problems in the Venusian atmosphere 412 have also been discussed. Thermospheric and Stratospheric Reactions.-The destruction by sunlight of C03-,H20and 03-,413 and the aeronomy of odd nitrogen in the thermosphere *14 and in auroral emissions415have been discussed. Reactions of O3 in the stratoV. S. Letokhov, Spectroscopy Letters, 1975, 8, 697. P. L. Kelley, N. M. Kroll, and C. K. Rhodes, Optics Comm., 1976, 16, 172. 393 M. Jura, Astrophys. J., 1976, 204, 12. 384 J. Silk and R. A. Sunyaev, Nature, 1976, 260, 508. 386 W. D. Watson, Astrophys. J., 1976, 204, 47. 3g6 A. E. Livingston, H. Garnir, Y. Baudinet-Robinet, P. D. Dumont, E. Biemont, and N. Grevesse, Astrophys. Letters, 1976, 17, 23. 397 (a) L. E. Snyder, J. M. Hollis, B. L. Ulich, F. J. Lovas, D. R. Johnson, and D. Buhl, Astrophys. J., 1975, 198, L81; (b) J. P. Liddy, C. G. Freeman, and M. J. McEwan, Astrophys. Letters, 1975, 16, 159. 388 L. M. Hobbs, Astrophys. J., 1975, 202, 628; R. M. Crutcher, ibid., 634. 888 W. D. Watson, R. M. Crutcher, and J. R. Dickel, Astrophys. J., 1975, 201, 102. 400 W. M. Jackson, J. Photochem., 1976, 5, 107. 401 L. Heroux and R. A. Swirbalus, J. Geophys. Res., 1976, 81, 436. 40a (a)R. S. Stolarski, P. B. Hays, and R. G. Roble, J. Geophys. Res., 1975, 80, 2266; (6) E. M. Reeves, J. E. Vernazza, and G. L. Withbroe, Phil. Trans. Roy. SOC., 1976, A281, 319; R. M. Bonnet, ibid., p. 305. 403 C.-C. Cheng and K. G. Widing, Astrophys. J., 1975, 201, 735. 404 J. E. Ross and L. H. Aller, Science, 1976, 191, 1223. 4 0 5 E. Chipman and E. C. Bruner, jun., Astrophys. J., 1975,200,765. (a) R. W. Milkey, Astrophys. J., 1975,199, L131; (6) P. Turon, Solar Phys., 1975,41,271. G. W. Adams, Planetary Space Sci., 1975, 23, 1293. 408 S. S. Prasad, L. A. Capone, and L. J. Schneck, Geophys. Res. Letters, 1975,2, 161. 409 J. P. Fems and C. T. Chen, Nature, 1975,258, 587. p10 T. Z. Martin, D. P. Cruikshank, C. B. Pilcher, and W. M. Sinton, Icarus, 1976, 27, 391. a1 R. E. Dickinson, J. Atmos. Sci., 1976, 33, 290. 41a N. D. Sze and M. B. McElroy, Planetary Space Sci., 1975, 23, 763. 413 J. R. Peterson, J. Geophys. Res., 1976,81, 1433. 414 E. S. Oran, P. S. Julienne, and D. F. Strobel, J. Geophys. Res., 1975, 80, 3068. 415 A. Vallancejones and R. L. Gattinger, J. Geophys. Res., 1976,81,497.

3g1 302

Gas-phasePhotoprocesses

149

sphere continue to dominate discussion of processes occurring in this region, particularly with regard to effects of chlorofluoromethanes (Freons) transported from the troposphere. There have been several papers reviewing the reactions of 0, in the ~ t r a t o s p h e r e , ~including ~ ~ - ~ ~ ~those with chlorine from F r e o n ~ . ~ ~ ~ Estimates of concentrations of Freons in the a t m o ~ p h e r e , 421 ~ ~ the ~ 1 u.v.-absorption of fluorocarbon^,^^^ and the photochemical reactions of Freons with regard to ozone depletion 423-430 have been discussed, and the sources of atmospheric chlorine c o n ~ i d e r e d . ~ ~The l - ~ ~U.S. ~ ban on use of aerosols has provoked 437 and Papers concerned with effects of 0, of nuclear explosions,436* of NO produced by SST aircraft438-440and other sources441v442 are often in contention, as are those on 0, healing mechanism^.^^^-^^^ Despite the very large amount of work published in this field, it is clear that further studies will be required to achieve the desired detailed understanding of stratospheric O3chemistry necessary before human activities can be regarded as inoffensive or otherwise. There have been several recent papers on modelling of stratospheric reaction^,^^^-^^^ 416

417

419 420 421

422

Oa4 426 426

S. Johnston, Ann. Rev. Phys. Chem., 1975,26, 315; (6) H. S. Johnston, Rev. Geophys. Space Phys., 1975, 13, 637. M. Nicolet, Rev. Geophys. Space Phys., 1975, 13, 593. S. Cieslik, La Recherche, 1976, 7, 510. G. B. Lubkin, Physics Today, 1975, 28, 34. L. E. Heidt, R. Lueb, W. Pollock, and D. H. Ehhalt, Geophys. Res. Letters, 1975,2,445. (a) P. W. Krey, R. J. Lagomarsino, and J. J. Frey, J. Geophys. Res., 1976,81, 1557; (b)A. L. Schmeltekopf, P. D. Goldan, W. R. Henderson, W. J. Harrop, T. L. Thompson, F. C. Fehsenfeld, H. I. Schiff, P. J. Crutzen, I. S. A. Isaksen, and E. E. Ferguson, Geophys. Res. Letters, 1975, 2, 339; (c) L. Zafente, N. E. Hester, E. R. Stephens, and 0. C. Taylor, Atmos. Environment, 1975,9, 1007; ( d ) R. J. Cicerone, D. H. Stedman, and R. S. Stolarski, Geophys. Res. Letters, 1975,2, 219. C. Sandorfy, Atmos. Environment, 1976, 10, 343. N. E. Hester, E. R. Stephens, and 0. C. Taylor, Atmos. Environment, 1975, 9, 603. Y. L. Yung, S. C. Wofsy, and M. B. McElroy, Geophys. Res. Letters, 1975, 2, 397. R. S. Stolarski and R. D. Rundel, Geophys. Res. Letters, 1975, 2, 443. F. S. Rowland and M. J. Molina, Science, 1975,190, 1038; T. L. Cairns and J. P. Jesson, ibid., (a) H.

1038. S. C. Wofsy, M. B. McElroy, and N. D. Sze, Science, 1975,187, 535. a8 A. Appelby, D. Lillian, and H. B. Singh, Geophys. Res. Letters, 1976, 3, 327; Y. L. Yung, M. B. McElroy, and S. C. Wofsy, ibid., 238. 42g D. E. Robbins, Geophys. Res. Letters, 1976, 3, 213. 430 (a) R. P. Turco and R. C. Whitten, Atmos. Environment, 1975,9, 1045; (6) T. M. Donohue, R. J. Cicerone, S. C. Liu, and W. L. Chameides, Geophys. Res. Letters, 1976, 3, 105. 431 K. A. Rahn, R. D. Borys, and R. A. Duce, Science, 1976,192,549. 43a T. E. Graedel and D. L. Allara, Atmos. Environment, 1976,10,385. 433 P. H. Howard and A. Hanchett, Science, 1975, 189, 217. 434 J. A. Ryan and N. R. Mukherjee, Rev. Geophys. Space Phys., 1975, 13, 650. 436 C. Norman, Nature, 1975, 255, 571. 436 E. Bauer and F. R . Gilmore, Rev. Geophys. Space Phys., 1975,13,451. R. C. Whitten, W. J. Borucki, and R. P. Turco, Nature, 1975,257, 38. 430 H. S. Johnston, D. Kattenhorn, and G. Whitten, J . Geophys. Res., 1976, 81, 368. H. S. Johnston, Accounts Chem. Res., 1975, 8, 289. 4 4 0 F. M. Luther, Science, 1976, 192, 49. 441 G. E. Sinelkikova, Fiz. Atmosfery i Okeana, 1976, 12, 106. 442 P. J. Crutzen, Geophys. Res. Letters, 1976, 3, 169. 44s W. L. Chameides and J. C. G. Walker, Science, 1976, 191, 338. 444 W. L. Chameides and J. C. G. Walker, Science, 1975,190, 1294. 446 R. S. Scorer, Atmos. Environment, 1976,10, 177. 446 R. N. Gupta and W. L. Grose, AIAA Journal, 1975, 13, 792. 447 B. Martin, J. Atmos. Sci., 1976, 33, 131. 448 D. J. Wuebbles and J. S. Chang, J. Geophys. Res., 1975, 80, 2637. 449 R. A. Beck, Science, 1976, 192, 559. 4 5 0 A. V. Artemyev, Fiz. Atmosfery i Okeana, 1975, 11, 1161. 427

Photochemistry effects of cosmic rays on ozone concentration^,^^^ heterogeneous reactions in the and stratospheric aerosol formation.463 150

Tropospheric Reactions and Pollutants.-There have been reports on various aspects of photochemical smog and ozone formation in Los A n g e l e ~ New ,~~~ Y ~ r kPhiladelphia,456 , ~ ~ ~ Britain,458The the S.E. AtlanticYae1 Iraq,462and the Several papers concerned with modelling of photochemical smog have appeared,464-46Q and general descriptions of the phenomenon have been r e p ~ r t e d . ~Articles ~ ~ - ~ concerned ~~ particularly with tropospheric O Z O ~ ~aerosol , ~ f~o ~ r r n- a~t i ~ n~, ~NO, ~~-~ ~~ reaction^,^^^-^^^ 451

4ba 453

4h4 465

456 467

Q68 468 460

462

463

465

466

467 468

Q6D 470 471 472

N. G. Skryabin, Y. G. Shafer, V. D. Sokolov, V. G. Moiseev, and Y. S. Semenov, Izuest. Akad. Nauk S.S.S.R., Ser. fiz., 1976, 40, 601. R. D. Cadle, P. Crutzen, and D. Ehhalt, J. Geophys. Res., 1975, 80, 3381. (a) R. G. Pinnick, J. M. Rosen, and D. J. Hofmann, J. Atmos. Sci., 1976, 33, 304; (b) A. B. Harker, J, Geophys. Res., 1975, 80, 3399. J. G. Calvert, Environ. Sci. Technol., 1976, 10, 256. (a) W. S . Cleveland, B. Kleiner, and J. E. McRae, Science, 1976, 191, 179; (b) D. C. McLean and R. E. Schneider, . I Enuiron. . Quality, 1976, 5, 75. W. S. Cleveland and B. Kleiner, Environ. Sci. Technol., 1975, 9, 869. J. S. Jacobsen, H. C. McKee, and F. Lebron, Atmos. Environment, 1975, 9, 1040. P. D. Moore, Nature, 1975, 256, 537. H. Nieboer and J. van Ham, Atmos. Environment, 1976,10, 115. P. Winkler, Meteorologische Rundschau, 1975, 28, 131. R. F. Stallard, J. M. Edmond, and R. E. Newell, Geophys. Res. Letters, 1975,2, 289. M. A. Abbas and M. K. Elnesr, Pure. Appl. Geophys., 1974, 112, 753. D. W. Reynolds, T. H. Vonder Haar, and S. K. Cox, J. Appl. Meteorology 1975,14,433. M.-K. Liu and J. H. Seinfeld, Atmos. Environment, 1975, 9, 555. W. L. Chameides and J. C. G. Walker, J. Geophys. Res., 1976, 81, 413. W. L. Chameides and J. C. G. Walker, Amer. J. Sci., 1975, 275, 737. T. J. Lyons and D. R. Cutten, Atmosph. Environment, 1975,9,731. W. S. Cleveland and B. Kleiner, J. Air. Pollution Control ASSOC.,1976, 26, 36. W. B. Sisson and M. M. Caldwell, Photochem and Photobiol., 1975, 21,453. E. R. Stephens, J. Air PolIution Control ASSOC.,1975, 25, 521. B. M. Smirnov, Uspekhi Fiz. Nauk, 1975, 117, 313. D. S. Covert, R. J. Charlson, R. Rasmussen, and H. Harrison, Rev. Geophys. Space Phys., 1975, 13, 765.

J. Heicklen, Nature, 1976, 259, 524. B. J. Finlayson and J. N. Pitts, jun. Science, 1976, 192, 111. 4 7 b M. Schmidt, P. Fabian, and H. Tiefenau, Pure Appl. Geophys., 1974, 112, 887; P. Fabian, ibid., p. 901; H. U.Dutsch, ibid., p. 915. 4 7 6 R. Chatfield and H. Harrison, J. Geophys. Res., 1976,81,421; W. L. Chameides and J. C. G. Walker, ibid., p. 424. 417 D. L. Fox and R. Kamens, Science, 1975, 188, 1113. 478 I. J. Barton and D. F. Robertson, Nature, 1975, 258, 68. 479 R. A. Cox, A. E. J. Eggleton, and R. G. Dement, Nature, 1975,255, 5504. o0 W.G. Zdunkowski and K.-N. Liou, Tellus, 1976, 28, 31. 461 J. B. Pollack, 0. B. Toon, A. Summers, W. Van Camp, and B. Baldwin, J. Appl. Meteorology, 473

474

1976, 15, 247.

482 453

484 496 4s6 487

4R8

49u

4R2 483

K.-N. Liou and T. Sasamori, J. Atmos. Sci., 1975, 32, 2166. R. F. Pueschel and P. M. Kuhn, J. Geophys. Res., 1975, 80, 2960. K.-N. Liou, J. Atmos. Sci., 1976, 33, 798. C. Tornasi, R. Guzzi, and 0. Vittori, J. Atmos. Sci., 1975, 32, 1580. A. S. Britayev and G. P. Faraponova, Fiz. Atmosfery i Okeana, 1976,12, 103. N. Fuchs, Atmos. Environment, 1975, 9, 697. R. B. Husar and W. H. White, Atmos. Environment, 1976,10, 199. R. J. O’Brien and J. H. Crabtree, Environ. Sci. Technol., 1975, 9, 577. R. J. O’Brien and J. R. Holmes, Environ. Sci. Technol., 1975, 9, 568. S. C. Liu, R. J. Cicerone, T. M. Donahue, and W. L. Chameides, Geophys. Res. Letters, 1976, 3, 157. C. W. Spicer and D. F. Miller, J. Air Pollution Control ASSOC.,1976, 26, 45. D. H. Stedman, R. J. Cicerone, W. L. Chameides, and R. B. Harvey, J. Geophys. Res., 1976, 81, 2003.

151

Gas-phase Pho toprocesses

ha loge no carbon^,^^^ ~ l e f i n SO2 , ~ ~pollution ~ and a e r o s o I ~ , ~and ~~-~~ inhibition of photochemical smog 6 0 0 ~610 have been published. Papers of particular kinetic and photochemical interest are discussed in more detail below. Readers are also strongly recommended to the proceedings of a symposium on ‘Chemical Kinetics Data for the Upper and Lower Atmosphere’ held in Warrenton, Virginia, September 1974.611 This valuable document contains a wealth of useful information too extensive to be incorporated into this review. Detection and Estimation of Atmospheric Pollutants and Constituents.-New methods of monitoring atmospheric constituents have been r e ~ i e w e d . ~ l ~ - ~ Differential absorption of laser radiation has been applied to sensing of ozone,616-618water v a p o ~ r61g, ~0~2, 616 ~ ~SO 2, 621 CO29 622 CH4, 623 propane and butane,624N02,525 and HC1,626and refractive index effects on such measurements 627 have been described. Laser-induced fluorescence methods have been applied to NO2528, 529 and other species emitted from aircraft engines,629 620s

4g4

S. C. Wofsy, M. B. McElroy, and Y. L. Yung, Geophys. Res. Letters, 1975, 2, 215. R. A. Cox, R. G. Denvent, A. E. J. Eggleton, and J. E. Lovelock, Atmos. Environment, 1976,

10, 305. C. K. K. Yeung and C. R. Phillips, Environ. Sci. Technol., 1975, 9, 732. 487 R. E. Train, Science, 1975, 189, 748. 4ga C.-H. Shen and G. S . Springer, Atmos. Environment, 1976, 10, 235. 499 J. R. Richards, D. L. Fox, and P. C. Reist, Atmos. Environment, 1976, 10, 211. R. Smith, R. G. De Pena, and J. Heicklen, J. Colloid Interface Sci., 1975, 53, 202. P. P. Smith and L. D. Spicer, Chemosphere, 1975, 4, 131. 6oa W. E. Clark and K. T. Whitby, J. Colloid Interface Sci., 1975, 51, 477. L. Newman, J. Forrest, and B. Manowitz, Atmos. Environment, 1975,9, 959,969. T . E. Graedel, Geophys. Res. Letters, 1976, 3, 181. M. T. Dana, J. M. Hales, and M. A. Wolf, J. Geophys. Res., 1975, 80, 4119. F. B. Smith and G. H. Jeffrey, Atmos. Environment, 1975, 9, 643. P. Haagenson, Atmos. Environment, 1975, 9, 770. N. B. Cuong, R. Bonsang, G. Lambert, and J. L. Pasquier, Pure Appl. Geophys., 1975, 113, 489. L. Stockburger, tert. and J. Heicklen, Atmos. Environment, 1976, 10, 51. & l o Y. V. Nguyen and C. R. Phillips, Chemosphere, 1975, 4, 125. 4g6

611

sls 614

616

&16

618

61s

620

&ll 622 628 624

s26

s2*

Proctedings of Symposium on Chemical Kinetics Data for the Upper and Lower Atmosphere, Warrenton, Virginia, September 1974, published in Internat. J. Chemical Kinetics, 1975, 7, No. 1 Supplement. R. Perry and R. M. Harrison, Chem. in Britain, 1976, 12, 185. A. Liberti, Pure Appl. Chem., 1975, 44, 519. H. Walter, jun. and D. Flanigan, Appl. Optics, 1975, 14, 1423. F. I. Shimabukuro, P. L. Smith, and W. J. Wilson, J. Geophys. Res., 1975, 80, 2957. P. Rabache and B. Rebours, Infrared Phys., 1975, 15, 179. B. Carli, D. H. Martin, E. Puplett, and J. E. Harries, Nature, 1975, 257, 649. J. Shewchun, B. K. Garside, E. A. Ballik, C. C. Y.Kwan, M. M. Elsherbiny, G. Hogenkamp, and A. Kazandjian, Appl. Optics., 1976, 15, 340. (a) J. W. Brault, J. S . Fender, and D. N. B. Hall, J. Quant. Spectrosc. Radiative Transfer, 1975, 15,549; (6)E. R. Murray, R. D. Hake, jun., J. E. van der Laan, and J. G. Hawley, Appl. Phys. Letters, 1976, 28, 542. R. T. Thompson, jun., J. M. Hoell, jun., and W. R. Wade, J. Appl. Phys., 1975,46,3040. D. A. Johnson and D. H. F. Atkins, Atmos. Environment, 1975,9,825. P. S. Gillespie, R. L. Armstrong, and K. 0 . White, Appl. Optics, 1976, 15, 865. K. 0. White and W. R. Watkins, Appl. Optics, 1975, 14, 2812. W. R. Watkins and K. 0. White, Appl. Optics, 1976,15,1114; F . I. Shimabukuro, S. R. King, T. S. Hartwick, E. E. Reber, and D. J. Spencer, ibid., p. 1115. J. F. Noxon, Science, 1975, 189, 547. M. Ackerman, D. Frimout, A. Girard, M. Gottignies, and C. Muller, Geophys. Res. Letters, 1976, 3, 81.

627 628 6ag

D. E. Snider, J. Atmos. Sci., 1975, 32, 2178. A. W. Tucker, M. Birnbaum, and C. L. Fincher, Appl. Optics, 1975, 14, 1418. D. A. Leonard, Opt. Quantum Electronics, 1975,7, 197.

Photochemistry

152

and other species,531,532 including polynuclear U.v.-laser sounding of the troposphere and lower stratosphere has been disLaser scattering techniques have been applied to N2+y534 CHI and NH3,6SO2 and 03.535 Other methods employed include spectrophone measurements (on air),536a Dobson spectrophotometer (on N02),537Lynian 01 radiation (on 02),538 chemiluminescence (on S02),539(and 03),540photolytic detection (on H2S),541and mass spectrometry/g.l.c. (on c h l o r ~ f l u ~ r ~ c a r b oInstrumental n~).~~~ methods for determination of photochemical smog have been d e s ~ r i b e d . ~ ~ ~ Specific measurements have been made on 0 atom^,^^^-^^^ H 554 Of and H+,555 N atomSy556,557 02, 551, 552, 554, 558 OH 648, 559, 560 NO 561-565 03,564, 565, 566-568 N 0 2 , 5 6 1 , 562, 566, 569, 570 CO 571 HF 572 HC' 573 HN03,561,574 N 0 421b H 0 562 Y Y Y 2 Y 2 Y 9

630 631 633 633

634 635

636 637 b38

638 640 641 642 643

644 646

s46 647

64a

649

550

661 652

s63 654

5s6 666

657

6s8 669

663

5e4

s66 566 667

668 SBg

570 5'1 672

673 574

Y

V. N. Bagratashvili and I. N. Knyazev, Opt. Comm., 1975, 14, 426. S. V. Babu and Y. V. Chalapati Rao, Chem. Phys. Letters, 1976, 37, 249. G . Heinrich and H. Gusten, Z. Analyt. Chem., 1976, 278, 257. A. J. Gibson and L. Thomas, Nature, 1975, 256, 561. J. E. Solomon and D. M. Silva, J. Appl. Phys., 1976, 47, 1519. W. B. Grant and R. D. Hake, jun., J. Appl. Phys., 1975, 96, 3019. B. G. Ageev, A. B. Antipov, A. A. Pomeshchenko, and Yu. N. Ponomarev, Optika i Spektroskopiya, 1976, 40, 600. R. N. Kulkarni, J. Atmos. Sci., 1975, 32, 1641. L. H. Weeks, J. Geophys. Res., 1975, 80, 3655, 3661. J. Stauff and W. Jaeschke, Atmos. Environment, 1975, 9, 10. H. C. McKee, J. Air Pollution Control ASSOC.,1976, 26, 124. J. J. Young and J. N. Zemel, Appl. Phys. Letters, 1975, 27, 45. E. P. Grimsrud and R. A. Rasmussen, Atmos. Enoironrnent, 1975,9, 1010, 1014. J. Oritz de Landaluce, Quim. Ind. (Madrid), 1975, 21, 521. S. Konno and T. Okita, Koshu Eiseiin Kenkyu Hokoku, 1974, 23, 50. R. L. Grob and M. A. Kaiser, Environmental Letters, 1975, 8, 235. J. Mulik, M. Cooke, and M. F. Guyer, Analyt. Letters, 1975, 8, 511. R. G. Roble, .I. F. Noxon, and J. V. Evans, Planetary Space Sci., 1976, 24, 327. R. E. Good, Planetary Space Sci., 1976, 24, 389. G. E. Thomas and D. E. Anderson, jun., Planetary Space Sci., 1976,24, 303; D. J. Strickland and G. E. Thomas, ibid., p. 313. D. W. Rusch, W. E. Sharp, and P. B. Hays, J . Geophys. Res., 1975, 80, 1832. E. S. Oran and D. F. Strobel, J. Geophys. Res., 1976, 81, 257. A. 0. Nier, W. E. Potter, and D. C. Kayser, J. Geophys. Res., 1976, 81, 17. J. G. Anderson, Geophys. Res. Letters, 1975, 2, 231. G. G. O'Connor, J . Atmos. Terrestrial Phys., 1976, 38, 377, 383. J. A. Murphy, G. J. Bailey, and R. J. Moffett, J. Atmos. TerrestrialPhys., 1976,38, 351. T. Ogawa and T. Shimazaki, J. Geophys. Res., 1975,80,3945. M. R. Torr, R. G. Burnside, P. B. Hays, A. I. Stewart, D. G. Torr, and J. C. G. Walker, J. Geophys. Res., 1976, 81, 531. R. C. Schaeffer and J. F. Noxon, Planetary Space Sci., 1975, 23, 1413. J. G. Anderson, Geophys. Res. Letters, 1976, 3, 165. C. C. Wang, L. D. Davis, jun., and C. H. Wu, Science, 1975, 189, 797. M. Ackerman, J. Atmos. Sci., 1975, 32, 1649. C. P. Chaloner, J. R. Drummond, J. T. Houghton, R. F. Jarnot, and H. K. Pascoe, Nature, 1975, 258, 696. D. W. Rusch and C. A. Barth, J. Geophys. Res., 1975, 80, 3719. M. Loewenstein and H. F. Savage, Geophys. Res. Letters, 1975, 2, 448. M. Loewenstein, H. F. Savage, and R. C. Whitten, J. Atmos. Sci., 1975, 32, 2185. R. N. Kulkarni, Quant. J. Roy. Meteorological Soc., 1976, 102, 461. J. G. Breiland, J. Geophys. Res., 1976, 81, 1991. M. A. Ruderman, H. M. Foley, and J. W. Chamberlain, Science, 1976,192, 555. H. Axelrod, J. Miller, D. Pack, and J. Thompson, Tellus, 1976, 28, 95. M. A. Goldman, Tellus, 1976, 28, 96. T. Kassal, Appl. Optics., 1975, 14, 1513. R. Zander, Compt. rend., 1975, 281, B, 213. C. B. Farmer, 0. F. Raper, and R. H. Norton, Geophys. Res. Letters, 1976, 3, 13. D. G. Murcray, D. B. Barker, J. N. Brooks, A. Goldman, and W. J. Williams, Geophys. Res. Letters, 1975, 2, 223.

Gas-phase Pho toprocesses

153

fluorocarbons,*zlb~ 421e and (HzS04) aerosol, SO3, and 575 in the thermosphere, stratosphere, and various points in the troposphere, using a variety of techniques. Rare Gases.-Photoionization of rare e x ~ i m e r and , ~ ~ exciplex ~ (alkali-noble gas) emissions arising from these atoms, their reactions with and calculations on the diatomic rare gas ions 681 and polyatomic lifetimes of the 3Pz and 3P0states of the species 683 have been reported. Other papers have discussed the energy dependence of the reaction between He+ and NH3,684the determination of the single-photon transition rate between Z3S1 and llSo states of He(I), measured as 1.1 x s-1,685 population measurements on excited He and Ne atoms in interactions of excited (3lP) and ground-state He atoms,687-58Q excited He (3%, 33S, and 33P) states with Ne, Ar, Kr, and Xe,690these being of importance in the upper atmosphere and in gas lasers; collisions between He and Ne, Ar, Kr, and Xe;691studies on He-Hg lasers;263.5 Q 2 and excitation of Ne atoms by 4.6 eV He r n e t a s t a b l e ~ . ~ ~ ~ Photoionization of neon 594 and fluorescence yields of multiply ionized neon 595 have been discussed. The lifetimes of the lP1and 3P1levels of Ne have been given as 1.65 k 0.16 and 20.5 k 1.5 ns, respectively 596 and calculations on lifetimes of higher levels have been The Doppler-free two-photon spectroscopy 576

R. L. Thomas, V. Dharmarajan, G. L. Lundquist, and P. W. West, Analyt. Chem., 1976,48, 639.

676

677 678

J. A. R. Samson and J. L. Gardner, Phys. Rev. (A), 1975,12, 1459. R. F. Stebbings, F. B. Dunning, and R. D. Rundell, Atomic Phys., 1975,4, 713. L. A. Lompre, G. Mainfray, C. Manus, S. Repoux, and J. Thebault, Phys. Rev. Letters, 1976, 36, 949.

579

680

681 682

683

m4 685

686

687 688

68g

690 6g1

692

693 694 6g6 696

687 698 69g

8oo 601

802

603 604 606

607

V. Yakhot and R. B. Gerber, Chem. Phys., 1975,8, 366. A. Tam, G. Moe, W. Park, and W. Happer, Phys. Rev. Letters, 1975, 35, 85. W. B. Maier, J. Chem. Phys., 62, 4615. T. Sakurai, I. M. Littlewood, and C. E. Webb, Appl. Phys. Letters, 1976, 2, 533. N. E. Small-Warren and L.-Y. C. Chiu, Phys. Rev. (A), 1975,11, 1777. W. Lindinger, D. L. Albritton, and F. C. Fehsenfeld, J. Chem. Phys., 1975,62,4957. J. R. Woodworth and H. W. MOOS,Phys. Rev. (A), 1975,12,2455. K. Miyazaki, R. Nakata, Y. Tomita, M. Suemitsu, S. Watanabe, and K. Fukuda, Japan J. Appl. Phys., 1975, 14, 1075. W. J. Steets and N. F. Lane, Phys. Rev. (A), 1975, 11, 1994. C. B. Collins and B. W. Johnson, J. Chem. Phys., 1976, 64, 2605. M. G. Payne, G. S. Hurst, M. H. Nayfeh, J. P. Judish, C. H. Chen, E. B. Wagner, and J. P. Young, Phys. Rev. Letters, 1975, 35, 1154. S. Kubota, C. Davies, and T. A. King, J. Phys. (B), 1975, 8, 1220. J. C. Brenot, D. Dhuicq, J. P. Gauyacq, J. Pommier, V. Sidis, M. Barat, and E. Pollack, Phys. Rev. (A), 1975, 11, 1933. E. Graham, M. A. Biondi, and R. Johnsen, Phys. Rev. (A), 1976,13,965. E. L. Leasure and C. R. Mueller, J. Appl. Phys., 1976,47, 1062. T. N. Chang and R. T. Poe, Phys. Rev. (A), 1975,12, 1432. C. P. Bhalla, J. Phys. (B), 1975, 8, 1200. N. D. Bhaskar and A. Lund, Phys. Rev. (A), 1976,13, 1484. N. V. Afans’eva and P. F. Grazdev, Optika i Spektroskopiya, 1975,38,1013. F. Biraben, E. Giacobino, and G. Grynberg, Phys. Rev. (A), 1975, 12,2444. P. K. Leichner, J. D. Cook, and S. J. Luerman, Phys. Rev. (A), 1975,12,2501. D. B. King and C. E. Head, Phys. Rev. (A), 1976,13, 1778. P. L. Chapovsky, V. N. Lisitsyn, and A. R. Sorokin, Optics Comm.,1976, 16, 33. W. M. Hughes, T. N. Olson, and R. Hunter, Appl. Phys. Letters, 1976, 28, 81. S. K. Searles and G. A. Hart, Appl. Phys. Letters, 1976,28, 384. M. J. Boxall, C. J. Chapman, and R. P. Wayne, J. Photochem., 1975, 4, 435. J. L. Fraites and D. H. Winicur, J. Chem. Phys., 1976, 64, 89. J. R. McNeely, G. S. Hurst, E. B. Wagner, and M. G. Payne, J. Chem. Phys., 1975,63,2717. L. G. Piper, D. W. Setser, and M. A. A. Clyne, J. Chem. Phys., 1975, 63, 5018.

154 Photochemistry of neon has been investigated.69*Rate constants for reactions (121) and (122) have been given as 1.02 x lo9 and 1.54 x lo8 1mol-1 s-l respectively.599 Ne 3- Ne(lPl)

Ne(8p,)

+ He

-

+ Ne Ne(?S',) + He Ne(3P1)

(121) (122)

Experimental lifetimes for selected laser and other levels of Arf vary between 4 and 9 ns,600and gas lasers based on Ar, Xe, Kr(1) transitions 601 and argon oxide (558 nm) 602 have been described. Energy transfer from Ar*(3P1) to N2,603s 604 reactions of Ar*("9 with HBr to give ArBr*,606and reactions of Ar*(3P1) and Ar*(lP') with N2, H,, and NO have been reported.606 In this last study rate constants were given as 5.4 x 10-l1 (Vl) and 0.8 x 10-l1 (V1) for N2, 22 x (T1) and 21 x (3P1)for H2, and 54 x 10-l1 (TI), 32 x 10-l1 (3P1)for NO, in units of ~ m ~ m o l e c u l e - ~ s Rate - ~ . constants for the transfer of energy from Ar*("3 to Kr to give Kr[Sp(#),] and Kr[Sp(#),] have been reported to be 5.6 x 10-l2 and 6.5 x cm3molecule-l s-l, Excimer emission at 756.5 nm was seen from collisions of Ar(3Po)and Kr.607 The exact mechanism of the energy-transfer process from AI-*(~P,) giving Kr*(3P0,z)has been found to be complex,6o8the direct process not occurring. Excitation of N2(C37ru) by collision with Ar*(3P2,0),603~ 609 U.V. radiation from Ar-Ar atomic collisions 610 and visible (400-500 nm) radiation from Ar2*611 have been observed. Ab initio calculations on Arz(S2&+)and (3&,+) have been carried out.s12 Recent studies on krypton have included discussion of the radiative lifetimes of the Kr(1) atom,613calculations on 614 and vacuum-u.v. emission from Kr2,615 and excitation of Hg(lS,) to Hg(3P1) by collision with Kr*, amongst other partners. 264 Photoionization of Xe metastable atoms616 and Xe atoms in high Rydberg states 617 has been studied. The radiative lifetime of the Xe2(lu)molecular species has been measured as 99 ns, and the rate constant for deactivation of Xe(3P1) to Xe(T,) reported to be 1.5 x lo81 mol-1 s-1.61s Production of XeBr* from collisions of excited Xe atoms with Br, 603 and xenon vacuum-u.v. lasers 619s 620 have been discussed. Atomic and Molecular Hydrogen.-Multiphoton ionization of the H atom 621* 632 and photodetachment of negative hydrogen ions 623 have been reported. Recomeo8 609

611

el2

el3 614

D. H. Winicur, J. L. Fraites, and J. Bentley, J. Chem. Phys., 1976, 64, 1724. W. Lee and R. M. Martin, J. Chem. Phys., 1975, 63, 962. H. L. Rothwell, jun., R. C. Amme, and B. Van Zyl, Phys. Rev. Letters, 1976, 36, 785. A. Birot, H. Brunet, J. Galy, and P. Millet, J. Chem. Phys., 1975, 63, 1469. R. P. Saxon and B. Liu, J. ChemPhys., 1976,64,3291. P. F. Gruzdev and A. V. Loginov, Optika i Spektroskopiya, 1975, 38, 1056. T. L. Barr, D. Dee, and F. R. Gilmore, J. Quant. Spectroscopy Radiative Transfer, 1975, 15,625.

610

H. A. Koehler, L. J. Ferderber, and D. L. Readhead, Phys. Rev. (A), 1975, 12, 968. R. D. Rundel, F. B. Dunning, H. C. Goldwire, jun., and R. F. Stebbings, J. Opt. Sac. Amer., 1975, 65, 628.

W. P. West, G. W. Foltz, F. B. Dunning, C. J. Latimer, and R. F. Stebbings, Phys. Reo. Letters, 1976, 36, 854. 618 P. K. Leichner, K. F. Palmer, J. D. Cook, and M. Thieneman, Phys. Reu. (A), 1976,13, 1787. 619 G. R. Fournier, Opt. Comm., 1975, 13, 385. eao D. J. Bradley, D. R. Hall, and M. H. R. Hutchinson, Opt. Comm.,1975, 14, 1. ezl S. V. Khristenko and S. I. Vetchinkin, Optika i Spekroskopiya, 1976, 40,417. B. J. Choudhury, J. Phys. (B), 1975,8, 1420. M. P. Ajmera and K. T. Chung, Phys. Rev. (A), 1975,12,475.

Gas-phase Pho topro cesses

155 bination reactions of hydrogen reactions of hydrogen atoms with other atoms,626NO, NzO, and C02,e2e02,627 HC1,e28HBr,62ghalogenated me thane^,^ acetylene,630substituted t h i ~ p h e n ,thiirane,e33 ~~~ and silane and germane 634 have been studied. Reactions (123)-(129) are of interest in the H-O2

system. These account for observed excited-state formation (denoted as *, t referring to vibrational excitation). The ratio of rate constants for reactions (130) and (131) was found to vary between 0.7 and 1.5 depending upon conditions.ezs D*

+ HBr

-

D H f Br

(1 30)

+H

(131)

DBr

The rate constant for the addition of hydrogen atoms to acetylene was given in Arrhenius form as k = 9.2 k 2.6 x 10-l2 exp(-2410 f 140)/1.987T).e30 Laser photodissociation of HD+ ions e36 and dissociative photodissociation of H2e3shave been reported. Other reports of interest are concerned with microwave transitions in triplet H2,e37reactions between H2 and N20,638 differences in rate constants for reactions involving vibrationless and vibrationally excited H2,830 and the H2 vacuum-u.v. laser.e4o Atomic and Molecular Oxygen and Ozone.-U.v. emission from the reaction between O+ and H2 has been Reactions of O ( T ) with the following S. Bediee and H. Tchen, J. Chim. Phys., 1976, 73, 61. J. F. Bukta and W. J. Meath, MoZ. Phys., 1975, 29, 1409. 626 (a) J. H. Birely and P. A. Johnson, J. Chem. Phys., 1975,62,4854; (b) J. H. Brophy and J. A. Silver, ibid., p. 3820. 627 D. J. Giachardi, G. W. Harris, and R. P. Wayne, Chem. Phys. Letters, 1975, 32, 586. 638 J. E. Spencer and G. P. Glass, J. Phys. Chem., 1975,79,2329. 628 H. Y. Su and J. M. White, J. Chem. Phys., 1975, 63, 499. 630 W. A. Payne and L. J. Stief, J. Chem. Phys., 1976,64,1150. w 1 R. W. Henderson and W. A. Pryor, J. Amer. Chem. Soc., 1975,97, 7437. 63a 0. Horie, N. H. Hanh, and A. Amano, Chem. Letters, 1975, 1015. 633 T. Yokota, M. G. Ahmed, I. Safarik, 0. P. Strausz, and H. E. Gunning, J. Phys. Chem., 1975, 024

ea6

79, 1758.

640

K. Y.Choo, P. P. Gaspar, and A. P. Wolf, J. Phys. Chem., 1975,79, 1752. N. P. F. B. Van Assett, J. G. Maas, and J. Los, Chem. Phys., 1975, 11,253. A. L. Ford and K. K. Docken, J. Chem. Phys., 1975, 62,4955. R. S. Freund, T. A. Miller, and B. R. Zegarski, J. Chem. Phys., 1976,64,4069. R. R. Baldwin, A. Gethin, J. Plaistowe, and R. W. Walker, J.C.S. Faraday I, 1975, 71, 1265. V. Ch. Bokun, and A. M. Chaikin, Doklady Akad. Nauk S.S.S.R., 1975, 223, 890. I. N. Knyazev, V. S. Letokhov, and V. G. Morshev, I.E.E.E. J. Quantum Electronics, 1975,

e41

H. H. Harris and J. J. Leventhal, J. Chem. Phys., 1976, 64, 3185.

634

636 636

638 Ose

11, 805.

Photochemistry

156

substrates have been studied: O(1S),642s 643 O(3P),s2403,644 noble gases and H2,646SO 2, 648 HCI,64Q DBr,650C12 9 6s2 Br2,651-653 atmospheric BrC1,662I, and ICl,653CS,647* 654 OCS,la6 NO,655propane,656 p ~ o p y l e n e658 ,~~~~ m e t h y l p r o p e n e ~ ,larger ~ ~ ~ ole fin^,^^^, 661 a ~ e t y l e n e s , ~ fluoro~ ~ - ~ and ~ ~ chloroe t h y l e n e ~ , ~ benzene ~ ~ - ~ ~ ~and gsg thiols and s ~ l p h i d e s ,alkyl ~~~ 2341

6519

6609

Table 7 Rate constants for reactions of O(3P) Substrate O(W ow) (+ M) H 2

H 2

NO(+ NO(+ SO,(+ SO,(+

c10

M = N,) M = N,O) M = He) M = SO,)

Benzene Toluene Cyclopentene a-Pinene /3-Pinene Dimethylbut-2-ene

Products W3P) 02(3&-)

OH + H OH H NO,

+

so3 so3

c1 + 0,

C

C C C C

c

Activation energy in cal mol-'. products. a

642 643

644 646 646

647 646

64D 66 0

661

66B 663

664 666

666 667 668

668 660

661

eea 663

664 661

666

667 668

669

87 0

Rate constant/cm3molecule-' s-l 5.0 x exp (-610/RT) a 1.4 x exp (- 1300/RT) a, 8.3 & 3.8 x 10-l2 exp (- 8570/RT) a 5.1 k 0.1 x 10-l'exp (-4950/T) 5 k 0.1 x lo-= exp (+900/T) 1.7 & 0.1 x exp (+ 1230/T) 1.08 x exp (- 1400/T) 7.9 x (at 298 K) 7 & 1.5 x 10-l1 (at 1250 K) 1.8 x exp (-4200/RT) 3.8 x 10-l1 exp (- 3860/RT) a 9.3 x 10-l2 exp (+430/RT) a 1.2 x 10-l1 exp (-910/RT) a 1 x lo-" exp (- 820/RT) a 2.0 x exp (+ 774/RT) a In units of cm6

s-l.

Ref. 642 642 646 647 647 655 648 648 234 668 668 660 660 660 655

Complex reaction

T. G. Slanger and G. Black, J. Chem. Phys., 1976,64,3763,3767. M. Krauss and D. Neumann, Chem. Phys. Letters, 1975,36,372. M.Yaron and A. von Engel, Chem. Phys. Letters, 1975,33, 316. P. B. Foreman, A. B. Lees, and P. K. Rol, Chem. Phys.Letters, 1976,12,213. R. N . Dubinsky and D. J. McKenney, Canad. J. Chem., 1975, 53, 3531. I. M.Campbell and B. J. Handy, J.C.S. Faraday I, 1975,71,2097. A. A. Westenberg and N. de Haas, J. Chem. Phys., 1975,63,541 1. Z.Karny, B. Katz, and A. Szoke, Chem. Phys. Letters, 1975, 41, 100. V. P. Balakhnin, A. P. Dementiev, and V. I. Egorov, Doklady Akad. Nauk S.S.S.R., 1975,223, 108. F. B. Moin, I. P. Iurkevich, andV. M. Drogobytskii, DokladyAkad. NaukS.S.S.R., 1976,866. M.A. A. Clyne, P. B. Monkhouse, and L. W. Townsend, Internat. J. Chem. Kinetics, 1976, 8, 50. D. St. A. G. Radlein, J. C. Whitehead, and R. Grice, Mol. Phys., 1975,29, 1813. G. T.Bida, W. H. Breckenridge, and W. S . Kolln, J. Chem. Phys., 1976,64, 3296. D. L. Singleton, S. Furuyama, R. J. Cvetanovic, and R. S. Irwin, J. Chem. Phys., 1975, 63, 1003. A. B. Harker and C. S. Burton, Internat. J. Chem. Kinetics, 1975, 7, 907. C.A. Arrington, jun. and D. J. Cox, J. Phys. Chem., 1975,79,2584. J. S. Gaffney, R. Atkinson, and J. N. Pitts, jun., J. Amer. Chem. Soc., 1975, 97, 5049. J. J. Havel and C. J. Hunt, J. Phys. Chem., 1976, 80, 779. J. S. Gaffney, R. Atkinson, and J. N. Pitts, jun., J. Amer. Chem. Soc., 1975, 97, 6481. J. J. Havel and K. H. Chan, J. Amer. Chem. SOC.,1975, 97, 5800. P. Herbrechtsmeier and H. G. Wagner, Ber. Bunsengesellschaft phys. Chem., 1975,79,673. P. Herbrechtsmeier and H. G. Wagner, Ber. Bunsengesellschaft phys. Chem., 1975, 79, 461. M.C. Lin, R. G. Shortridge, and M. E. Umstead, Chem. Phys. Letters, 1976,37, 279. E. Sanhueza and J. Heicklen, J. Photochem., 1975,4,1. R . Atkinson and J. N. Pitts, jun., Internat. J. Chem. Kinetics, 1976,8,475. J. R. Gilbert, I. R. Slagle, R. E. Graham, and D. Gutman, J. Phys. Chem., 1976,80, 14. A. J. Colussi, D. L. Singleton, R. S. Irwin, and R. J. Cvetanovic, J. Phys. Chem., 1975,79, 1900. J. S. Gaffney, R. Atkinson, and J. N. Pitts, jun., J. Amer. Chem. Soc., 1976,98,1828. I. R. Slagle, R. E. Graham, and D. Gutman, Internat. J. Chem. Kinetics, 1976, 8, 451.

157 nitrites,s71n i t r ~ m e t h a n e ,and ~~~ Selected rate constants from this vast body of information are given in Table 7. Several of the reactions with olefins have positive activation energies. Various sources of 0(1D),s74-e77 including photolysis of ozone,676,677 have been outlined. It was shown in two studies that the yield of O ( l D ) from O3photolysis at 313 nm is strongly temperature-dependent, demonstrating that rotational excitation of the O3 contributes to the photodecomposition.6'8 Reactions of O('0) with C0,678N2,679, 680 C12 9 681 CH4,682 cyclob~tane,~~~ and chlorinated 246 and c h l o r ~ f l u ~ r ~ m e t h a n6 8e6 ~s686, ~ have ~ ~ ~ been reported. Rate constants for the deactivation of O ( l D ) by the gases listed have been measured (in units of cm3molecule-ls-l x 10-lo) as N, (0.30), O2 (0.41), CO, (1.2), O3(2.4), H2(1.3), D2(1.3), CH, (1.3), HCl (1.4), NH3 (3.4), H 2 0 (2.1), N,O (1.4), and Ne (0.0013),680ClO(u = 9) (2.0).s81 It has been found that excess kinetic energy in O('D) atoms does not affect reaction Absorption of O('D) at 115.2 nm 688 and emission at 557.7 nm 689 has been studied. Collision-induced emission from O(lS) by rare gases, N2, and H, has been r e p ~ r t e d691 , ~intensities ~~~ increasing linearly with gas pressure except for xenon, which forms XeO. Decay times of O(lS) have been Ionospheric 02+(221T,) and 02+(G411n,) reactions with NO,693 CO 69* (to give C02+);photodissociation of O,+(H,O) 6g5 and of 02-,6Q6 and absorption coefficients of 0, near the hydrogen Lyman d i n e 607 have been discussed. Several oxidation reactions of importance in polluted atmospheres have been discussed which involve molecular oxygen. In the case of methoxyl radicals,120 reaction (132) is of importance, followed by (133) and (134). The H N 0 3 from Gas-phase Photporocesses

671 67a 678 674 676 676

677

678

680

6*1

w2 884

tias f~~~

687

J. A. Davidson and B. A. Thrush, J.C.S. Faraday I, 1975,71,2413. I. M. Campbell and K. Goodman, Chem. Phys. Letters, 1975,34, 105. C. M. Owens and J. M. Roscoe, Canad. J. Chem., 1976,54,984. W . E. Sharp, D. W. Rusch, and P. B. Hays, J. Geophys. Res., 1975,80,2876. M. Cacciatore and M. Capitelli, J. Quant. Spectroscopy Radiative Transfer, 1976, 16, 325. (a) 0. Kajimoto and R. J. Cvetanovic, Chem. Phys. Letters, 1976, 37, 533; (6) S. Kuis, R. Simonaitis, and J. Heicklen, J. Geophys. Res., 1975, 80, 1328. G. K. Moortgat and P. Warneck, Z. Naturforsch., 1975, 30a, 835. R. G. Shortridge and M. C. Lin, J. Chem. Phys., 1976,64,4076. 0. Kajimoto and R. J. Cvetanovic, J. Chem. Phys., 1976, 64, 1005. J. A. Davidson, C. M. Sadowski, H. 1. Schiff, G. E. Streit, C. H. Howard, D. A. Jennings, and A. L. Schmeltekopf, J. Chem. Phys., 1976,64, 57. K. Freudenstein and D. Biedenkapp, Ber. Bunsengesellschaftphys. Chem., 1976, 80, 42. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, Internat. J. Chem. Kinetics, 1976, 8, 107. A. J. Colussi and R. J. Cvetanovic, J. Phys. Chem., 1975, 79, 1891. T. L. Osif, R. Simonaitis, and J. Heicklen, J. Photochem., 1975, 4, 233. H. M. Gillespie and R. J. Donovan, Chem. Phys. Letters, 1976, 37, 468. I. S. Fletcher and D. Husain, Chem. Phys. Letters, 1976, 39, 163. R. Overend, G. Paraskevopoulos, J. R. Crawford, and H. A. Wiebe, Canad. J. Chem., 1975, 53, 1915.

@88

BS9 6O0 6S1

693

L. F. Phillips, Chem. Phys. Letters, 1976, 37, 421.

P. S. Julienne, M. Krauss, and W. Stevens, Chem. Phys. Letters, 1976, 38, 374. G. Black, R. L. Sharpless, and T. G. Slanger, J. Chem. Phys., 1975, 63, 4546. K. H. Welge and R. Atkinson, J. Chem. Phys., 1976,64, 531. K. Henriksen, J. Atinos. Terrestrial Phys., 1975, 37, 1491. W. Lindinger, D. L. Albritton, F. C. Fehsenfeld, and E. E. Ferguson, J. Geophys. Res., 1975, 80, 3725.

604

J. M. Ajello, J. Chem. Phys., 1975, 63, 1863.

asL J. A. Vanderhoff and R. A. Beyer, Chem. Phys. Letters, 1976, 38, 532. 698

~7

P. C. Crosby, R. A. Bennett, J. R. Peterson, and J. T. Moseley, J. Chem. Phys., 1976,63, 1612. V. Dose, U. Schmocker, and G. Sele, Z. Physik. (A), 1975,274, 1.

158 CHSO

+02

HO,+NO

OH

+ NO, + M

__I_+

Photochemistry HOZ

+ HCHO

(1 32)

OH+NO,

(133)

+M

(1 34)

HNO,

(134) is produced as nitrate aerosol in smog. Oxidation of acetyl radicals 121and photo-oxidation of azomethane 149 also produces methoxyl radicals. Absorption cross-sections for 02(3Cu-)and O,(lA,) in the region 108.7170 nm,698and transition moments for the (b3Z,- +- Z3Cu-) and (3rIn, t %Z,-) systems in 0,s99 have been measured, as have diffusion coefficients of OZ(lAu) in 02(3Cg-).700The energy-pooling reaction (135) has been studied,701 and

luminescence from pairs of laser-excited 0, molecules Quenching rate constants for OZ(lAu)by NO and COzhave been reported to be 2.5 2 0.2 x 10-17 and 2.6 k 0.1 x 10-l8 cms molecule-l s-l, respectively.703In the reactions of O,(IAU) with olefins in the vapour phase pre-exponential factors were found to be constant, the large differences in room-temperature rates of reactions being due to variation in activation energies.7o4Chemiluminescence from reactions of 02(1Au)with ethylene has been discussed earlier.89 Reactions of ground-state ozone with the following substrates have been studied: 706 a l k e n e ~ , ~ ~alkyl ~-~ll s ~ l p h i d e s ,and ~~~ chlorine atoms.ls6*714 In the reactions with alkenes, chemiluminescence from CHO*, OH*, and C2 707 and a-diketones 708 was observed. The rate constant for the stratospherically important reaction (136) has been measured as

2.17 f 0.5 x exp(-171 k 30/7‘) over the temperature range 210360 KY7l4and 2.94 k 0.49 x 10-l1 exp (-298 k 39/T) over the range 213298K.lS6 As a result of the temperature dependence of this rate constant, 688 680

700 7 01

704

S. Ogawa and M. Ogawa, Canad. J. Phys., 1975,53, 1845. P. S. Julienne, D. Neumann, and M. Gauss, J. Chem. Phys., 1976, 64,2990. P. H. Vidaud, R. P. Wayne, and M. Yaron, Chem. Phys. Letters, 1976,38, 306. U. Schurath, J. Photochem., 1975, 4, 215. I. S. Sil’dos, L. A. Rebane, A. B. Treshchalov, and A. E. Lykhmus, J.E.T.P. Letters, 1975,22, 151.

703 7 04

7 05

700 707 7 08 7 09 710 711 719

713

714

M. Yaron, A. von Engel, and P. H. Vidaud, Chem. Phys. Letters, 1976,37, 159. R. D. Ashford and E. A. Ogryzlo, J. Amer. Chem. SOC.,1975,97,3604. S . D. Razumovskii, S. K. Rakovski, and G. E. Zaikov, Zzvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 24, 1844. T. H. Varkony, S. Pass, and Y. Mazur, J.C.S. Chem. Comm., 1975,709. D. A. Hansen and J. N. Pitts, jun., Chem. Phys. Letters, 1975, 35, 569. U. Schurath, H. Gusten, and R. D. Penzhorn, J. Photochem., 1976,5, 33. F. S. Toby, S. Toby, and H. E. O’Neal, Internat. J. Chem. Kinetics, 1976, 8, 25. F. J. Dillemuth, B. D. Lalancette, and D. R. Skidmore, J. Phys. Chem., 1976, 80,571. G. V. Pukhal’skaya and G. P. Zhitneva, Kvantovaia Elektronika, 1975,2, 1701. D . R. Hastie, C. G . Freeman, M. J. McEwan, and H. I. Schiff, Internat J. Chem. Kinetics, 1976, 8, 307. S. D. Razumovskii, E. I. Shatokhina, A. D. Malievski, and G. E. Zaikov, Zzvest. Akad. Nuuk S.S.S.R., Ser. khim., 1975, 24, 469. M. S. Zahniser, F. Kaufman, and J. G. Anderson, Chem. Phys. Letters, 1976,37,226.

Gas-phase Photoprocesses

159

notwithstanding the differences between results from the two studies, it is evident that at the reduced temperatures in the stratosphere loss of ozone by reaction (136) may have been overestimated in previous models. An erratum to a previous paper on enhancement of O3 reactions by vibrational excitation of 0,71s and a report on intermode vibrational energy transfer in vibrationally excited ozone ?16 have appeared. Excited states of O3 near the dissociation limit ?17 and excitation of O3 by recombination ?18 have been reported on. As stated above, the quantum yield of O(l0) from O3 photolysis at 313 nm is temperature dependent,s76varying from 0.29 at 293 K to 0.11 at 221 K.676bRelative yields of O(l0) from O3as a function of excitation wavelength have been and flash 719 and laser 720 photolysis of O3 discussed, the latter study in ambient air leading to observation of fluorescence from the hydroxyl radical. HO, Reactions.-Recent papers have been concerned with lifetimes of OD+,721 and production of OH from reactions of atomic hydrogen with N02.626b Kinetics of reactions of ground-state OH with the following substrates have been studied: deuterium H2,723, 724 N0,725, 726 N02,727 03,728 alkanes,723-730 ole fin^,^^^, 72g, 730-734 a l k y n e ~ , ?aromatic ~~ 73s alcohol^,^^^^ 736 nitr~methane,?~~ and c h l o r ~ f l ~ ~ r ~ c a r 724, b o 738 n ~ .Selected ~ ~ ~ ~ ~ rate constants from these extensive studies are given in Table 8. The observed pressure dependence of the rate constant for the reaction with ethylene is sufficient to reconcile nearly all previously reported and conflicting r e ~ u l t s . ~Taking s~ into account atmospheric nitrogen concentrations and temperature variations, the rate constant for reaction (137) has been measured as

(a) M. J. Kurylo, W. Braun, C. N. Xuan, and A. Kalder, J. Chem. Phys., 1975, 63, 1042; 1975, 62, 2065; (b) R. J. Gordon and M. C. Lin, ibid., 1976, 64, 1058. 718 K.-K. Hui, D. I. Rosen, and T. A. Cool, Chem. Phys. Letters, 1975, 32, 141. N. Swanson and R. J. Celotta, Phys. Rev. Letters, 1975,35, 783. C. W. von Rosenberg, jun. and D. W. Trainor, J . Chem. Phys., 1975, 63, 5348. 719 J. A, Hanvey and W. D. McGrath, Chem. Phys. Letters, 1975,36, 564. C . C. Wang, L. I. Davis, jun., C. H. Wu, and S . Japar, Appl. Phys. Letters, 1976,28, 14. 721 J. Brzozowski, P. Erman, and H. Lew, Chem. Phys. Letters, 1975,34,267. 7ea J. J. Margitan, F. Kaufman, and J. G. Anderson, Chem. Phys. Letters, 1975,34, 485. (a) R. P. Overend, G. Paraskevopoulos, and R. J. Cvetanovic, Cunad.J. Chem., 1975,53,3374; (b) C . J. Howard and K. M. Evenson, J. Chem. Phys., 1976, 64, 197. 724 R. Atkinson, D. A. Hansen, and J. N. Pitts,jun., J. Chem. Phys., 1975,63, 1703. 72s R. Overend, G. Paraskevopoulos, and C. Black, J. Chem. Phys., 1976, 64,4149. 7 3 ~B. K. T. Sie, R. Simonaitis, and J. Heicklen, Internut. J. Chem. Kinetics, 1976, 8, 85, 99. 7a7 C. Anastasi, P. P. Bemand, and I. W. M. Smith, Chem. Phys. Letters, 1976, 37, 370. 728 G. E. Streit and H. S . Johnston, J. Chem. Phys., 1976, 64, 95. 729 A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, jun., J. Phys. Chem., 1976,80,789. 7~ R. Zellner and W. Steinert, Znternat J. Chem. Kinetics, 1976, 8, 397. 731 (a) H. B. Palmer, J. Chem. Phys., 1976,64,2699; (b) D. D. Davis, S. Fischer, R. Schiff, R. T. Watson, and W. Bollinger, J. Chem. Phys., 1975, 63, 1707. 7~32 R. Atkinson, R. A. Perry, and J. N. Pitts, jun., Chem. Phys. Letters, 1976,38, 607. 733 J. P. L. Henri and R. W. Carr, jun., J. Photochem., 1976, 5, 69. 734 R. Atkinson and J. N. Pitts, jun., J. Chem. Phys., 1975, 63, 3591. 736 D. A. Hansen, R. Atkinson, and J. N. Pitts, jun., J. Phys. Chem., 1975, 79, 1763. 7a6 I. M. Campbell, D. F. McLaughlin, and B. J. Handy, Chem. Phys. Letters, 1976, 38, 362. 737 I. M. Campbell and K. Goodman, Chem. Phys. Letters, 1975,36, 382. 738 R. A. Perry, R. Atkinson, and J. N. Pitts, jun., J. Chem. Phys., 1976, 64, 1618. 715

Photochemistry

160

Table 8 Selected rate constants for reactions of OH radicals Substrate

D NO NO 0 3

0 3

H2 H2 'ZH4

(4% C3H6

C3H6

C2H2 C6H6

1,3,5-TrimethyIbenzene

Methanol CH2C12 CHF2CI CF,CI, CFCI, CHFCI, CH&l

Rate constant/cm3 molecule-l s-l

1.3 k 3 x 10-lo (298 K) 1.82 x 10-l1 1.2 x 10-1'" 1.1 x 10-1' 3.7 x 10-12 5.8 x 10-15 (at 295 K) 5.9 x 10-l2 exp (- 3990/RT) 2.24 x 2.0 x 10-l2 (295 K) 2.9 x 10-l1 (305 K) 4.1 x 10-l2 exp (1080/RT) 1.65 -t 0.15 x 10-13 1.24 x 10-l2 4.7 x 10-11 9.4 x 10-13 (292 K) 1.45 x 10-13 (298.5 K) 1.2 x 10-l2 exp (-3250/RT) < 1 x 10-15(297-424K) < 1 x 10-15(297-424K) 1.75 x 10-l2 exp (-2490/RT) 4.1 x 10-l2 exp (-2700/RT)

Rex 722 725 726 728 728 723a 724 7316 723a 729 734 731b 735 735 736 738 724 724 724 738 738

a At high-pressure limit of Lindemann mechanism. OH(v" = 9). OH(v" = 4). ActivaAt 3 Torr added He rising to 5.33 X 10-l2at 300 Torr He. tion energy in cal mol-'.

9.5 x 10-l2 ~ m ~ m o l e c u l e - ~at s - ~15 km altitude, falling to 5 x 10-14 cm3 molecule-l s-l at 50 km.727 The results for the Freons 724, 738 have some import for the controversy surrounding chlorine-atom-sensitized destruction of stratospheric ozone, since previous estimates were based on tropospheric lifetimes of Freons of as long as 50 years. If the estimated ambient tropospheric OH concentration 660 of 3 x lo6 molecule cm-, is correct, the results shown in Table 8 give tropospheric half-lives of 0.3 years, 0.2 years, 0.05 years, and 1.5 years for CHFCI2, CH3Cl, CH2C12,and CHF,CI, respectively. Thus only the last compound and CF2C12and CFCI3 may reside long enough for transport to the stratosphere, where OH reaction provides a further destruction mechanism for CHF,Cl. When coupled with the fact that the rate of C1 attack on ozone is less than previously thought (see above), this suggests that recently expressed alarm at the use of Freons may have been an o v e r - r e a ~ t i o n although ,~~~ many authors suggest the contrary. Decay of electronically excited OH has atrracted some attention of late, and in one publication lifetimes of OH and OD(C2Z+) were measured as 2.9-1.8 ns, with no deuterium isotope effect, suggesting that radiative decay to O H ( Z 2 n i ) dominates the decay.73QPredissociation in the B-2 system of OD 740 and transition probabilities for the 2-2 system of OH741have been discussed. The radiative lifetimes of 0H(L2XC+)have been measured as 693 k 30 ns for u' = 0, 736 ns for v' = 1, with corresponding values in OD for v' = 0, u' = 1, and 730

740 741

W. H. Smith and G. Stella, J. Chem. Phys., 1975,63, 2395. C. Carlone, Phys. Rev. (A), 1975, 12, 2464. D.R. Crosley and R. K. Lengel, J. Quant. Spectroscopy Radiative Transfer, 1975, 15, 579.

Gas-phase Photoprocesses 161 v’ = 2 of 691, 712, and 736 ns, respectively.74zThe v’ = 2 level of OH is predissociated, but can be collisionally quenched.743 Early reports on electronic quenching of OH(XzZ+,u‘ = 1) have been shown to involve serious experimental error.744Luminescence of OH(XzZ+)produced in the vacuum-u.v. photolysis of Hz0,745predissociation in H 2 0 ions,746reactions of HzOz+with atmospheric gases,747HzOz in chemiluminescent reactions,748and the dependence of the HOz-HO, reaction on H 2 0 749 have been discussed. The role of water in forming active species, principally OH, in the primitive atmosphere of the earth has been discussed earlier.8

Atomic and Molecular Nitrogen, NO, Reactions-Reactions of N(4S)with itself oz(3&-),752 and NO and (and third body),6z4with C(3P),750with 0(313),751 NOz 753 have been investigated. The carbon-atom reaction is two-step [reactions (138) and (139)], which considered as a single-step process has an overall rate C(3P)

+

N(4S) CN(4C+)

+M +M

---+

+

CN(4X+) M CN(@P, v 2 7)

+M

(138) (139)

constant of 9.4 k 2.5 x cms molecule-z s-1.750 Quenching of N(2P) by a variety of small atoms and molecules have rate constants varying from < 8 x 1 0 - 1 8 for Hz to 2.8 x 10-l1 cm3molecule-l s-l for Quenching of N(zD) by O(3P)has been Lifetimes of N2+(B2Zu+,v’ = 0) 692 and the N2+(4Z,++ 2&,+) transition 756 have been discussed. Photoionization in Nz 757s 758 and oscillator strengths in Nz(&:Xu- + zlZg+)and (?A, f- zlZg+) 759 transitions have been reported. Nz(A3E,+) interactions in afterglows,7eowith COz,761 with Oz, 0, N, and H atoms,76z with rare gases,7s3 and with mercury 264 have been investigated. Quenching of Nz(B3rIg)has also been ~ t u d i e d . ~ ~ ~ - ~ ~ ~ 742

743

744

746 748 747

’ 748

76* 761 762 763

764

766 766

767 768

76D 760

761

763 764 7t36 766

K. R. German, J. Chem. Phys., 1975, 63, 5252. K. R. German, J. Chem. Phys., 1976, 64, 4065. R. K. Lengel and D. R. Crosley, J . Chem. Phys., 1976,64,3900; P. Hogan and D. D. Davis, ibid., 1975, 62, 4574; 1976, 64, 3901. I. P. Vinogradov and F. I. Vilesov, Optika i Spektroskopiya, 1976, 40, 58. J. H. D. Eland, Chem. Phys., 1975, 11, 41. W. Lindinger, D. L. Albritton, C. J. Howard, F. C. Fehsenfeld, and E. E. Ferguson, J. Chem. Phys., 1975, 63, 5220. K. D . Gundermann, Chem. Z., 1975,99,279. E. J. Hamilton, jun., J. Chem. Phys., 1975, 63, 3682. N. Washida, D. Kley, K. H. Becker, and W. Groth, J. Chem. Phys., 1975, 63,4230. T. W. Dingle and P. A. Freedman, Chem. Phys., 1975, 8, 171. C. W. Wilson, jun., J. Chem. Phys., 1975, 62, 4842. (a) M. A. A. Clyne and I. S. McDermid, J.C.S. Furaday I, 1975,71, 2189; (b) D. W. Rusch, D. G. Tom, W. E. Sharp, T. M. Donahue, and K. Henriksen, J . Atmos. TerrestrialPhys., 1975, 37, 1173. R. A. Young and 0. J. Dunn, J. Chem. Phys., 1975,63, 1150. J. E. Davenport, T. G. Slanger, and G. Black, J . Geophys. Res., 1976, 81, 12. K. Dressler, J . Chem. Phys., 1976,64,3493; J. d’Incan and A. Topouzkhanian, ibid., 3494. J. W. Davenport, Phys. Rev. Letters, 1976, 36, 945. C. Duzy and R. S. Berry, J . Chem. Phys., 1976, 64, 2421. S. G. Tilford and N. M. Benesch, J. Chem. Phys., 1976,64,3370. D. E. Shemansky, J. Chem. Phys., 1976, 64, 565. W. T. Rawlins and F. Kaufman, J. Chem. Phys., 1976, 64, 1128. 0. J. Dunn and R. A. Young, Internat. J. Chem. Kinetics, 1976, 8, 161. C. R. Roy, J. W. Dreyer, and D. Pernei, J. Chem. Phys., 1975,63,2131. B. A. Garetz, J. I. Steinfeld, and L. L. Poulsen, Chem. Phys. Letters, 1976, 38, 365. E. M. Gartner and B. A. Thrush, Proc. Roy. SOC.(A), 1975,346, 103, 121. R. F. Heidner, tert., D. G. Sutton, and S. N. Suchard, Chem. Phys. Letters, 1976, 37, 243.

162 Photochemistry 76g HO2, l 2 O , 770 Reactions of NO with N(4S),7530 and C atoms,767 NHy126NH 136 NH3,136* 137 CH3, ll1 C F and CF2,104and vibrationally excited Oa715have largely been discussed in earlier sections. Rate constants for the competing aeronomically important reactions (133) and (140) have been measured 7679

29

HO,+NO

---+ ---+

OH+NO, HON02

as 1.O k 0.2 x 10-la and < 2 x 10-15 cm3molecule-l s-l, Fluorescence of NO excited by the 184.9 Hg resonance line 771 and using synchrotron radiation 772 has been observed. Nitrogen dioxide has been further investigated in the past year. Thus pressurebroadened linewidths in the molecule 773 have been reported, and the spectroscopy considered from a theoretical standpoint .774 The peculiarities of the system have long been known, and the theoretical treatment suggests that the x 2 A l and A2B2states are strongly coupled to give 2A’ states when asymmetric distortions are considered. This coupling manifests itself upon excitation to the nominal x 2 B 2state in the observation of two types of excited state which behave independently. For excitation in a beam with the 514.5 and 488 nm argon-ion lines,776 one set of states is observed with lifetimes of 1-3 p s whereas others have lifetimes of 30-100 ps, and these clearly decay principally by radiative routes. The perturbation by the ground state of the 2B2state has been further evidenced in another elegant beam study with the gas cooled to 3 and in the predissociation region (397.9-420 nm) again two sets of states, one fluorescent s) and one dissociative ( < ( 7 x s), are reached which behave independ e n t l ~ .In~ the ~ ~ last study it was shown that molecules may predissociate even if excited to vibrational levels below the threshold (398.0 nm) provided rotational excitation is present. Electronic self-quenching had a rate-constant of 4 x 10-l1 cm3molecule-l s-l. In a study in which KB = 0 rotational levels of the 2B,states of NO2 were selectively excited all states had collision-free lifetimes of 33 5 4 ps, and quenching parameters greater than gas kinetic.778 Two distinct photodissociation mechanisms have been postulated in the excitation of NO2, one in the 458-630 nm region, the other between 400 and 430 nm.77s Two types of emission have been seen in NO2 excited by a ruby laser at 694.3 nm, one electronic due to multiphoton absorption, the other i.r.780 N

767 768

76Q 770

T71

K. Glanzer and J. Troe, J. Chem. Phys., 1975,63,4352. D. Golomb and J. H. Brown, J. Chem. Phys., 1975,63,5246. H. Ando and T. Asaba, Internat. J. Chem. Kinetics, 1976, 8, 259. R. Simonaitis and J. Heicklen, J. Phys. Chem., 1976, 80, 1. T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. Ichimura, and Y. Mori, J. Chem. Phys., 1975,

776 777

63, 5470. 0. Benoist D’Azy, R. Lopez-Delgado, and A. Tramer, Chem. Phys., 1975,9,327. G . D. T. Tejwani and E. S. Yeung, J. Chem. Phys., 1975, 63, 4562. C. F. Jackels and E. R. Davidson, J. Chem. Phys., 1976, 64,2908. F. Paech, R. Schmiedl, and W. Demtroder, J. Chem. Phys., 1975. 63,4369. R. E. Smalley, L. Wharton, and D. H. Levy, J. Chem. Phys., 1975, 63, 4977. W. M. Uselman and E. K. C. Lee, J. Chem. Phys., 1976, 64, 3457.

778

Y.Haas, P. L. Houston, J. H. Clark, C. B. Moore, H. Rosen, and P. Robrish, J. Chem. Phys.,

779

1975,63,4195. C. L. Creel and J. Ross,J. Chem. Phys., 1976,64, 3560. D . F. Hakala and R. R. Reeves, Chem. Phys. Letters, 1976,38, 510.

772 77a 774 776

780

163

Gas-phase Photoprocesses

Rate constants for processes (141) and (142) have been measured as 2-6 x lofoImol-ls-l for (141) and 2.7 f 0.6 x lo4lmol-ls-l for (142).781 H 0 2 formation in shock-heated HN03-NO, mixtures has been r e p 0 ~ t e d . l ~ ~ NO2* O,(lA,) NO2 + O,(l&+) (141) O~FA") NO o,(3zu-) NO (142) Electronic fluorescence in the 105-180 nm region in N,O excited by synchrotron radiation,782 and rovibrational fluorescence excited by active nitrogen,783s784 have been observed. Absorption by NzO in the vacuum u.v.785#788 and vibrational relaxation of N 2 0excited in the v1 mode 787# 788 have been studied. In the photodissociation of N,O, quantum yields for production of O(lS) were -1.0 throughout the 128-138 nm region, with yields of N(2D) large for A,, 6 120 nm, and those of N2(x3E:,+)< 0.2 over the entire 110-150 nm region.7sD The N,03-H20-HNO equilibrium has been and the thermal decomposition of HNO reported earlier.13D

+

+

-

+

CO, Reactions.-Production of CO+(A2rI)was mentioned early in this chapter,l and lifetimes (and fluorescence spectra) of this species 792 and its reaction with CO [reaction (143)] have been Photoionization of CO 7D3 and 791e

7679

(CO+)*

+ co

____3

C20t

+ co

(143)

transition moments for the Z3C---E3Il system of C 0 7 D 4have been discussed. Production of the e" state by atomic sulphur radiation shows that relaxation to the LlIl state is very rapid.7D6Vacuum-u.v. fluorescence from photodissociation fragments of CO and CO,has been CO(ii31-I)has been produced in a mercury discharge,7D7and mercury excited to the 3P1state by excited C0.264 Reaction of CO with N2H+is of importance in interstellar space.140 Photoionization of CO,798-801 and various aspects of the CO, 802-805 have been 781 782 783

784

786

'a6

D. J. Giachardi, G. W. Harris, and R. P. Wayne, J.C.S. Faraday II. 1976, 72, 619. L. C. Lee, R. W. Carlson, and D. L. Judge, J. Phys. (B), 1975,8,977. A. Picard-Bersellini and C. Rossetti, J. MoZ. Spectroscopy, 1975, 58, 216. A. Picard-Bersellini and C. Rossetti, Chem. Phys. Letters, 1975, 36, 647. H. S. Johnston and G. Selwyn, Geophys. Res. Letters, 1975, 2, 549. R. H. Huebner, R. J. Celotta, S. R. Mielczarek, and C. E. Kuyatt, J. Chem. Phys., 1975, 63, 4490.

787 788

7e9 791

793 7g4

795 797 7g8 799

8oo SO1 802

803

806

R.T. V. Kung, J. Chem. Phys., 1975,63,5313. R. T. V. Kung, J. Chem. Phys., 1975,63, 5305. G. Black, R. L. Sharpless, and T. G. Slanger, J. Chem. Phys., 1975, 62,4266. R. Varrna and R. F. Curl, J. Phys. Chem.. 1976.80.402. R. Anderson and M. Jursich, Amer. J. Phys., 1975, 43, 535. L. C. Lee, R. W. Carlson, and D. L. Judge, J. Phys. (B), 1976,9, 855. M. S. Yur'ev and V. S . Yarunin, Optika i Spektroskopiya, 1975,39, 672. T. G. Slanger and G. Black, J. Chem. Phys., 1976, 64,219. T. G. Slanger and G. Black, J. Chem. Phys., 1975, 63,969. L. C. Lee, R. W. Carlson, D. L. Judge, and M. Ogawa, J. Chem. Phys., 1975,63,3987. K. M. Monahan and R. Goldstein, J. Chem. Phys., 1975, 62, 4954. E. P. Gentieu and J. E. Mentall, J. Chem. Phys., 1976, 64, 1376. J. T. Moseley, P. C. Crosby, R. A. Bennett, and J. R. Peterson, J. Chem. Phys., 1975, 62, 4826. P. C. Crosby and J. T. Moseley, Phys. Rev. Letters, 1975, 34, 1603. R. Fabbro, J. Bruneteau, and E. Fabre, J. Phys. (Paris), 1975, 6, 25. A. P. Peterson, C. Wittig, and S. R. Leone, J. AppZ. Phys., 1976, 47, 1051. G. D. Downey and D. W. Robinson, J. Phys. Chem., 1976, 80, 1234. J. Finzi and C. B. Moore, J. Chem. Phys., 1975, 63, 2285. H. Guegen, F. Yzambart, and A. Chakroun, Chem. Phys. Letters, 1975, 35, 198.

164

Photochemistry

discussed in many reports. The relative yield of O(lS) from CO, photodissociation is greatest in the 109.5-113.5 nm region, falling off at both shorter and longer wavelengths.s06At longer wavelengths, the reaction sequence (144)-( 153)

+

CO, kv1I3s.s O ~ D ) CO,

+ M + OCP) + OCP) O+O,+M 0

0

+

+ 0, €320

O H + CO

H+O,+M

HO,

+0 20H

---+

wall

>

co + O W )

(144)

o(3q

(145)

+ CO,

+M

(146)

03+M

(147)

20,

(1 48)

0,

+H

(149) (1 50)

____+

HO,+M

(151)

I___,

OH

20H CO,

+ 0,

H,O,

(1 52) (1 53)

occurs.8o7These reactions account for the fact that 13C0, is formed in the presence of I3CO. Surface effects on the photodissociation of COzin relation to the Martian atmosphere have been discussed.808 SO, and H,S Reactions.-The quenching rate constant of H2S+ions by H2S has been reported to be 2.3 rt 0.3 x 10-Bcm3molecule-l s-1.809 Brief comment on the lifetimes of H2S and dimethyl sulphide in polluted (hours) and non-polluted (days) atmospheres has been made.s1o The flash photolysis of SO, produces a transient species whose lifetime is determined by the diffusion rate to the The species is formed by interaction between excited singlet SO, and its ground state, and has not been completely characterized, but may be a loosely bound dimer. Thus the photochemistry of this small but aeronomically important molecule, as Alice said, grows ‘curiouser and curiouser,’ given that the three triplet states are also implicated in atmospheric reactions. The phosphorescent ii3B, state has a zero-pressure decay rate of 3.8 +_ 0.6 x 10, s-l, with a rate constant for interaction with the ground state of 4.5 rt 0.1 x los 1mol-1 s-1.812 In a static system, the apparent quantum yield of photo-oxidation of SO, decreases owing to film formation in the vessel and a b a c k - r e a ~ t i o n .Recent ~ ~ ~ papers have reported oxidation of SO, in aqueous reaction of SO, with organic halogen compounds,816 and isomerization of cis-but-2-ene photosensitized by Finally, and wearily, it can be reported that i.r. laser studies on energy transfer in S80, have been carried I. Koyano, T. S. Wauchop, and K. H. Welge, J. Chem. Phys., 1975,63, 110. L. F. Loucks and R. C. Michaelson, J. Chem. Phys., 1975, 63,404. P. Papacosta and S. J. B. Corrigan, Chem. Phys. Letters, 1975, 36, 674. G. R. Mohlmann and F. J. de Heer, Chem. Phys. Letters, 1975, 36, 353. 810 R. D. Cadle, Atmos. Environment, 1976, 10, 417. 811 J. W. Bottenheim and J. G. Calvert, J. Phys. Chem., 1976, 80, 782. *la J. P. Briggs, R. B. Caton, and M. J. Smith, Canad. J. Chem., 1975, 53, 2133. P. A. Skotnicki, A. G. Hopkins, and C. W. Brown, J. Phys. Chem., 1975, 79, 2450. 814 S. Beilke, D. Lamb, and J. Muller, Arrnos. Environment, 1975, 9, 1083. 816 B. Gostisamihelcic and B. Kastening, 2. phys. Chem. (Frankfurt), 1975, 98, 443. 816 R. D. Penzhorn and W. G. Filby, J. Photochem., 1975,4, 91. 817 B. L. Earl, A. M. Ronn, and G. W. Flynn, Chem. Phys., 1975,9,307. 806

Part 11 PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS By J. M. KELLY

1 Photochemistry of Transition-metal Complexes Recent publications of general interest include a monograph on inorganic photochemistry,l which nicely complements the earlier text of Balzani and Carassiti,2 a review of metal complex photochemistry covering the 1971-72 l i t e r a t ~ r e ,a~ summary of photochemical syntheses of inorganic compound^,^ and a discussion of spectroscopic investigations of transition-metal complex excited state^.^ The annual appearance of a report on luminescence properties of inorganic compounds in a sister Volume will be of interest to inorganic photochemists.6 The potential of transition metals for catalysis of the photodissociation of water has been recognized for many years. Balzani and co-workers have performed a useful service to inorganic photochemists by analysing the possible cyclic pathways for this process.’ (See also last year’s Report, p. 564.) This year has seen the realization of efficient production of molecular hydrogen and oxygen using visible light and monolayer-bound ruthenium@) bipyridyl complexes.* This remarkable discovery should provide an even greater stimulus to further research in this area. The quenching of electronically excited states of either organic or inorganic compounds by transition metal complexes is still an imperfectly understood process, despite the substantial number of studies carried out. Difficulties arise because of the variety of possible mechanisms (e.g. electron transfer, energy transfer, exciplex formation, catalysed inter-system crossing), and because of the sensitivity of the rate constant for quenching to such factors as solvent, transition metal involved, nature of the ligand, charge of the complex, and type of excited

a 3

4

a

‘Concepts of Inorganic Photochemistry’, ed. A. W. Adamson and P. D. Fleischauer, Wiley, New York and London, 1975. V. Balzani and V. Carassiti, ‘Photochemistry of Co-ordination Compounds’, Academic Press, London and New York, 1970. C. H. Langford and N. A. P. Kane-Maguire, in ‘M.T.P. International Review of Science’, Inorganic Chemistry Series Two, ed. M. L. Tobe, Butterworths, London, 1974, Vol. 9, p. 135. J. R. Wasson, in ‘Annual Reports in Inorganic and General Synthesis - 1974’, ed. K. Niedenzu and H. Zimmer, Academic Press, New York and London, 1975, Vol. 3. G. A. Crosby, Accounts Chem. Res., 1975,8, 231. A, J. Thomson, in ‘Electronic Structure and Magnetism of Inorganic Compounds’, ed. P. Day (Specialist Periodical Reports), The Chemical Society, London, 1976, Vol. 4, p. 149. V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta, and M. Gleria, Science, 1975, 189, 852. G. Sprintschnik, H. W. Sprintschnik, P. P. Kirsch, and D. G. Whitten, J. Amer. Chem. SOC., 1976,98, 2337.

167

168 Photochemistry state of the quencher available. A recent report considers the quenching of anthracene triplet by Cu2+,Ni2+,Co2+,and Mn2+,and of phenanthrene triplet by Ce3+,Pr3+,and Nd3+in methanol-water solutions over a wide temperature range (120-293 K).O At low temperatures, where the medium is viscous, the reaction rate is diffusion controlled but, in agreement with earlier work, the rate constants for quenching at higher temperatures are substantially below this limit. Under these latter conditions it is possible to calculate the intra-cage reaction rate constant k, [equation (1); T = aromatic molecule triplet state, Q = quenching kd +

k-d

~

(T-Q)

Products

(1)

metal ion]. The proposed mechanism for quenching is by energy transfer via exchange interaction. The low rate constants for quenching by the metal ions, despite favourable energy factors, are attributed to the relative inefficiency of transmission of the exchange interaction through the ligand solvent molecules. Energy transfer via exchange interaction is also the favoured mechanism for the quenching of the triplet states of organic sensitizers by tris(acety1acetonato)Fe"', -RulI1, - A P , and tris(dipiva1oylmethanato)Fe"' in benzene solution.l0V In this study the authors have recorded the efficiency of the quenching process as a function of sensitizer energy (Figure 1). This correlates well with the excited state energy of the metal complex acceptor determined spectroscopically. Thus it is apparent that energy transfer to metal-centred (ligand field) states of the Fell1 complex is substantially less efficient than transfer to its CT or intra-ligand states. These conclusions are confirmed by examination of the [Ru(acac),] complex where the ligand field states are at higher energy than the CT state, and for the [Al(acac),] species where only intra-ligand states are available. The considerable differences in rate constants for [Fe(acac),] and [Fe(dpm),] are attributed to the steric effect of the bulkier ligand on the efficiency of the exchange process. The influence of solvent environment on quenching rates has been examined for the interaction of anthracene triplet and Co2+ in mixed THF-water and t-butanol-water mixtures of various proportions.12 As was noted previously by the same authors for the naphthalene triplet, the value of the quenching rate constant passes through a minimum as the proportion of water in the organic phase is increased. This effect is ascribed to the changing nature of the solvation of the organic and ionic species. The general importance of electron transfer as a mechanism for the quenching of excited states of co-ordination compounds has been stressed by several reports this year. An illustration that this process may be responsible for the quenching of all classes of metal complex excited states has been given.13 Thus paraquat (1) deactivates the MLCT excited states of Ru"' complexes, the f-f* state of [Eu(phen),13+(phen = 1,lo-phenanthroline), and the intraligand excited state of Pd(octaethy1porphyrin). In all cases the mechanism proposed involves electron

* 10 l1 12

E. J. Marshall, N. A. Philipson, and M. J. Pilling, J.C.S. Faraday 11, 1976, 72,830. F. Wilkinson, Pure Appl. Chem., 1975,41, 661. F. Wilkinson and A. Farmilo, J.C.S. Faraday ZZ, 1976, 72, 604. V. A. Rogov, Y. I. Naberukhin, and Y. N. Molin, Zzvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1095.

R. C. Young, T. J. Meyer, and D. G. Whitten, J. Amer. Chem. Sac., 1976, 98, 286.

Photochemistry of Inorganic and Organornetallic Compounds

169

transfer. Although porphyrins and related compounds including chlorophyll are known to undergo both oxidative and reductive quenching, this behaviour has not previously been reported for co-ordination compounds. Balzani and co-workers l4 and Creutz and Sutin l6 have recently observed such behaviour for

Figure 1 Rate constants for quenching of Fe(acac),, Fe(dpm),, Ru(acac),, and Al(acac), as a function of ET, the energy of the trblet states being quenched (Reproduced from J.C.S. Furaday 11, 1976,72,604)

the excited state of [Ru(bipy),12+ (bipy = 2,2’-bipyridyl) when quenched by a variety of metallocyanide c o m ~ l e x e s and , ~ ~ also by reductants such as Eu”, S2042-, and [ R U ( N H ~ ) ~ ] ~Thus + . ’ ~ it could be shown that some metallocyanide complexes quench by energy transfer [equation (2)] {e.g. [Cr(CN),I3-}, some by oxidative electron transfer [equation (3)] {e.g. [Fe(CN)B]3-),and yet others by reductive electron transfer [equation (4)] {e.g. [MO(CN)~]~-) (Table 1). In l4

l6

A. Juris, M. T. Gandolfi, M. F. Manfrin, and V. Balzani, J. Amer. Chem. SOC.,1976,98,1047. C. Creutz and N. Sutin, Inorg. Chem., 1976,15,496.

170

*[Ru(bipy),12+ + Q *[Ru(bipy)J2+

+Q +

*[Ru(bipy)s12+ Q

-

Photochemistry

+ Q* [Ru(bipy),I3+ + Q [Ru(bipy),]+ + Q+ [Ru(bipy),12+

(2)

(3) (4)

Table 1 The rates oj'quenching of the luminescence of [Ru(bipy),12+by various metallocyanide complexes l4 E(*Q) "I E(Q+/Q)/ E(Q1Q-Y k, bl kJ mol-1 V V dm3mol-l s-l 234 148 28 3 28 1 31 1 561 275 27 1 230 = 204

kJ mol-l.

+-0.73

+0.36 > +0.75

-

+0.75 > +1.0 > +1.0

< -1.8 - 1.28 < -1.7 +0.36 -0.83 -

- 1.35

< -1.8

At 23 "C,ionic strength

3.4 7.5 3.3 6.5

x x x x

108 108

lo@ lo@ c 106 1.2 x lo@ 5.6 x loE < 106 < 106

= 0.50.

another paper it has been demonstrated that the MLCT excited state of [Ru(bipy),I2+ is deactivated by [Cr(bipy),13+,and that the metal-centred luminescent excited state of [Cr(bipy),13+is quenched by [ R ~ ( b i p y ) , ] ~ +In . ~both ~ cases the mechanism is one of electron transfer, even though in the first example, energy transfer is energetically favourable. The rate constant for the dynamic quenching of UOZ2+excited state by metal ions in aqueous solution correlates well with the ionization potential for the metal ion involved, and therefore a mechanism involving electron transfer seems most suitab1e.l' Electron transfer has also been shown to occur on the quenching of the triplet state of phenothiazine by Cu2+and Eu3+ ions in aqueous sodium lauryl sulphate micellar The influence of transition metal ions on the photochromism of a spiro-indolene-2,2'-benzopyran has been discussed.l@ Other examples of sensitization and quenching studies are discussed in the section dealing with the particular metal. As noted in last year's Report, picosecond laser-flash photolysis should prove to be a valuable technique for investigations of the properties of the excited states of co-ordination compounds, particularly under conditions similar to those used in photochemical studies. The results of such an investigation with Fe", Ru", and Cr"' compounds have been reported recently,20and are especially interesting in showing that non-radiative processes such as inter-system crossing are very fast indeed (7 < 5 ps). Burdett 21 has presented a theory for predicting the course of non-dissociative photochemical isomerizations of transition metal compounds. The method 16 17

18 20

21

F. Bolletta, M. Maestri, L. Moggi, and V. Balzani, J.C.S. Chem. Comm., 1975, 901. H. D. Burrows, S. J. Formosinho, M. da Graca Miguel, and F. Pinto Coelho, J.C.S. Furaday I, 1976, 72, 163. S. A. Alkaitis, G. Beck, and M. Graetzel, J. Amer. Chem. SOC.,1975, 97, 5723. D. Walther and E. G. Jager, 2. Chem., 1975, 15,236.

A. D. Kirk, P. E. Hoggard, G. B. Porter, M. G. Rockley, and M. W. Windsor, Chem. Phys. Letters, 1976, 37, 199. J. K. Burdett, Inorg. Chcm., 1976, 15, 212.

Photochemistry of Inorganic and Organornetallic Compounds 171 utilizes a simple molecular orbital approach based on consideration of metal d orbitals-ligand interactions. The site preference for the ligands in both excited and ground states is calculated from the particular d orbital configurations involved. For d s octahedral complexes it is presumed that the reaction proceeds by a thermal rearrangement of the excited state species to give the more stable excited state isomer. This then relaxes to the ground state. Good agreement is found with the experimental results for [Co(CN),(H,O),]- and Ru(PPh,),(C0)J2. For d8 square-planar compounds (e.g. Pt" complexes) the theory predicts that excitation of either the cis- or trans-isomer leads to the same distorted tetrahedral intermediate. This then decays back into the ground state, the proportion of cis- and trans-isomers depending on the position of intersection of the excited state and ground-state potential energy surfaces. As in previous Reports, the photochemistry of compounds of each transition metal will now be considered systematically. Transition metal organometallics, low oxidation-state compounds, and metalloporphyrins are the subjects of subsequent sections. Titanium.-A study of the photolysis of TiCI, in ethanol or methanol solution has been reported.22 Under these conditions the main species present in solution is solvated Ti(OR),Cl,. Irradiation at 300 nm leads to Ti"' and alkoxy radicals, which have been detected at low temperatures by a combination of e.s.r. and u.v.-visible spectroscopic techniques. The initial step in the reaction is presumed to be ( 5 ) (L = OR- or C1-). This is presumably also the first step in the lightTiIVL-

-

TiIx1L*

(5)

induced reaction of &unsaturated ketones in the presence of TiC14 in methanol An example in this report is the conversion of (2) into (3) in 65%

(2)

(3)

yield. It will be noticed that an extra (solvent-derived)carbon atom fragment has been incorporated into the product. It is reported that photolysis of an oxalato-TiIV complex in aqueous oxalic acid solution leads to reduction and hydrolysis of the species.24

Vanadium.-Methoxo-oxobis(8-quinoyloxo)vanadium(v), (VO)(OMe)Q,, has been used as a photo-initiator for the polymerization of methyImetha~rylate.~~~ 26 The polymerization is radical-initiated, and this suggests that the primary photochemical step is reaction (6). 21

83 84

2s 26

A. I. Kryukov, S. Y. Kuchmii, A. V. Korzhak, and 2. A. Tkachenko, Doklady Akad. Nauk S.S.S.R., 1975,222, 1134. T. Sato, G. Izumi, and T. Imamura, TetrahedronLetters, 1975, 2191. J. Shiokawa and A. Matsumoto, Asahi Garasu Kogyo Gijutsu Shoreikai Kenkyu Hokoku, 1974, 207 (Chem. Abs., 1976, 84,97 745). S. M. Aliwi and C. H. Bamford, J.C.S. Faraday I, 1975, 71, 1733. S. M. Aliwi, C. H. Bamford, and S. U. Mullik, J. Polymer Sci.,Part C, Polymer Symposia, 1975, 50, 33.

172 (VO)(OMe)Q2

-

Photochemistry (VIV0)Q2

+ *OMe

(6)

The phosphorescence of vanadium(v) species supported on silica gel has been m ~ n i t o r e d . It ~ ~ was further shown that this emitting species is reduced to vanadium(1v) by adsorbed ammonia or methanol. The photoreduction of vanadium(v) to vanadium(1v) in aqueous solution has been followed by polarography and by e.s.r.28 Chromium.-It is proposed that the photo-oxidation of pinacol [Me,C(OH)C(OH)Me,] by dichromate ion involves an initial two-electron transfer to give Crlv, as radicals could not be detected either by e.s.r. at low temperatures or by trapping with methylmetha~rylate.~~ In spite of the very considerable amount of work carried out on Cr"' complexes, the detailed mechanism of their photosubstitution reactions, and in particular photo-aquation, is still not fully understood. For most complexes it is clear that it is the lowest quartet state (4T2,in octahedral symmetry), and not the doublet (2Eu),which is photoactive, but this may not always be the case. Similarly, although stereochemical studies and experiments with macrocyclic ligands suggest that the substitution process proceeds via an associative rather than dissociative mechanism, conclusive proof of this is still lacking. These matters have been discussed in a short review,,O and in a detailed survey.31 Riccieri and Zinato 32 have published the results of a thorough investigation of the photoaquation reactions of tran~-[Cr(NH,),(H,0)Cl]~+ (4), trans[Cr(NH,)4(H,0)(NCS)]2+ (9,trans-[Cr(NH,),(H,0),l3+ (6), trans-[Cr(NH,),Cl,]+ (7), and trans-[Cr(NH,),(NCS)Cl]+ (8). In all cases, excitation in the ligand trans-[Cr(NH,),( H,O) C1I2+ trans-[Cr(NH,),Cl,]+

hV

hv

HIO

>

cis-[Cr(NH,),( H,O)C1I2+

'

cis-[Cr(NH3)JH,O)ClI2+

(7)

+ C1-

(8)

field bands of the complexes leads to aquation of the acido-groups, while NH3 aquation accounts for less than 10% of the reaction. Release of the axial ligand is accompanied by trans to cis isomerization. For example, the main products from (4) and (7) are those shown in equations (7) and (8). From a study of the wavelength dependence it could be shown that population of the lowest quartet state (4E)led almost exclusively to the labilization of the axial ligand, while the low yield of ammonia originated from the state, which lies at somewhat higher energy. (,E and 4B arise from the 4T,ustate under the non-octahedral symmetry of these complexes.) This is in accord with simple theory, which suggests that population of a a-antibonding orbital (the d , ~ orbital in the ,E state, or the d,+a orbital in the 4B state) should lead to axial or equatorial labilization respectively. As the authors point out, it is remarkable that cis-[Cr(NHJ427 Z8

A. M. Gritscov, V. A. Shvets, and V. B. Kazansky, Chem. Phys. Letters, 1975, 35, 511. M. Kitamura and H. Imai, Bull. Chem. SOC.Japan, 1975, 48, 1459. P. R. Bontchev, M. Mitewa, K. Kabassanov, and A. Malinovski, Inorg. Nuclear Chem. Letters, 1975, 11, 799. Kutal, J. Chem. Educ., 1975, 52, 502. E. Zinato, in ref. 1, p. 143. P. Riccieri and E. Zinato, J. Arner. Chem. SOC.,1975, 97,6071.

an C. 91 32

Photochemistry of Inorganic and Orgaltometallic Compounds 173 (H20)ClI2+,which is the common product of the ligand field photolysis of [Cr(NH,)6C1]2+,of (4) and of (7) is formed in all cases with a quantum yield close to 0.4, despite the differencesin the labilized ligands (NH,, H 2 0 , and C1-). This leads them to propose that the constancy of this quantum efficiency may be indicative of a common photophysical process. For compounds (6)-(8) U.V. irradiation leads to population of CT states. Bond fission resulting from such excitation was much less selective than from that caused by population of ligand-field states. [For example, for (4) at 254 nm Ocl- = 0.21 and O)NB,= 0.29.1 In this case homolytic rupture of the bonds is implicated. However, before the fragments so formed can escape from the solvent cage, the oxidized ligand recaptures an electron from the Cr", and the net products are therefore those of heterolytic cleavage. The extent of the stereoselectivity of the photo-aquation reactions of CrII' complexes is well exemplified in a study of the ligand-field band photolysis of trans-[Cr(en),X,]+ (X = Br, Cl) (en = 1,2-ethylenediamine).,, In both cases the principal product is [Cr(en),(H2O)XIa+. Using quantitative ion-exchange chromatography, it was possible to show that for X = C1 the product is >99.2% cis-complex, and that for X = Br it is >95% cis-complex. It is unlikely that such stereochemical preference can arise from thermodynamic or kinetic factors following dissociation of the excited state. The authors therefore propose that this stereospecificity is strong evidence for a mechanism in which entry of the solvent is concerted with halide loss. In agreement with the results obtained with related compounds, the small amount of NH, substitution (a678 = 0.003; OD,,, = 0.075; @4oe = 0.042) appears to arise from reaction of the 4Bstate. Ligand-field band excitation of 1 ,6-[Cr(en)(H,O),Cl,]+ (9) leads exclusively to chloride aquation (see Scheme l).,* Although photo-induced exchange of the aquo-ligand is also possible, arguments are presented to demonstrate that this is unlikely. Products (10) and (11) are formed in similar amounts. [For (lo), O,,, = 0.12, 0 4 0 0 = 0.22; and for (ll), @589 = 0.10, 0 4 0 0 = 0.15.1 Although it might appear that species (10) is formed by a stereo-retentive process, it is more probable that it too has arisen by a cis-trans isomerization step involving the H 2 0 ligands. The absorption spectrum of complex (9) exhibits essentially no splitting of the 4A2g-+4T2g band, implying that the 4E and 4B states are close in energy. This compound is therefore suitable for testing the relative importance of 0- and n-bonding in the excited state. As only axial labilization occurs, it appears that a-antibonding along the ClCrCl axis is the predominant effect, and that n-bonding factors are of minor importance. excited state of [Cr(bipy),],+ is remarkably long-lived in solution, and The its decay can be monitored by emission spectroscopy or by conventional flash photolysis. As discussed earlier (ref. 16) this excited state is deactivated by [R~(bipy),]~+, and in turn [Cr(bipy)J3+ may quench the luminescent state of [Ru(bipy),12+. For both reactions it has been proposed that electron-transfer mechanisms are operative, and this has been confirmed by flash photolysis.s6 33

36

W. J. Rosebush and A. D. Kirk, Canad. J. Chem., 1976,54,2335. R. T. Walters and R. G. Linck, Znorg. Chem., 1975, 14, 2098. R. Ballardini, G. Varani, V. Carassiti, and F. Scandola, '6th IUPAC Symposium of Photochemistry', Aix-en-Provence, July 1976, Abstract No. 4.

7

174

Photochemistry

OH2

C1

(9)

A OH2 (1 1) Scheme 1

More recently it has been found in quenching 36 and flash photolysis 37 experiments that this [Cr(bipy)J2+ doublet state reacts with water to form a [Cr(bipy),(H20)l3+transient species. The quenching of the photoracemization and phosphorescence of ( + )D[Cr(phen)$+ (phen = 1,lO-phenanthroline) by SCN- and O , have been investigated, and that by I- r e - e ~ a m i n e d .It~ ~ is shown that the efficiency for 4T2,-+ 2Eg intersystem crossing is greater than 95%, and that the photoracemization in acid solution proceeds via thermal population of the 4T2gstate from the 2Eg species. Hydroxide ion also partially quenches the phosphorescence, but causes an increase in the quantum yield for racemization. This is attributed to a reaction of the 2Egstate with OH-. Thus, from this observation and from those mentioned a b o ~ e it, appears ~ ~ ~ ~that ~ at least for these Cr"' chelate complexes, the 'rule' that only the quartet state is photoreactive is invalid. The importance of 4T2g+ 2Euintersystem crossing in Cr"' photochemistry has long been recognized. Reports this year indicate clearly that at room temperature in fluid solution this process is extremely rapid. Picosecond laser flash photolysis investigations with Cr(acac),, [Cr(NCS),I3-, and ~ ~ ~ ~ S - [ C ~ ( N H , ) , ( N Chave S)~]been carried out.20 In each case only the 2Eustate could be observed, indicating that the 4T2,species must undergo intersystem crossing in times less than the laser pulse width (5 ps). Independent evidence for the extreme rapidity of ISC in CrlI1complexes has been presented by Kane-Maguire and c o - ~ o r k e r s40. ~ The ~~ 36

37

33 39 40

M. Maestri, F. Bolletta, M. F. Manfrin, L. Moggi, and V. Balzani, see ref. 35, Abstract No. 65. M. S. Henry and M. Z . Hoffman, ref. 35, Abstract No. 43. N. A. P. Kane-Maguire and C. H. Langford, Inorg. Chem., 1976,15, 464. N. A. P. Kane-Maguire, J. E. Phifer, and C. G. Toney, Inorg. Chem., 1976, 15, 593. N. A. P. Kane-Maguire, D. E. Richardson, and C. G. Toney, J . Amer. Chem. SOC.,1976,98, 3996.

Photochemistry of Inorganic and Organometallic Compounds

175

+

photo-aquation of ( )=-[Cr(en)J3+ [equation (9)] has been followed by polarimetry in the presence of OH-. Hydroxide ion, selectively, and at the concentrations used, completely, deactivates the state. Variation of excitation wavelength had a marked influence on the percentage of reaction quenched, even though only the 4A2,-+ 4T2,band was excited (Figure 2). Further, the quantum

440

480

520

Figure 2 (A) Absorption spectrum of [Cr(en),Is+. (B) Variation of percentage reaction photoracemization as a function of excitation quenching by OH- of (+)~-[Cr(en),]~+ wavelength (Reproduced by permission from Inorg. Chem., 1976, 15, 593)

yield for phosphorescence in the absence of quencher shows a similar dependence; that upon excitation at 514 nm being only 63% of that recorded for 436 nm excitation. Both these observations indicate that crossing to the doublet state is more efficient at wavelengths shorter than 496nm. The authors suggest that this is evidence for effective competition of ISC processes with vibrational relaxation of the quartet state. An alternative possibility is that ISC might be competing with solvent-restricted relaxation of the quartet state to its thermally equilibrated state. However, this possibility may be ruled out as it has been demonstrated that the activation energies for phosphorescence after excitation at 460 or 514nm are identical. The explanation proposed for this variation in quantum yield is that excitation at wavelengths less than 490 nm corresponds to promotion to a point on the 4T2,surface above the intersection with the 2E, state, whereas at longer wavelengths the point reached is below the crossover, and the configuration of the 4T2,state is close to that of its vibrationally relaxed state. To rationalize the efficiency of crossing, it is also necessary to assume that at the crossover the 4T2,state is already substantially distorted from Oh

176

Photochemistry

Attempts to determine the lifetime of the 4T2,state in crystalline or rigid solvent matrices at low temperature have been performed using laser e ~ c i t a t i o n .42~ ~ , It was found that Cr(acac), and [Cr(CN),I3- in alcoholic glasses emit only phosphore~cence.~~ As no 'grow-in' of this emission can be observed, the lifetime of the initially formed 4T2, state must be less than 20 ns at 77 K, and less than 1 ps at 18 K. At 77 K [Cr(~rea)~],+ and [Cr(antipyrene),lS+show both fluorescence and phosphorescence, although in these cases the decays are none~ponential,~l1 4 2 and in the case of [Cr(antipyrene),13+ concentration dependent .41 The non-exponential nature of the [ C r ( ~ r e a ) ~ emission ]~+ decay has been found to be due to complexes in different environments. Most interestingly, it appears that the fluorescence is not prompt, but rather arises by thermal repopulation of the 4T2gstate from the 2E, state. This requires that the intersystem crossing from the 4T2,state must be very rapid indeed (k > log s - ~ ) . * ~However, Watson et al. state that because of the non-exponential nature of these emissiondecay processes, these compounds should be quoted as examples of delayed fluorescence 'only with due The c.d. spectra of a number of tris(fl-diketonato)Cr"' complexes have been determined following partial photo-induced resolution using circularly polarized light.43-45 The quantum yields for the photo-inversion are largest for tris(propanedialato)Cr"', and lowest for bulky-ligand-containing species such as tris(dipivaloy1methanaf~)Cr"'.~~ The influence of ligand deuteriation on the phosphorescence lifetimes of [Cr(NH3),]3+, [Cr(en),13+, and [Cr(NH,),(NCS),]- in rigid solution at 77 K has been in~estigated.~~ Effects are pronounced: e.g. for [Cr(NH,),],+ in methanolwater, T = 54 ps; for [Cr(ND3)J3+ in CD30D-D,O, r = 4350 ps. By comparison of these lifetimes with the frequencies for the ligand vibration overtone bands, it has been shown that a dipole-dipole mechanism is responsible for the non-radiative deactivation of these complexes. The phosphorescence lifetime of [Cr(CN),I3- has been studied in fluid organic solvents as a function of temperat ~ r e . ~The ' variation in lifetime for different solvents (e.g. at 25 "C, in D M F r = 6060 ps; in methanol, r = 19 ps) correlates with the solvent polarity parameter ET. This effect is ascribed to an enhancement of the radiationless decay of the complex produced by electrostatic perturbation of the solvent dipoles. Phosphorescence lifetimes in aqueous solution at room temperature have also been reported for [Cr(en),13+, [Cr(NH3)J3+, and [Cr(bipy),]3+.48 Other recent reports on luminescence from CrIII species include those on [Cr(en),I3+ and its deuteriated analogue,49 on [Cr(en),13++,[Cr(en),(ox)]+, and [ C ~ ( O X ) , ] ~on - , ~[Cr(NCS),(H20)6-,](3-n)+ ~ (n = 0_6),51 on CrS+ on various 41

p2 43

44

46 46

47 48

4B 6o

W. M. Watson, Y. Wang, J. T. Yardley, and G . D. Stucky, Znorg. Chem., 1975, 14, 2374. F. Castelli and L. S. Forster, J . Amer. Chem. Soc., 1975, 97, 6306. K. L. Stevenson and R. L. Baker, Znorg. Chem., 1976,15, 1086. H. Yoneda, U. Sakaguchi, and Y. Nakashima, Bull. Chem. Soc. Japan, 1975,48, 1200. B. Norden, Znorg. Nuclear Chem. Letters, 1975, 11, 387. I. B. Neporent, E. B. Sveshnikova, and A. P. Serov, Izvest. Akad. Nauk S.S.S.R., Ser. fiz., 1975,39, 1959. R. Dannoehl-Fickler, H. Kelm, and F. Wagestian, J. Luminescence, 1975, 10, 103. A. W. Adamson, C. Geosling, R. Pribush, and R. Wright, Znorg. Chim. Acta, 1976, 16, L5. C. D. Flint and A. P. Matthews, J.C.S. Faraday ZZ, 1976, 72, 579. P. E. Hoggard and H. H. Schmidkte, Spectrochim. Acta, 1975, 31, A, 1389.

Photochemistry of Inorganic and Organometallic Compounds

177

crystal lattices,62and on the magnetically induced circular emission of CrS+doped magnesium Molybdenum.-E.s.r. spectra have been recorded after the photolysis of several potassium diperoxomolybdates at low temperature^.^^ Manganese.-No excited state could be observed following picosecond laser photolysis of Mn0,- at room temperature, implying either that the excited state is very short-lived (T < 3 ps), or that its absorption is masked by that of the ground state.20 A report of reduction following irradiation of solid-state K[Mn(cydta)],3Hz0 (H,cydta = trans-1 ,ZcycIohexylenedinitrotetra-acet ic acid) has been noted .6Q

-

Rhenium.-As previously reported, photolysis of in acetonitrile leads to rupture of the quadruple Re-Re bond (equation 10). The reaction quantum [Re2C18]2-

+ 2MeCN

2[ReC14(MeCN),]-

(10)

yield is wavelength dependent, and irradiation in the longest wavelength band (Amx = 680 nm) causes no photocleavage. The results of laser flash-photolysis experiments with and [Re2Br8I2-in dichloromethane and acetonitrile have now been communicated.66 For [RezClsI2-,a transient (formed with 90% efficiency) having similar absorption spectrum and decay characteristics in either CH,Cl, or MeCN is observed, both following 337 and 615 nm excitation. This is assigned to a 0 ~ 7 7 ~ 6 ~ ( 6excited * ) ~ state, but it could be shown that this species is not that responsible for reaction (10). The authors speculate that the reaction pathway involves a halide-bridged intermediate formed from upper-excited states. Another report confirms that the long wavelength band of [Re,C1,I2- is indeed due to a 8-6* tran~ition.~' Iron.-Although the literature concerned with the photoredox chemistry of FeIrl complexes is very substantial, the precise nature of the primary photochemical processes has been, and still remains, a matter of controversy. This year, for example, the photoreduction of FeCl, or Fe(ClO,), in aqueous media is the subject of apparently contradictory reports.68,59 However, comparison of separate investigations is not easy as the course of the reaction is very dependent on the conditions used. Thus the quantum yield for Fe" production has been found to depend on the Fe"', Fell, and chloride ion concentrations, on pH, on excitation wavelength, on time of irradiation, and on the presence of radical scavengers (intended or accidental).68 Some of these problems arise because of 61

E. A. Solov'ev, G. P. Tikhonov, and E. A. Bozhevol'nov, Zhur. priklad. Spektroskopii, 1975, 23,434.

6a 63 64

s6

W. F. Coleman, J. Luminescence, 1975, 10, 72, 163. R. A. Shatwell and A. J. McCaffery, Mol. Phys., 1975, 30, 1489. G. L. Smorgonskaya, G. A. Bogdanov, G. L. Petrova, and M. V. Savina, Zhur. obshchei Khim., 1975,45, 2745. T. Takeuchi and A. Ouchi, Nippon Kagaku Kaishi, 1975, 7 , 1175 (Chem. Abs., 1975, 83, 106 150).

67 68 69

R. H. Fleming, G . L. Geoffroy, H. B. Gray, A. Gupta, G. S. Hammond, D. S. Kliger, and V. M. Miskowski, J. Amer. Chem. SOC.,1976, 98, 48. F. A. Cotton, B. A. Frenz, B. R. Stults, and T. R. Webb, J. Amer. Chem. SOC.,1976,98,2768. F. David and P. G. David, J. Phys. Chem., 1976, 80, 579. C. H. Langford and J. H. Carey, Canad. J. Chem., 1975, 53, 2430.

178

Photochemistry

the variety of Fe"' species which may be present in the solution, (e.g. [Fe(H20),l3+ (121, [WH20)60H12+(1 3), [Fe(Hzo)4(oH),Fe(H2o)4I4+ (14), and [Fe(H20),C1I2+(15)). Their relative concentrations depend on the pH, and on the concentrations of Fe"' and chloride ion. Further, the quantum yield for the initial process is difficult to estimate, because of the recombination reaction (11). For [Fe(H20),l2+

+ OH*+ H+

-

[Fe(H,0),I3+

+ H,O

(11)

FeCI, solutions at pH = 2.5, David and Davids8 have reported that the quantum yield for Fell production, following irradiation at 350 nm, decreases with chloride ion concentration. They therefore suggest that under these conditions the primary process is (12) and not (13). Langford and Carey60 have [Fe(H2O),OHl2+ [Fe(H,0),C1]2C

+

+ OH* [Fe(H20),l2+ + Cl.

[Fe(H20),12+

hv

(12) (1 3)

used t-butanol as a scavenger for hydroxyl radicals and chlorine atoms. (Other alcohols exhibit more complex behaviour-see below.) By this means and by selective excitation of the species concerned, they have determined the quantum yields for steps (13) and (14) to be 0.093 (at 350 nm) and 0.065 (at 254 nm) respectively. These values have been derived assuming that the mechanism for [Fe(H,O),],+ decomposition, under the conditions used, is as shown in steps (14)-(16), while that for [Fe(H20),Cl]a+ involves reactions (13) and (17)--(19). [Fe(H2o),I3+

+ MqCOH *CH,CMe,OH + Fe3+ C1* + Me,COH c1- + c1C1;- + [Fe(H20)s]2+ OH*

-

[Fe(H,0),l2+

+ OH* + H+

+ *CH,CMe,OH Fe2+ + H+ + HOCH,CMe,OH

H,O

HCI

+ -CH,CMe,OH

a,*[Fe(H2O)&1I2+

(14) (1 5 )

(16) (17)

(1 8)

+ C1- + H,O

(19)

The authors also remark that the apparent contradictions on the nature of the primary photochemical step in the literature arise partly because of neglect by other authors of steps (18) and (19). For certain scavengers (methanol, 2-propanol, formic acid, but not t-butanol) the yield of Fe" increases linearly with the concentration of the organic compound.eo This behaviour is ascribed to outersphere oxidation by the CT states of (12)-(14) of non-co-ordinated scavenger. Very interesting results, which may be relevant to the above discussion, have been reported by Plyusnin and Bazhin for the low temperature (77 K) photolysis of Fe'II in the presence of high concentrations of bromide ion.g1 Under these conditions, the species present are FeBr, and [FeBr4]-. The effects observed in frozen acidic aqueous solutions are quite different from those found in rigid alcohol glasses. In the first case Br2*-is formed, whereas in the ethanolic solutions this species is not produced, but [FeBr,12- and alcohol radicals may be 1o

J. H. Carey and C. H. Langford, Canad. J. Chem., 1975,53, 2436. V. F. Plyusnin and N. M. Bazhin, Khim. vysok. Energii, 1974, 8, 316.

Photochemistry of Inorganic and Organometallic Compounds

179

identified after irradiation. These results suggest that no bromine atom actually leaves the co-ordination sphere of the iron atom. It is proposed that after the initial photo-induced charge transfer, oxidation of species present in the second co-ordination sphere takes place, leading to the observed results. Photolysis (350-600 nm) of aqueous solutions of ferric bromide, under conditions where the main species present is [Fe(H20)6Br]2+,produces bromine with a quantum efficiency of 7.5 x lo-, at room temperature.62 Cox and Kemp have studied the e.s.r. spectra of radicals formed on photooxidation of alcohols, carboxylic acids, amides, and ketones by ferric chloride and ferric perchlorate at 77 K. The behaviour of these systems is in general similar to that observed for CeIV photo-oxidations. As identical results are obtained with ferric chloride and ferric perchlorate, it is presumed that the ferric chloride photo-oxidations in these experiments are not induced by chlorine atoms. Other authors have described the e.s.r. investigations of the photo, ~ ~of cellobiose 65 by ferric ions. oxidation of a l c ~ h o l sand Photolysis of hydrated ferric perchlorate in acetonitrile solution apparently yields FeIV compounds, as the products obtained when this reaction is carried out in the presence of cyclohexanol parallel those found when [Fe0I2+is generated by other means.66 Irradiation of (16) in the presence of anhydrous ferric perchlorate in acetonitrile gives (17)-(19).67 The ratio of (18) to (19) is very different

from that observed with other free radical initiators, suggesting that the stereochemistry of the products is controlled by co-ordination of the radical to the iron atom. Other reports on the photolysis of FeIII compounds in the presence of organic reagents describe the aerobic photodegradation of Fe"'(edta) 68 and Fe"'(nitri1otriacetate) 60 chelates, the photochemical reactions of Fe"'(citrate) cornplexe~,~~ the light-induced formation of radicals on irradiation of FeCI, in the presence of methacrylate esters 71 and he~-l-ene,'~ and the initiation of photopolymerization of methylmethacrylate by a Fe"l-triethy1enetetramine-carbon tetrachloride mixture.', 6a

6a 64 66

S.-N. Chen, N. N. Lichtin, and G. Stein, Science, 1975, 190, 879. A. Cox and T. J. Kemp, J.C.S. Faraday I, 1975, 71, 2490. 0. Hinojosa, J. A. Harris, and J. C. Arthur, Carbohydrate Res., 1975, 41, 31. E. Y. Davydov, G. B. Pariiskii, and D. Y. Toptygin, Zzuest. Akad. Nuuk S.S.S.R.,Ser. khim., 1974, 1747.

67

68

'O

71 7a

73

J. T. Groves and W. W. Swanson, Tetrahedron Letters, 1975, 1953. J. T. Groves, Tetrahedron Letters, 1975, 3113. H. B. Lockhart and R. V. Blakeley, Enoiron. Sci. Technol., 1975, 9, 1035. R. J. Stolzberg and D. N. Hume, Enuiron. Sci. Technol., 1975, 9, 654. N. A. Kostromina, N. V. Beloshitskii, and V. F. Romanov, Koord. Khim., 1975, 1, 1367. A. A. Nosonovich, S. V. Sogonova, S. Y. Kuchmii, L. E. Mazur, V. P. Sherstyuk, and A. I. Kryukov, Ukrain. khim. Zhur., 1975,41, 1330. A. I. Kryukov, S. Y. Kuchmii, A. V. Korzhak, and Z. A. Tkachenko, Doklady Akad. Nauk S.S.S.R., 1975, 222, 882. Y . Inaki, M. Takahashi, and K. Takemoto, J . Macromol. Sci., 1975, A9, 1133.

180

Photochemistry

Transient ground-state bleaching has been observed following picosecond laser photolysis of [ F e ( ~ h e n ) ~and ] ~ + [Fe(bi~y)~]~+.~O This has been assigned to either ligand field state. a 3(MLCT), lq,, or 3T1B Irradiation in the ligand-field bands of [Fe(CN),C0I3- causes reaction (20) to take place with a quantum yield of 0.90.74CT band excitation also induced CO [Fe(CN),C0I3-

+ H,O A

[Fe(CN),(H,0)]3-

+ CO

(20)

expulsion, although at present it is not clear whether this is due to non-radiative conversion to the reactive ligand-field excited state or to photoelectron ejection with consequent production of [Fe(CN),C0I2-, which is expected to release CO spontaneously. In another study it has been confirmed that on laser flash photolysis the ejection of an electron from [Fe(cN),l4- occurs by a single photon process.76 U.V. light accelerates the evolution of hydrogen from ferrous hydroxide gels, apparently by causing the disproportionation of the Fe(OH), to elemental iron.’, Ruthenium and Osmium.-Reports on the photochemistry of ruthenium complexes continue to be dominated by those dealing with [Ru(bipy),12+. This arises because of the remarkable properties of its lowest excited state, which luminesces in solution at room temperature and which is a very strong reducing agent. Although it is commonly classified as a triplet (d-n*) MLCT state, Crosby and co-workers have previously warned about the inaccuracy of this ‘spin label’, because of the dominant role of spin-orbit coupling. This research group has now published a series of detailed articles on the CT excited states of [Ru(bipy),12+ and related c ~ m p l e x e s . ~ ~From - ~ @ consideration of the lifetime and quantum yield for the emission at low temperatures, it may be deduced that the lowest excited state consists of a manifold of three energy levels. The second of these (the E level) lies about 10 cm-l above the first (the A l level), but decays approximately ten times faster, while the A 2 level, which is about 60 cm-1 above the Al, decays about 250-300 times more rapidly than it. The low values for the energy gaps between these levels is an indication of the large separation of the promoted electron from the metal, a feature which is apparent in the role of these states as strong reducing agents. The absorption of this lowest excited state of [Ru(bipy),12+has been monitored both in water and in acetonitrile solution following pulsed laser excitation at 265, 353, and 530nm.80 From a determination of its extinction coefficient, and by quenching with retinol, it has been shown that the quantum yield for formation of the excited state is 0.5 k 0.1 (excitation at either 265 or 353 nm). This contrasts with the previously accepted value of 1.O. The excited state of [Ru(bipy),I2+ could not be detected in a picosecond laser flash photolysis study,20presumably 76

76 77 78 78 *O

A. Vogler and H. Kunkely, 2. Naturforsch., 1975,30b, 355. U. Lachish, A. Shafferman, and G. Stein, J . Chem. Phys., 1976,64, 4205. G. N. Schrauzer and T. D. Guth, J . Amer. Chem. SOC.,1976,98, 3508. G. D. Hager and G. A. Crosby, J . Amer. Chem. SOC.,1975,97,7031. G.D.Hager, R. J. Watts, and G. A, Crosby, J. Amer. Chem. SOC.,1975, 97, 7037. K.W. Hipps and G. A. Crosby, J . Amer. Chem. SOC.,1975,97,7042. R. Bensasson, C. Salet, and V. Balzani, J. Amer. Chem. SOC.,1976,98,3722.

Photochemistry of Inorganic and Organometallic Compounds

181

because the excited state absorbs only weakly in the wavelength region (550650nm) available under the conditions of the experiment. However, the nonphosphorescent MLCT excited state of [Ru(bipy),(MeOH)J2+ was detected and its lifetime shown to be 620 ps. Interest in the photo-induced electron transfer reactions of ruthenium(I1) complexes continues this year. One of the most notable papers is that by Whitten

=O

Figure 3 Surfactant compounds used for the photochemical cleavage of water (Reproduced by permission from J. Amer. Chem. Soc., 1976, 98, 2337)

and co-workers on the properties of monolayers of the surfactant molecules shown in Figure 3.8 The absorption and emission properties of these monolayers on glass slides are very similar to those of [Ru(bipy),12+in solution. However, immersion of the slide in water completely quenches the luminescence of the complex, although addition of water has no effect on its emission in dioxan solutions. Further, irradiation of the monolayer assembly immersed in water with Pyrex-filtered light from a medium-pressure mercury lamp leads to the evolution of molecular hydrogen and oxygen (0E 0.1). The photosensitized decomposition of water appears to cause little permanent damage to the monolayers as over a thousand molecules of gas are evolved per molecule of complex. Recent work has demonstrated, however, that monolayers of highly purified substrates are inactive in inducing water photolysis. Although the mechanism for the process has not yet been elucidated, it is quite probable that the corresponding Ru"'

182

Photochemistry

-

complex is involved. Indeed, Creutz and Sutin 81 have shown that [Ru(bipy),],+ oxidizes OH-, resulting in the evolution of oxygen [equation (21)]. This reaction

+

[R~(bipy)~],+ OH-

[Ru(bipy),12+

+ 40, + Hf

(21)

exhibits a marked pH dependence, peaking in efficiency (80%) at pH9. Investigations using pulse radiolysis and stopped-flow methods suggest that species of the type (20) might be important.

(20)

[Ru(bipy),12+has been found to act as a photocatalyst for the oxidation of Fe2+to Fe3+by molecular oxygen.82 The quantum yield for the reaction depends upon the pressure of oxygen and upon the effective hydrogen ion concentration, but is insensitive to the concentration of Fe2+. The mechanism proposed for the reaction is that detailed in steps (22)-(28). While the electronic structure of (21) *[Ru(bipy),12+

+ 0, (21)

+ (21) + (21) [Ru(bipy),13+ + *02H [Ru(bipy),I3+ + Fe2+ 3H+ + *02H+ 3Fe2+ H+

H+

___+

*[Ru(bipy),.O2I2+ (21)

(22)

+ [Ru(bipy),P+ +

[Ru(bipy),I2+

+ [ R ~ ( b i p y ) ~ ]+ ~+ [Ru(bipy),12+ + 3Fe3+ + 2H,O [Ru(bipy)J2+

is not established, it is presumed to be a cage complex of [Ru(bipy),],+ and 0,-, although the extent of charge transfer need not necessarily be so great. As was mentioned in Section 1, electron transfer appears to be a quite general route for the intermolecular deactivation of the excited states of ruthenium(r1) complexes. Thus this mechanism has been shown to be operative in the quenching of [Ru(bipy),I2+, [Ru(phen),12+, [Ru(terpy)(bipy)(NH,)l2+ (terpy = 2,2',2"terpyridine), or [Ru(bipy),(CN)J by Fe3+ or paraquat.13 One of the most notable observations this year has been that [Ru(bipy),12+ excited state may undergo both reductive and oxidative quenching 149 l5 (see Table 1). Lin and Sutin 83 have communicated the results of experiments on the quenching of the MLCT excited states of [Ru(bipy),12+and [Os(bipy),12+ by oxygen, 8a

8s

C. Creutz and N. Sutin, Proc. Nat. Acad. Sci. U.S.A., 1975,72, 2858. J. S. Winterle, D. S. Kliger, and G. S. Hammond, J. Amer. Chem. SOC.,1976, 98, 3719. C.-T. Lin and N. Sutin, J. Phys. Chern., 1976, 80,97.

Photochemistry of Inorganic and Organornetallic Compounds

183

Fe3+, [Co(phen),13+, [ R u ( N H ~ ) ~ ]and ~ + , [Fe(CN)6]3-.83 It was found that the osmium complex is quenched 60-100% faster than its ruthenium analogue, probably because it is a better reducing agent. Steady-state photolysis of [Ru(bipy),12+in the presence of Fe3+ leads to the build-up of [Ru(bipy),],+ and Fez+. By determining this concentration as a function of irradiation intensity, it is possible to estimate k29/k30 (= 2.6 x lo3 at 25 "C).As the value of k30 is *[Ru(bipy),Ia+ [Ru(bipy)J3+

+ Fe3+ + Fe2+

-

___+

[Ru(bipy),13+

+ Fe2+ +

[ R ~ ( b i p y ) ~ ] ~ +Fe3+

(29) (30)

known from stopped-flow studies, it may be calculated that kze is 1.9 x log dm3 mol-ls-l. A photogalvanic cell has been constructed in which the cathode compartment containing a [Ru(bipy),12+-Fe3+ mixture is irradiated. This cell gives a current output comparable to that of the well-known iron-thionine system. Photo-induced electron-transfer reactions may be utilized in the study of highly reactive oxidized and reduced species such as organic radical-cations and anions.s4 For example, the recombination reaction (3 1) has been monitored following reduction of paraquat (P") (1) by flash-excited [Ru(bipy),12+, and subsequent oxidation of triphenylamine by the Ru"' complex [equations (32) and (33)]. NPh,'++ P'+ + NPha Pa+ (31)

+ P2+ + NPh,

*[Ru(bipy),12+ [Ru(bipy),13+

-

+

+ + NPh,'+

[ R u ( b i p ~ ) ~ ] ~ +P'+

(32)

[Ru(bipy),12+

(3 3)

Electron transfer from *[Ru(bipy),12+to the conduction band of SnO, gives rise to anodic photocurrents.8S The photocurrents may be quenched by Fe3+. No electron transfer from the valence band of semiconductors such as S i c to * [ R ~ ( b i p y ) ~could ] ~ + be detected. At least 95% of the quenching of *[Ru(bipy),lz+ by [Cr(bipy),13+proceeds via electron transfer, even though energy transfer would be energetically favourable.16 [R~(NH,)~(pyrazine)]~+ reacts with Cu2+ in solution to form compound (22).86 Flash-photolysis studies have indicated that this species undergoes

photo-induced electron-transfer forming (23), and thermal relaxation back to (22). The postulate that this process occurs by an inner-sphere mechanism is supported by the observation that [Ru(NH&,(~~Ac)]~+ (pyAc = 4-acetylpyridine) is not reversibly oxidized even in the presence of 0.35 mol dm-, Cu2+, although its spectroscopic properties are similar to those of [R~(NH~)~(pyrazine)]~+.

86

R. C. Young, T. J. Meyer, and D. G. Whitten, J . Amer. Chem. Soc., 1975, 97, 4781. M. Gleria and R. Memming, Z . phys. Chem. (Frankfurt), 1975, 98, 303. V. A. Durante and P. C. Ford, J. Amer. Chem. SOC.,1975,97, 6898.

184

Photochemistry

Irradiation of [R~(bipy),(N~)~]+ causes photoreduction of the complex [equation (34)].*' Although the lowest excited state has LMCT character, no free azide radical could be trapped by acrylamide. In a second light-induced

step, photosolvolysis of the Ru" complex occurs [equation (35)]. Photolysis of [Os(NZ)(NH3),]CI2has been reported to give [OS(NH,)~C~]~+ following initial photo-oxidation of 0s" to Os"', and subsequent photosubstitution of Nz by CI-.S8 Cobalt.-Photoredox reactions of Co"' complexes and theoretical models for these processes have been discussed in a recent review.8g Endicott and FerraudigOhave presented some new results for the photosubstitution reactions of Co"' and Rh"' complexes following ligand-field band excitation, and on the basis of these, they have critically discussed earlier theories 91 for the prediction of the products and quantum yields of these reactions. The experimental observations in this area are summarized by the authors as: (i) the quantum yields for pure LF band excitation of [Co(NH3),XI2+are in general small but very strongly wavelength-dependent, whereas those for [CO(CN)~X]~and for [Rh(NH3),XI2+are nearly wavelength-independent;(ii) complexes whose reactions are wavelength-independent have their lowest LF states at energies large compared to the activation energy for thermal ligand substitution; (iii) some correlations exist between non-radiative relaxation rates and photosubstitution quantum yields, but photoreactivity does not apparently increase with the lifetime of the excited state; (iv) the quantum yields are functions of medium conditions; and (v) the experimentally determined quantum yields do not correlate with ligand-field parameters such as the tetragonality parameter Dt. Therefore they propose an alternative model for these processes in which the reaction takes place from a vibrationally excited ground state. The role of the thermally equilibrated excited state is to provide a stereospecific distortion, which determines the configuration and momentum of the species when entering the ground state, and in this way it controls the nature of the products and the magnitude of their yields. Thus coupling between the excited state and the ground state may lead to reaction if the vibrational energy in the ground-state modes is greater than the activation energy for the thermal process and if a sufficient component of the momentum of the vibrationally excited system is along the critical reaction co-ordinate. Of course, for such a theory to be of predictive applicability much more detailed knowledge of both ground-state and excited-state potential energy surfaces will be required.

89

G. M. Brown, R. W. Callahan, and T. J. Meyer, Inorg. Chem., 1975, 14, 1915. A. P. Pivovarov, Y . V. Gak, G. I. Kozub, Y . M. Shul'ga, I. N. Ivleva, L. S. Volkova, and Y . G. Borod'ko, Koord. Khim., 1975,1, 1061. J. F. Endicott, in ref. 1, Chapter 3, p. 81. J. F. Endicott and G. J. Ferraudi, J. Phys. Chem., 1976, 80, 949. M. J. Incorvia and J. I. Zink, Inorg. Chem., 1974, 13, 2489.

Photochemistry of Inorganic and Organometallic Compounds

185

[Co(NH3),N3I2+exhibits unusual properties for cobalt(I1I)ammines in that LF band excitation leads to quite high quantum yields for ammonia aquation [equation (36)].g2 Irradiation at other wavelengths induces both substitution hv

[C0(NH3)5N3l2+

hv

[C0(NH3),N3l2+

[Co(NH3),(HzO)N3lZ+-t NH3 Co2+

(36)

+ 5NH3 + QNZ

(37)

[equation (36)] and redox decomposition [equation (37)]. The relative efficiency of these processes is markedly wavelength dependent (Figure 4). A comparison

0.6

0.5

0.4

0.3

0.2

0.1

I

I

I

300

400

X

n.-

1 50 0

1

m

Figure 4 Absorptioii spectrum of [CO(NH,),N,]~+,and quantum yields .for photoaquation [reaction (36)] (0) and photoredox [reaction (37)] (0) processes as a function of wavelength (Reproduced by permission from J. Amer. Chern. Soc., 1975,97, 6406)

of the photochemical properties of [Co(NH3),NCSI2+with those of [Co(NH,),N3I2+is instructive as both the ligand field strength and oxidation potential of NCS- and N3- are similar. However, the reactions observed for [Co(NH,),NCSI2+ are NCS- aquation and photoredox decomposition but not ammonia aquation. The quantum yields for Co" formation and those for the aquation reaction are greater for [ C O ( N H , ) ~ N ~ than ] ~ + for [Co(NH3),NCSlZ+,and this feature has been attributed to the sulphur atom increasing the rates of radiationless deactivation processes, possibly by a heavy-atom effect. The quantum efficiencies for Co" production are also markedly dependent on the excitation wavelength and the solvent (Figure 5). In particular, photolysis at h < 280 nm in glycerol-water solutions results in a pronounced increase in the quantum 82

G. J. Ferraudi, J. F. Endicott, and J. R. Barber, J. Amer. Chem. Soc., 1975, 97, 6406.

186

Photochemistry

yield for Co", and this effect appears to be quite general for compounds of the type [CO(NH,),X]~+,being found for X = N3, Br, C1, and NCS. Evidence has been presented to demonstrate that this phenomenon is due to photo-oxidation of the solvent. The primary process is presumed to be similar to that shown in

0.3

t

10

20

30 Excitation Energy I k K

40

50

Figure 5 Variations in the quantum yields for Co" production as a function of excitation energy for [Co(NH,),N3I2+ and [Co(NH3),NCSI2+in water ( 0 and A respectiuely) and in 50% water-glycerol (+ and 0 respectively) (Reproduced by permission from J. Amer. Chem. SOC.,1975, 97,6406)

[Co(NH,),NCSI2+ (24)

(24) H,O+

+ -OH + NCS*OH + R1R2CHOH R ~ R ~ ~+O H

H+

~0111

hv

___+

([Co(NH,),NCS]+, H,O+) (24)

(38)

[Co(NH,),NCSI2+

(39)

+ 5NH3 + NCS- + H,O+ *OH + H+ H,O + NCS. H 2 0 + R1R2eOH R~R~C=O + H+ + CO" Co2+

(40) (41) (42)

(43)

(44)

equation (38) for [CO(NH,)~NCS]~+, and secondary reactions such as those represented in equations (39)-(44) are predicted (R1R2CHOH = glycerol). Excitation of the CT bands (330 nm) of [Co(NH3),SCN12+ causes both photoredox (a = 0.48) and photo-isomerization reactions (@ = 0.24).93

8s

A. Vogler

and H. Kunkely, Inorg. Chim.Actu, 1975, 14, 247.

Photochemistry of Inorganic and Organometallic Compounds 187 Photolysis in the solid state leads exclusively to isomerization, and no Colt could be detected. By analogy with the case of [CO(NH,),NO,]~+,the isomerization is presumed to take place in the initially formed radical pair. The spectra of [CO(CN)~X]~and [CO(NH,),X]~+exhibit CT bands separated In~ contrast with this, the threshold by only between 2000 and 4 0 0 0 ~ m - l . ~ energy for photoredox activity in [Co(CN),N3I3- is 7000 cm-l higher than that in [ C O ( N H , ) , N ~ ] ~ This + . ~ ~apparent anomaly may be rationalized when account is taken of the spin state of the Co" fragment formed.e6 In the case of the cyanocomplex this will be the low-spin (doublet) state which correlates with the lCT state. However, for the Co" ammine complex the high-spin (quartet) species is the stable state, and as this correlates with the reactive ,CT state, a low-energy pathway for the reaction is available. This difference accounts for the experimental observations. Although medium effects have been considered for photoredox processes, much less work has been carried out with photosubstitution reactions. An interesting study has now been reported by Scandola et al., for the photoaquation of [Co(CN),I3- [equation (46)].Q6The equation is known to proceed via

dissociation of the TIg LF state. Examination of the variation in quantum yield for the reaction in a number of alcohol-water mixtures reveals a strong correlation with solvent viscosity but no apparent dependence on the percentage water or dielectric constant (Table 2). This viscosity effect is attributed to the solvent

Table 2 Solvent efects on [ c O ( c N ) 6 I 3 - photo-aquation Q6 DieIectric % Alcoholic Solvent

Water Methanol-wa ter G1ycerol-water Ethanol-water Glycerol-water 1 ,Zpropanediol-water Glycerol-water

soIvent 60 20 60 40 60 60

constant 78 54 73 50 67 57 60

Viscosity 1 .o 1.7 2.0 2.6 4.7 5.3 14.7

Photo-aquation

a)

0.31 0.27 0.24 0.25 0.17 0.1 4 0.10

preventing diffusive escape of the reaction pair formed after dissociation of the excited state. As the authors point out, this role of the solvent cage may cause the measured quantum yield to differ substantially from the primary value for bond cleavage. The photosolvation of [Co(CN),13- has also been studied in a variety of organic solvents (methanol, ethanol, acetonitrile, dimethylformamide and ~ y r i d i n e ) .In ~ ~all cases the quantum yield was in the range 0.28-0.32. The importance of the solvent cage has also been emphasized in a report on the photo-anation reactions of [CO(CN),(H,~)]~-with I-, N3-, HNs, and O4

O6 O7

V. M. Miskowski and H. B. Gray, Inorg. Chem., 1975, 14,401. J. F. Endicott and G. J. Ferraudi, Inorg. Chem., 1975, 14, 3133. F. Scandola, M. A. Scandola, and C. Bartocci, J. Amer. Chem. SOC.,1975, 97, 4757. K. Nakamura, K. Jin, A. Tazawa, and M. Kanno, Bull. Chem. SOC.Japan, 1975,48, 3486.

188

Photochemistry HCN.98 With the anions as reactants, it appears that five-co-ordinate [Co(CN)JZhas a definite existence, whereas with the uncharged compounds, an interchange mechanism between water and the HX species in the second co-ordination sphere is assumed. Photosubstitution of [Co(CN),IS- by HzO and OH- is a convenient synthetic route to the corresponding complex [CO~~'(CN),X,-,]~- (m = 3 or 4, X = OH or H20).9gPhotochemical isomerization of [Co(CN),(H,O),]- favours the formation of the thermally less stable trans-compound (e.g. at pH = 3, @ = 313 nm, @)cje*ans = 0.30, @)ttonlrcje = 0.04).100In contrast with the thermal trans-cis isomerization, the photochemical interconversion proceeds via a nondissociative twist mechanism. A theory to predict the course of this reaction has been developed by Burdett.21 Several publications on the photochemistry of [Co(phen),(ox)]+ and [Co(bi~y)~(ox)]+ have been p ~ b l i s h e d . ~ Langford ~ ~ - ~ ~ ~and co-workers lol have shown that LF band excitation of [Co(phen),(ox)]+ I- in acidic solution causes reaction (47) to take place. The presumed mechanism is represented in equations (48)and (50). Under the acidic conditions employed, the Co" products of steps 2[Co(phen),(ox)]+

-

* [Co(phen),(ox)]+ A [Co"(phen),(ox-)]+ [Co(phen),(ox)]+

+

OX*-

2C02+

+ 4phen + ox2- + 2C02

[Co1Yphen),(ox*)I+

(47)

(48)

[Co(phen),l2+

+ OX*-

(49)

[Co(phen),ox]

+ 2COZ

(50)

(49) and (50) decompose to give the overall stoicheiometry of reaction (47). While the reactive excited state cannot be unambiguously identified, the authors favour a ligand-field state (Tlg, 3T,,, or possibly even 6T2g).The quintet species might well be lowest in energy if the excited state is considerably distorted. More details on the outer-sphere redox reaction of [Co(phen),13+ and oxalate ion have been reported.lof Hennig et al. have also examined the redox decomposition of [Co(phen),(ox)]+ and [Co(bipy),(ox)]+ in solution lo2and in the solid state.lo3#lo4In neutral solution on irradiation at X < 400 nm (in bands variously assigned to CT, intra-ligand, or ligand field transitions) they observe that the quantum efficiency of the reaction is both wavelength and counter-ion dependent.lo2 In the solid state, the effect of the anion is such that the quantum yield diminishes in the order C3H,COz- > HC02- > F- > C1- > Br- > I- > Clod-. The reaction appears to proceed via steps (48) to (50), and as evidence for this the oxalate radical has been identified by e.s.r. In a study of the products formed on photolysis of various oxalato-Co"' complexes at low temperatures, a complex identified as a Co"' species has been observed.106 This L. Viaene, J. D'Olieslager, and S. De Jaegere, Bull. SOC.chim. belges., 1976, 85, 89. L. Viaene, J. D'Olieslager, and S. De Jaegere, Znorg. Nuclear Chem., 1975, 37, 2435. loo L. Viaene, J. D'Olieslager, and S. De Jaegere, Znorg. Chem., 1975, 14, 2736. lol C. P. J. Vuik, N. A. P. Kane-Maguire, and C. H. Langford, Canad. J . Chem., 1975,53,3121. lo* H. Hennig, K. Jurdeczka, and P. Thomas, 2. Chem., 1976, 16, 161. lo3 H. Hennig, K. Hempel, and P. Kertscher, 2. Chem., 1975, 15, 491. lo4 H. Hennig, K. Hempel, M. Ackermann, and P. Thomas, 2.anorg. Chem., 1976,422, 65. lo6 A. L. Poznyak, S. I. Arzhankov, and B. A. Budkevich, Doklady Akad. Nauk Belarus. S.S.R., 98

99

1975, 19, 905.

Photochemistry of Inorganic and Organometallic Compounds 189 complex decomposes to form CO" at temperatures above 200K. The triplet biacetyl- or triplet 9-carboxyanthracene-sensitizeddecomposition of [CO(OX),]~has been described.lo6 The process which is several orders of magnitude slower than the diffusion-controlled rate is presumed to proceed via energy transfer.lo6 Other reports deal with the e.s.r. and optical spectra of intermediate species formed on the low-temperature photolysis of cobaltammine carboxylate and chelate c o m p l e ~ e slo* , ~ ~the ~ ~reactions of aromatic compounds and C03'generated from [ C O ( N H ~ ) & O ~ ]the + , ~oxidation ~~ of Alizarin S by hydrogen peroxide catalysed by Co2+formed photochemically from [ C O ( N H ~ ) ~ N O ~ ] ~ + and the accelerating effect of U.V. light on the racemization of [Co(phen),l3+.ll1 Rhodium and Iridium.-The only photochemical reaction following LF band excitation of [Rh(NH3)6L]3+in aqueous solution at room temperature is the photo-aquation reaction (51).l12 The reactive excited state is the ,E species. At

+

CRh(NHa)5L]3+ H 2 0

A

[Rh(NH&(H20)l3+

+L

(51)

77 K in methanol-water glasses no photochemical reaction is observed, but detailed information on the energies and lifetimes of the thermally relaxed 3E states may be obtained. These data are presented in Table 3. It may be noticed

Table 3 Photo-aquationquantum yields and photoemission data for [Rh(NH,) &I3+ '12 Absorption Ligand L

La*lnm

Ammonia 4-Methylpyridine Pyridine

305 302 302 302 300 301 316

3-Chlorop yridine

Benzonitrile Acetonitrile Water

Free ligand PKB 9.3 6.0 5.3 2.8 - 10 - 10

-

Quantum yield at 25 "C 313 nm 0.075 0.091 0.14 0.34 0.35 0.47 0.43

Emission lifetime T at 77 K/(ps) 18.7 18.6 17.1 13.6 7.6 5.0 2.7

that the change in photo-aquation quantum yield parallels that for the ligand donor ability, although showing no correlation with parameters such as the absorption maxima in the absorption spectra. A relationship is also apparent between the photoreaction efficiency and the rate constant for non-radiative transition (/in). (As the emission is weak, T = l / k n . ) Of the possible mechanisms for the photosubstitution reactions the authors give serious consideration to the hot ground-state mechanism proposed by Endicott.DOHowever, no way of distinguishing between this and dissociative reaction of the excited state is obvious at present. S. Sakuraba, A. Kakuta, and R. Matsushima, Bull. Chem. SOC.Japan, 1975,48,2660.

lo6

lo7

A. L. Poznyak and V. V. Pansevich, Vestsi Akad. Navuk Belarus. S.S.R., Ser. khim. Navuk, 1975, 50.

A. L. Poznyak and S. A. Arzhankov, Doklady Akad. Nauk Belarus. S.S.R., 1975, 19, 439. log S.-N. Chen, M. Z. Hoffman, and G. H. Parsons, J. Phys. Chem., 1975,79, 1911. l10 S. D.Varfolomeev, S. V. Zaitsev, T. E. Vasil'eva, and I. V. Berezin, Doklady Akad. Nauk S.S.S.R., 1974, 219, 895. ll1M.Yamamoto and Y. Yamamoto, Inorg. Nuclear Chem. Letters, 1975, 11, 691. 112 J. D.Peterson, R. J. Watts, and P. C. Ford, J. Amer. Chem. SOC.,1976, 98, 3188. lo8

190

Photochemistry

The results of the photolysis of trans- and cis-[RhN,X,]+ [N4 = 2-en or cyclam (1,4,871l-tetra-azocyclotetradecane); X = C1, Br, or I] have been communi~ated.ll~ ?~~ ~ and compared with those of earlier reports on these syst e m ~11.6 ~With ~ ~the~ exception of cis-[Rh(en),Cl,]+, halide photo-aquation is the only process occurring. This reaction proceeds with retention of configuration. Quantum yields for the trans-complexes lie in sequence 0 1 - > @ ) B ~ - > @cI-, whereas the reverse order holds for cis-[Rh(cyclam),X,]+. In contrast with a previous report,l16 quantum yields are higher for CT than for LF band excitation. For cis-[Rh(en),Cl,]+ both ethylenediamine and chloride ion aquation were observed, although determination of the quantum yields was hampered by competing dark reactions. LF band photolysis of [RhC1613- yields [RhC1,(H2O)l2- with a quantum yield of 0.024.l'' This observation supports the hypothesis that for Rh"' complexes with weak-field ligands the photochemical reactions will proceed inefficiently. For photoredox reactions of Rh"' compounds it is important to understand more about the lability of the powerfully reducing Rh" species formed. This problem has been studied by following the decomposition of [Rh(NH3),C1]+, [Rh(NH3),(H20)I2+, and [Rh(NH3)4Br2]formed on electron capture by the corresponding Rh"' complex after pulse radiolysis.lls In all cases loss of two ligands occurs very rapidly to yield [Rh(NH3)J2f, which exchanges NH, molecules only on a millisecond time scale. Photochemical isomerization of L,RhHX, (L = tertiary phosphine or arsine ; X = Br or C1) has been described.ll@A mechanism involving dissociation of the arsine or phosphine ligand seems most probable. Emission from thermally non-equilibrated levels of [IrCl,(phen)(4,7-Me2phen)]Cl and [IrClz(phen)(5,6-Me,phen)]Cl has been analysed in 121 From these studies it is possible to derive selection rules for non-radiative transitions. These may be summarized as dn* -f dn*, or mm* + mm* but not dn* -+ mn*, and they are particularly strict when the energy gaps between the states are small, or when the states are localized on different parts of the molecule. With [IrCl,(phen),]Cl and [IrC1,(5,6-Me2phen)]C1at 77 K, emission is from the dn* (MLCT) and mn* states, respectively.lZ2However, at higher temperatures dd* emission predominates in both cases. This observation is ascribed to slow, thermally activated, radiationless transition between the CT- or ligand-centred states and the dd* (LF) states. Caution must therefore be exercised in assuming that the state emitting at low temperatures is the same as that involved in photochemistry under ambient conditions. J. Sellan and R. Rumfeldt, Canad. J . Chem., 1976, 54, 519. J. Sellan and R. Rumfeldt, Canad. J. Chem., 1976, 54, 1061. C. Kutal and A. W. Adamson, Inorg. Chem., 1973, 12, 1454. 116 M. M. Muir and W.-L. Huang, Inorg. Chem., 1973, 12, 1831. 11' N. A. P. Kane-Maguire and C. H. Langford, Inorg. Chim. Acra, 1976, 17, L29. n8 J. Lilie, M. G. Simic, and J. F. Endicott, Inorg. Chem., 1975, 14, 2129. llS C. E. Betts, R. N. Hazeldine, and R. V. Parish, J.C.S. Dalton, 1975, 2215. R. J. Watts, M. J. Brown, B. G. Griffith, and J. S. Harrington, J. Amer. Chem. SOC.,1975,97, 6029. 121 R. J. Watts, B. G. Griffith, and J. S. Harrington, J. Amer. Chem. SOC., 1976, 98, 674. 12a R. J. Watts, T. P. White, and B. G . Griffith, J. Amer. Chem. SOC., 1975, 97, 6914. llS

114 ll6

Photochemistry of Inorganic and Organometallic Compounds

191 Nickel.-The photochromism of [Ni(CS,N(CH,Ph),},]+ has been investigated using an n.m.r. detection method.123 The light-induced step (@ z 0.19) is a photoreduction of the NiIVto Ni" fragments [equation (52)]. 2[NiL3]+

+ 2Br-

NiL,

+ NiBr, + L,

(52)

Platinum.-On photolysis in acidic solution, [Pt(NH&I4+undergoes both aquation and reduction, whereas under alkaline conditions the primary products are reported to be [Pt(NH3)5NH2]3+and [Pt(NH3)4(NH2)2]2+.124 Other workers and of [Pt(en)have investigated the photolysis of C~S-[P~B~,(NO,)(NH,),],~~~ (py)(N02)2C1]+.126 Emission spectra of salts of the type K,PtC&-,Br, (n = 0-6) have been Photolysis of Pt" doped silver bromide crystals causes the formation of e.s.r. detectable centres, formulated as [PtBr6]5-.128 Recent studies on luminescent Pt" systems include those on [PtC1412- at 4 K,lagon the emission lifetime of MgPt(CN)4,7H20 as a function of temperature,130 and on the effects of pressure on the emission of various [Pt(CN),I2

salt^.^^^^ 13, Copper, Silver, and Gold.-A report on the photochemical decomposition of bis(diethyldithiocarbamato)copper(zI) has been Copper salts dramatically improve the yield of formylpyrroles produced on irradiation of pyridine 0 ~ i d e s . lTriplet ~~ quenching effects have been ruled out, and a probable explanation is that the copper ion interacts with one of the intermediates formed during the reaction. Further examples of CuI-amine complexes, which exhibit fluorescent thermochroism, have been r e p ~ r t e d . l ~ ~ - ~ ~ ~ Photolysis of silver salts of carboxylic acids provides a convenient route to alkyl ~adica1s.l~~ Silver ions enhance the rate of photopolymerization of N-vinylc a r b a z ~ l e .140 ~~~, A study of the photoreduction of [AuCI,]- by oxalate ion has been reported.141 123

D. P. Schwendiman and J. I. Zink, J. Amer. Chem. SOC.,1976, 98, 1248. R. M. Orisheva, S. P. Gorbunova, and G. A. Shagisultanova, Zhur. neorg. Khim., 1975, 20,

lZ4

1934.

126

128

R. I. Rudnyi, I. F. Golovaneva, 0. N. Evstaf'eva, A. V. Babaeva, and L. I. Solomentseva, Zhur. neorg. Khim., 1975,20, 422. R. I. Rudyi, I. F. Golovaneva, and 0. N. Evstaf'eva, Isuest. Akad. Nauk S.S.S.R.,Ser. khim., 1975, 1480.

V. Lipnitskii, N. M. Ksenofontova, A. B. Kovrikov, V. G. Popov, and D. S. Umreiko, Izvest. Akad. Nauk S.S.S.R., Ser. fiz., 1975, 39, 2241. lZ8 R. S. Eachus and R. E. Graves, J. Chem. Phys., 1975, 63, 83. lZQ H. H.Patterson, T. G. Harrison, and R. J. Belair, Inorg. Chem., 1976, 15, 1461. I3O G. Gliemann, H. Otto, and H. Yersin, Chem. Phys. Letters, 1975, 36, 86. lS1 Y. Hara, Chem. Letters, 1975, 1063. 132 M. Stock and H. Yersin, Chem. Phys. Letters, 1976, 40, 423. 133 K. K. M. Yusuff, P. M. Madhusudanan, and C. G. R. Nair, Current Sci., 1975, 44,221. lS4 F. Bellamy, P. Martz, and J. Streith, Heterocycles, 1975, 3, 395. ls6 H. D. Hardt and H. Gechnizdjani, Inorg. Chim. Acta, 1975, 15, 47. lSe H.D.Hardt and A. Pierre, Naturwiss., 1975, 62, 298. ls7 M. A. S. Goher, Naturwiss., 1975, 62, 237. 138 E. K. Fields and S. Meyerson, J. Org. Chem., 1976, 41, 916. 13s Y.Takeda, M. Asai, and S. Tazuke, Polymer J., 1975, 7 , 366. 140 M. Asai, Y.Takeda, S. Tazuke, and S. Okamura, Polymer J., 1975, 7 , 359. 141 B. S. Maritz, R. V. Eldik, and J. A. Van den Berg, J. S. African Chem. Inst., 1975, 28, 14. lZ7 1.

192

Photochemistry

Mercury.-The light-induced formation of dimethylmercury from inorganic mercury in aqueous acetic acid in the presence of HgO, HgS, or elemental sulphur has been Lanthanides.-The photo-oxidation of tryptophan and methionine in lysozymelanthanum(1n) complexes has been i n v e ~ t i g a t e d .Other ~ ~ ~ authors have utilized the luminescence of Eu"' in studies of its interaction with transfer RNA,144and with pyridoxylidene-amino-acid 146 Energy transfer from Tb3+to anthracene and 9-anthrylmethylketone has been studied in solvents of varying p01arity.l~~ It has been shown that in poor donor solvents, the rate of energy transfer to the 9-anthrylmethylketone is far greater than that to anthracene, because co-ordination to the Tb3+ion is possible with the former compound. Shakhverdov has investigated the quenching of fluorescent organic dyes by lanthanide ions in a number of organic solvents (alcohols, DMSO, and pyridine).148-150The quenching takes place in ion-pairs of the negatively charged dye molecule and the lanthanide ion. Dipole-dipole energy transfer is responsible for the quenching in many cases [e.g. with Nd"' and Hot''], but with some lanthanides the deactivation process probably involves either reversible photoreduction (e.g. with Eu"') or reversible photo-oxidation (e.g. with Ce"'). Energy transfer between the triplet state of coumarinium ion and Eu3+ in alkanesulphonic acid glasses at 77 K,151 between triplet phenanthrene and Ce3+,Pr3+,and Nd3+ in methanol-water solutions at temperatures between 120 and 293 K,g and between benzopyranopyridine derivatives and Eu3+ or Tb3+,152 have been reported. The technique of circularly polarized emission has been employed in the study of Tb"' and Ed1' complexes of optically active carboxylic acids in In the same publication it has been recorded that the efficiency of the quenching of Tb"' emission by Eu"' in solutions of L-malic acid is a function of pH. It is suggested that under certain pH conditions, the transfer of energy occurs between ions complexed to the same malic acid molecule. Intermolecular energy transfer between Tb(acac), and La(acac), or other tris(acety1acetonato)lanthanide complexes has been described.154,155 The rate of transfer is sensitive to the solvent used, being negligible for strongly co-ordinating compounds such as pyridine, but increasingly markedly for non-polar solvents such as benzene. This effect is attributed to the formation of mixed dimers in the non-polar solvent. Quenching of the excited state of U022+ by Eu3+proceeds only in part by energy H. Agaki, Y . Fujita, and E. Takabatake, Chem. Letters, 1976; 1, Nbpon Kagaku Kaishi, 1975, 1273. 14s G. Jori, M. Folin, G. Gennari, G. Galiazzo, and 0. BUSO, Photochem. and Photobiol., 1974, 19, 419. 144 J. M. Wolfson and D. R. Kearns, Biochemistry, 1975, 14, 1436. 145 V. F. Zolin, L. G. Koroneva, and V. I. Tsaryuk, Biofizika, 1975, 20, 194. 146 V. F. Zolin and L. G. Koreneva, Biofizika, 1975, 20, 198. 147 V. L. Ermolaev and V. S. Tachin, Optika i Spektroskopiya, 1975, 38, 1138. 14* T. A. Shakhverdov, Optika i Spektroskopiya, 1975, 38, 1228. us T. A. Shakhverdov, Optika i Spektroskopiya, 1975, 39, 786. lSo T. A. Shakhverdov and Z . N. Turaeva, Izvest. Akad. Nauk S.S.S.R.,Ser. fiz., 1975,39, 1952. lS1 P. G. Tarassoff and N . Filipescu, J.C.S. Chem. Comm., 1975, 208. lS2 A. Fujimoto, A. Sakurai, and E. Iwase, Bull. Chem. SOC.Japan, 1976, 49, 809. lSs C. K. Luk and F. S. Richardson, J. Amer. Chem. SOC.,1975, 97, 6666. lS4 G. D. R. Napier, J. D. Neilson, and T. M. Shepherd, J.C.S. Faraday IZ, 1975, 71, 1487. J. D. Neilson and T. M. Shepherd, J.C.S. Faraday ZZ, 1976, 72, 557.

142

Photochemistry of Inorganic and Organometallic Compounds 193 transfer; the rest of the deactivation occurs via some other unidentified pathway.lS6 The first report of fluorescence lifetime studies of a rare-earth chelate in the gas phase [Tb(ButCOCHCOBut),] indicates that the lifetime is much shorter than in Thus it decreases from solution and also markedly ternperat~re-dependent.~~~ approximately 1 ps at 235 "C to about 0.2 ps at 290 "C. The mechanism proposed requires intramolecular energy transfer from the rare earth ion to the chelate, followed by chelate relaxation. Tb3+Ions luminesce from the SD3as well as the 6D4excited state. (The states are separated by approximately 6000 cm-l.) By using a pulsed laser (265 nm) source for excitation, the 5D3to 5D4interconversion has been investigated both in borate glasses and D 2 0 solution.16* In D,O solution the rate constant for this process is rather low (1.5 x los s-l), and in concentrated solutions it is accompanied by an efficient Tb3+-catalysedinterconversion. In POC1,-SnCI, solution the decay kinetics of the 6D4luminescence of Tb3+ have been found to depend on the wavelength of e ~ c i t a t i 0 n . l This ~ ~ effect is also attributed to the slow 6D3--f 5D4radiationless transition. The rate of non-radiative crossing between excited states has been investigated for Eu"' and Tbl" p-diketone chelates using nanosecond laser excitation.160 For complexes such as Eu(PhCOCHCOPh), where the ligand triplet level lies above the 6D1state, initial excitation into a ligand-localized state is followed by population of both the and the lowest 5D0excited states. The subsequent internal conversion from the level to the state has been monitored.160 Several other reports on the photophysical properties of luminescent rare earth P-diketonates and related chelates have been published.1s1-16D It is well known that the luminescence of rare earth ions is markedly affected by the solvent. In particular, solvents with high-energy vibrations (e.g. O-H) cause rapid deactivation of the excited state. For Nd3+ in solutions of tributyl phosphate it has been demonstrated that small quantities of water quench the luminescence. Stern-Volmer kinetics are obeyed, and from this relationship the rate constant for quenching at 293 K (1.9 x lo6 dm3mol-1 s-l) has been deduced.170 The effect of water on the emission lifetime of Eu"' in acetone solution has been studied, and from the data so obtained it has been possible to 168

Is7 lS8 lS8 160 161

162 163

16*

B. D . Joshi, A. G . I. Dalvi, and T. R. Bangia, J. Luminescence, 1975, 10, 261. R. R. Jacobs, M. J. Weber, and R. K. Pearson, Chem. Phys. Letters, 1975, 34, 80. C. R. Goldschmidt, G. Stein, and E. Wuerzberg, Chem. Phys. Letters, 1975, 34, 408. P. Tokousbalides and J. Chrysochoos, J . Chem. Phys., 1976, 64, 1863. W. M. Watson, R. P. Zerger, J. T. Yardley, and G. D. Stucky, Inorg. Chem., 1975, 14,2675. N. S. Poluektov, I. I. Zheltvai, G. I. Gerasimenko, M. A. Tishchenko, and A. A. Kucher, Zhur. priklad. Spektroskopii, 1976, 24, 276. B. A. Knyazev, V. M. Moralev, and E. P. Fokin, Optika i Spektroskopiya, 1976, 40, 93. M. A. Tishchenko, N. S. Poluektov, and I. I. Zheltvai, Ukrain. khim. Zhur., 1975, 41, 197. T. Fukuzawa, N. Ebara, M. Katayama, and H. Koizumi, Bull. Chem. Sac. Japan, 1975, 48, 3460.

165 166 167

168 169

170

A. P. Aleksandrov, Optika i Spektroskopiya, 1975, 38, 561. G. E. Malashkevich and V. V. Kuznetsova, Zhur. priklad. Spektroskopii, 1975, 22, 230. E. T. Karaseva, A. P. Golovina, M. I. Gromova, and I. P. Efimov, Koord. Khim., 1975,1,260. N. S. PoIuetkov, V. N. Drobyazko, S. B. Meshkova, S. V. Bel'tyukova, and L. I. Kononenko, Doklady Akad. Nauk S.S.S.R.,1975, 224, 150. E. T. Karaseva and V. E. Karasev, Koord. Khim., 1975,1, 926. E. M. Zinina and A. V. Shablya, Optika i Spektroskopiya, 1975, 39, 686.

1 94

Photochemistry

derive the stability constants for the aquo-complexes of Eu"' under these cond i t i o n ~ The . ~ ~lifetimes ~ of various lanthanide tributyl phosphate complexes are lengthened by four to seven times on perdeuteriation of the ligand.17, Other authors have studied the photophysical properties of Eu"' in acetic acidchloroform or carbon tetrachloride solvent^,^^^ the effect of solvent on the spectra of Ed1' chelate and the influence of pulse intensity on the luminescence lifetimes and intensities of rare earth ions in solid matrices and in POC1,SnCl, Actinides.-Kemp and co-workers 176 have carried out a particularly detailed investigation of the interaction of photo-excited UOzz+with a wide variety of substituted carboxylic acids, and also with some esters, alkenes, and unsaturated ketones. A selection of these data is presented in Table 4. It has been observed Table 4 Stern- Volmer constants for the quenching of UOZ2+ emission by carboxylic acids (RC0,H) 176 R K&/dm3 mol-1 R Ksv/dm3mol -l H Me Et cyclohexy1 Ph EtO(CH,),

6.5 0.28 1.28 89 4150 203

CICH, BrCH, ICH, ICH2CHz MeCH=CH CH,=CHCH,

0.15 0.32 3 600 1960

8.74 1600

that the quenching of *UOZ2+ may proceed by a number of mechanisms depending on the substrate. For example, for 10 substituted benzoic acids, a Hammett plot yields a p value of -0.88. This is consistent with formation of an exciplex involving only a small amount of charge transfer. For olefinic compounds the authors also support the exciplex formation mechanism previously proposed by Matsushima for other systems. Iodine-containing compounds (e.g. ICH2CH2C0,H) quench very efficiently, probably due to charge transfer interaction of *U0,2+and the iodine atom of the substrate via a short-lived exciplex. Alkoxycarboxylic acids also cause effective deactivation of the excited UOz2+. However, in this case the quantum yield for UIV is found to be 1.89, suggesting that the principal route for quenching is by chemical reaction. Matsushima and co-workers have examined the UOZ2+-sensitized isomerization of stilbene in more The ratio of trans- to cis-stilbene is sensitive to the concentrations of stilbene and uranyl ion, varying from 0.96 to 3.0. While it has been shown that free radical processes are not important, and that triplettriplet energy transfer within an exciplex is the most probable mechanism, no thoroughly satisfactory explanation for the above phenomenon has been proposed. 171

J73 174

176 176

17'

V. P. Gruzdev and V. L. Ermolaev, Zhur. neorg. Khim.,1975,20, 2650. E. B. Sveshnikova, A. P. Serov, and V. P. Kondakova, Optika i Spektroskopiya, 1975, 39, 285. J. Chrysochoos, Spectroscopy Letters, 1975, 8, 771. L. I. Kononenko, S . V. Bel'tyukova, S. B. Meshkova, V. N. Drobyazko, and N. S . Poluektov, Dopovidi Akad. Nauk Ukrain. R.S.R., Ser. B, 1975, 816. A. V. Aristov, V. P. Kolobkov, P. I. Kudryashov, and V. S . Shevandin, Optika i Spektroskopiya, 1975, 39, 281. M. Ahmad, A. Cox, T. J. Kemp, and S. Quaisar, J.C.S. Perkin IZ, 1975, 1867. R. Matsushima, T. Kishimoto, and M. Suzuki, Bull. Chem. SOC.Japan, 1975, 48, 3028.

Photochemistry of Inorganic and Organometallic Compounds 195 The light-induced reaction of UOz2+and benzaldehyde in aqueous acetone solution gives Urv(0= 0.14) and benzil (0= 0.12), as well as small amounts of condensation products of benzaldehyde and a~et0ne.l'~This is consistent with the initial photochemical process (53). Other authors studying the photo-

+

*U022+ PhCHO

U02+

+ H+ + PhkO

(53)

reduction of uranyl ion by organic conipounds have described the detection of U02+following irradiation of solid uranyl f ~ r m a t e a, ~polarographic ~~ investigation of the photoreduction of U022+in ethanol,lEOand an attempt to induce photoreduction of U02+,prepared by laser photolysis of UOZ2+in ethanol.lE1 As the logarithm of the rate constant for quenching of *U022+by metal ions (Ag+ Ce3+, Co2+, Cu2+, Fe2+, Fe3+, Hg22+,Mn2+, Ni2+, and Pb2+) decreases linearly with increasing ionization potential of the metal ion, it is proposed that an electron-transfer process is operative.17 Possible mechanisms are illustrated in equations (54) and ( 5 5 ) . In the case of Mn2+as quencher, Mn3+was identified 9

after flash photolysis suggesting that at least in some reactions complete electron transfer ( 5 5 ) occurs. Self-quenching of U022+luminescence has been reported ( k , = 4 x lo6 dm3mol-1 s-l).lS2 In the same study it has been shown that the *UOZ2+lifetime is markedly temperature dependent (Eact = 41 kJmol-l), and it is proposed that the deactivation process may involve reversible photooxidation of water. It has been reported that the quenching of *UOZ2+by Ce3+ proceeds via energy transfer and another unidentified radiationless process.156 When mixed, solutions of U022+and Eu2+ he mi luminesce.^^^ The emitting species is formed by disproportionation of the UOz+ ions [reaction (57)]. Further investigation of

this system also revealed that U02+is an efficient quencher of U022+excited Photodimerization of (25) gives (26), whereas in the presence of UOz2+the product is (27).lS5 The stereospecificity is attributed to the bulk of the uranyl group to which one molecule of (25) co-ordinates. R. Matsushima, K. Mori, and M. Suzuki, Bull. Chem. SOC.Japan, 1976, 49, 38. B. Claudel, J. P. Puaux, and H. Sautereau, Compt. rend., 1975, 280, C, 169. 180 M. Feve, Compt. rend., 1974, 279, C, 721. 181 J. T. Bell and M. R. Billings, J . Inorg. Nuclear Chem., 1975, 37, 2529. lSa P. Benson, A. Cox, T. J. Kemp, and Q. Sultana, Chem. Phys. Letters, 1975, 35, 195. lS3 R. G. Bulgakov, V. P. Kazakov, G . S. Parshin, D. D. Afonichev, and G . L. Sharipov, Khim. uysok. Energii, 1975, 9, 92. la4 R. G. Bulgakov, V. P. Kazakov, and S. V. Lotnik, Khim. uysok. Energii, 1975, 9, 555. N. W. Alcock, N. Herron, T. J. Kemp, and C. W. Shoppee, J.C.S. Chem. Comm., 1975,785. 178

196

Photochemistry

PhCH=CHyO

COCH=CHPh

LIZPh

PhCH=CHkO

I

PhCH=CHCO

The experimental difficulties and the theory of laser-induced photochemistry for separation of 235Uand 238Uhave been discussed in a series of report^.^^^-^*^ In an interesting discussion of the quantum properties of the lowest excited state of U022+,it is mooted that the question whether the excited state is singlet or triplet is of little significance because of spin-orbit coupling. Arguments are presented to show that the state has i2 = 4 and is of even parity.lsO Other publications dealing with the spectroscopic properties of Uvl compounds discuss the influence of co-ordination geometry on the lifetime of U022+in crystalline environments,lsl SCF calculations of the electronic structure of UOa2+,ls2and ls4 the fluorescence of UF6.1g3The photochemical reduction of the plutonyl ion (PuO,~+)on irradiation in ethanol produces Pu4+ and Pu3+.lg5As in the case of U022+,the initial photochemical act appears to be one-electron transfer to give Pu02+. 2 Transition-metal Organometallics and Low-oxidation-state Compounds Wrighton lg6and Strohmeier lg7have summarized their groups' contributions to the photochemical generation of catalysts from organometallic compounds.

Titanium, Zirconium, and Hafnium.-Interest continues this year in the photochemistry of titanocene derivatives. Vitz and Brubaker have examined in more detail the photo-exchange reaction (58) reported earlier.198~ lg9 Quantum yields B. B. Snavely, Report UCRL-75725, 1974 (Chem. Abs., 1975, 83, 169 712). A. Hartford, Report UCRL-76601, 1975 (Chem. A h . , 1975, 83, 210 559). B. B. Snavely, R. W. Solarz, and S. A. Tuccio, Report UCRL-76923, 1975 (Chem. Abs., 1976, 84, 113 220). ISB B. B. Snavely, R. W. Solarz, and S. A. Tuccio, Lecture Notes in Physics, 1975, 43, 268. l 9 O C. K. Joergensen and R. Reisfeld, Chem. Phys. Letters, 1975, 35, 441. lgl G. C. Joshi, Indian J. Pure Appl. Phys., 1976, 14, 180. lg2 M. Boring, J. H. Wood, and J. W. Moskowitz, J. Chem. Phys., 1975, 63, 638. lQ3 A. Andreoni and H. Buecher, Chem. Phys. Letters, 1976, 40,237. 194 P. Benetti, R. Cubeddu, C. A. Sacchi, 0. Svelto, and F. Zaraga, Chem. P h p . Letters, 1976, 40, 240. lg6 J. T. Bell and H. A. Friedman, J. Znorg. Nuclear Chem., 1976, 38, 831. 196 M. S. Wrighton, D. S. Ginley, M. A. Schroeder, and D. L. Morse, Pure Appl. Chem., 1975, 41, 671. lg7 W. Strohmeier, J. Organometallic Chem., 1975, 94, 273. IBa E. Vitz and C. H. Brubaker, J. Organometallic Chem., 1976, 104, C33. lD9 E. Vitz, P. J. Wagner, and C. H. Brubaker, J. Organometallic Chem., 1976, 107, 301. lE6

lS7

Photochemistry of Inorganic and Organometallic Compoundr hv

Cp,TiCl, 4- (C6D6)2TiCl2

197

2Cp(C6D6)TiC&

(58)

are wavelength dependent (e.g. at 313 nm, 0 = 0.02; at 520 nm, @ = 0.007). In contrast with the observations of other workers,200no decomposition of Cp,TiCl, to give CpTiCl, could be detected when the solvent was carefully purified. This indicates that formation of cyclopentadienyl radicals is not a major reaction pathway for the excited state, and the mechanism for the exchange reaction is therefore still uncertain. Transfer of cyclopentadienyl ligands between (28) and Cp2TiClzto form the (CH,),-bridged dimer species proceeds only in low yield. Photo-induced interchange reactions similar to (58) have been observed for the Ti"' compound (29) and for Cp,VCl,.les Photolysis of the dialkyl complexes Cp,MR2 (M = Ti, Zr, and Hf) leads to the corresponding, highly reactive, co-ordinatively unsaturated metallocene MCp,. /

,TiCI,

(HzQ

CP

cp\

c1

cp

/ \ /

Ti

Ti

/ \ / \

cp

c1

cp

s-s /,s-s

,s/

Cp,Ti

Irradiation of the dialkyl compound in the presence of some suitable ligand L affords a most convenient route to the derivative Cp,TiL2. Recent examples are the preparation of Cp,Ti(CO), and its bis(7-indenyl) analogue by photolysis of the corresponding dimethyl compound in the presence of C0,,O1 the synthesis of (30) in 70% yield from Cp2TiMe, or Cp,Ti(CH,Ph), with elemental sulphur,202 and the formation of a polymeric bis(fluoreny1)zirconium compound from its dimethyl derivative.203The electronic structure of titanocene and derivatives is of considerable interest to theoreticians and is the subject of recent p ~ b l i c a t i o n s,05 .~~~~ The photopolymerization of styrene in the presence of TiC14, Ti(CH,Ph),, or CpTiClR (R = C1, Et or CPh3) has been monitored both by dilatometry and electrical conductivity measurements.206With Cp,TiClEt and Cp,TiC1CPh3 the polymerization is radical-initiated, confirming that the initial photoprocess is homolytic cleavage of the Ti-C(alky1) bond. For Cp2TiC1, and Ti(CH2Ph)4,no unambiguous mechanism could be proposed. However, for Ti(CH,Ph), the photochemical step is most probably a styrene-to-metal electron transfer within a styrene-Ti(CH,Ph), complex. TiC14, TiBr,, and VCl, are photo-initiators for the polymerization of isobutylene with visible light.207The propagating species are isobutylene radical cations produced on irradiation of the MX4-olefin complex [reaction (59)]. 200

201 2 oa 2 03

B04 206

2 06

2 07

R. W. Harrigan, G. S. Hammond, and H. B. Gray, J. Organometallic Chem., 1974, 81, 79. H. G . Alt and M. D. Rausch, 2.Naturforsch., 1975, 30b, 813. E. Samuel and C. Giannotti, J. Organometallic Chem., 1976, 113, C17. E. Samuel, H. G . Alt, D. C. Hrncir, and M. D. Rausch, J. Organometallic Chem., 1976,113, 331. J. W. Lauher and R. Hoffmann, J . Amer. Chem. SOC.,1976, 98, 1729. J. L. Petersen, D . L. Lichtenberger, R. F. Fenske, and L. F. Dahl, J. Amer. Chem. SOC., 1975, 97, 6433. T. S. Dzhabiev, F. S. D'yachkovskii, and L. I. Chernaya, Izvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1091. M. Marek, L. Toman, and J. Pilar, J. Polymer Sci., Part A-1, Polymer Chem., 1975,13, 1565.

198

Photochemistry [Me,C=CH,.MX,]

hv

[Me,C.CH,]’+

+ [MX.,]-

(59)

Vanadium, Niobium and Tantalum.-Emission from salts of [Ta(CO),]- and [Nb(CO),]- {but not [v(Co)&}has been recorded for samples both in the solid state and in rigid Interestingly this behaviour contrasts with that of the isoelectronic Mo(CO), and W(CO), from which no luminescence has been detected. The emission quantum yields and lifetimes are markedly temperature dependent (falling by approximately a hundred-fold between 22 and 100 K), as previously noted for other heavy-atom co-ordination compounds such as ruthenocene. Although the emission is assigned to that from the ‘triplet’ LF (3T1,) state, the apparent lack of distortion in the excited state suggests that it also has substantial (M-r* CO) CT character. Photo-substitution of [V(CO),]by acetonitrile or pyridine in solution proceeds with quantum yields of 0.5-0.6. Chromium, Molybdenum, and Tungsten.-Photosubstitution of carbon monoxide by other ligands continues to be widely used in the syntheses of substituted metal carbonyl compounds (some recent examples are collected in Table 5, p. 219), and for the generation of catalytically active species.1g6The prototype of this reaction has been that involving the Group VI metal hexacarbonyls. Since the pioneer work of Strohmeier, it has been assumed that the quantum yield for the photodecomposition reaction (60) is unity. However, Nasielski and Colas 209 have

now shown that at least in the case of Cr(CO),, the quantum yield is only 0.67. This was measured by allowing the co-ordinatively unsaturated Cr(CO), to be scavenged by pyridine, forming Cr(CO),py. Photolysis of Cr(CO), in low temperature matrices produces Cr(CO), having C,, 211 The visible spectrum of this species is very sensitive to the matrix material (Ne, Ar, Kr, Xe, CF4, or CH,), and this phenomenon has been attributed to a stereospecific interaction between the Cr(CO), and the matrix species. As the absorption bands of these ‘complexes’ are well separated [e.g. Amax Cr(CO),*- Ne, 628 nm; Cr(CO), **. Kr, 487 nm] selective excitation experiments are possible. In these cases, irradiation in mixed matrices (e.g. Xe-Ne) allow ‘ligand‘ exchange reactions such as (61) to be observed. The weak bond Cr(CO)5-.Ne

hv(619 nm) 7 Cr(CO)6--Xe hv(432 nm)

between Cr(CO), and methane in low-temperature matrices suggests that a similar interaction may be present at ambient temperatures for Cr(CO), in solvents such as cyclohexane which are normally considered to be non-coordinating. Cr(CO), may also be synthesized by the reaction of chromium atoms in carbon monoxide or mixed carbon monoxide-argon matrices. It had previously been suggested from i.r. evidence that the trigonal bipyramidal Dah 208

2oB 210

211

M. S. Wrighton, D. I. Handeli, and D. L. Morse, Znorg. Chem., 1976, 15, 434. J. Nasielski and A. Colas, J. Organometallic Chem., 1975, 101, 215. R. N. Perutz and J. J. Turner, J. Amer. Chem. Soc., 1975, 97, 4791. J. K. Burdett, M. A. Graham, R. N. Perutz, M. Poliakoff, A. J. Rest, J. J. Turner, and R. F. Turner, J . Amer. Chem. SOC.,1975,97,4805.

Photochemistry of Inorganic and Organometallic Compounds 199 isomer is also produced under the latter conditions.212 It has now been unambiguously demonstrated that the bands assigned to the species belong to Cr(CO), *- Ar, Cr(CO),-- CO, and Cr(CO)4.211 The structures of Mo(CO), and Mo(CO),, formed by irradiation of Mo(CO), in a methane matrix, have been found to be surprisingly It has long been felt that one of the restrictions in comparing photochemistry in matrices and in solution might be that the rigid environment of the matrix would prevent the escape of bulky ligands from the matrix cage. A recent report 214 on W(CO),L (L = pyridine or 3-bromopyridine) illustrates that matrix photochemistry may parallel that observed in Thus irradiation at long wavelengths (320 c h c 390nm) leads to ligand expulsion [equation (62)], whereas at shorter wavelengths (e.g. 254 nm) CO dissociation [equation (63)] is also observed. As reaction (62) is photoreversible it is probable that, as in other examples of matrix photochemistry, the expelled ligand remains in the second co-ordination sphere of the complex. W(CO),L W(CO),L

-

A hv

+L W(CO),L + co

W(CO),

(42) (43)

An interesting comparison of the photosubstitution chemistry of W(CO),(L-L) (L-L = bipy, phen, 5-MephenYor 5-Brphen) and W(CO),(py), has been published.21e Absorption and luminescence data establish that the lowest excited states of W(CO),(L-L) are of MLCT character, but are of the L F type for W(CO),(py),. It is to these distinguishing features that their differing photoreactivities are ascribed. Thus, irradiation of W(CO),(py), causes pyridine substitution (0z 0.23, essentially independent of A) and very inefficient (0z dissociation of CO. This is consistent with the predicted enhanced M-N bond lability in the LF excited state. On the other hand, the chelate complexes W(CO),(L-L) show only wavelength dependent CO-expulsion, and = 1.6 x 0 3 1 3 = 2.2 x no ligand substitution. [For W(CO),(phen) This suggests that CO-labilization is the reaction of upper excited LF states, and that the MLCT state is essentially unreactive. It is not clear, however, whether the observed lack of amine substitution is due to the nature of the reactive excited states or to chelate effects. The photosubstitution reactions of (arene)Cr(CO), complexes are of considerable interest, in part because the thermal (arene replacement) and photochemical (CO expulsion) processes are so different. An important example of the application of the photochemical substitution of CO in (arene)chromium carbonyls by phosphines is that which led to the first reported isolation of enantiomers of chiral CrO compounds Two reports on the quantum efficiency 21a a13 a14

216

216

a17

E. P. Kuendig and G. A. Ozin, J. Amer. Chem. SOC.,1974, 98, 3820. R. N. Perutz and J. J. Turner, J. Amer. Chem. SOC.,1975, 97, 4800. A. J. Rest and J. R. Sodeau, J.C.S. Chem. Comm., 1976, 696. M. S. Wrighton, Inorg. Chem., 1974, 13, 905; M. S. Wrighton, G. S. Hammond, and H. B. Gray, Mol. Photochem., 1973, 5, 179. M. S. Wrighton and D. L. Morse, J. Organometallic Chem., 1975, 97, 405. G. Jaouen, A. Meyer, and G. Simonneaux, Tetrahedron, 1975, 31, 1889.

200

Photochemistry for CO dissociation have been published 219 For the substitution in (benzene)Cr(CO), by pyridine, the quantum yield is 0.72 (at 313, 366, or 436 nm),218and that in (mesitylene)Cr(CO), by maleimide is 0.90.21QNo evidence was found in either study for exchange of the aromatic ligand. Apparently anomalous behaviour is exhibited by (toluene)Cr(CO), in the presence of cycloheptatriene.220 The reaction found was (64), which is particularly surprising as (toluene)Cr(CO),

+ CHT A

(CHT)Cr(CO),

+ toluene

(64)

its analogue CpMn(CO), reacts to give CpMn(CHT). Two groups of workers have described the photochemical conversion of compounds of type (31) 222 into (32).221* 2

p"" Et

- p"' Et

+

P""

Me

(65)

As noted in last year's Report, olefin metathesis reactions [e.g. (65)] may be catalysed by photolysis of W(CO), in carbon tetrachloride. Flash photolysis of W(CO), in CCl, leads to W(CO),Cl, which is a possible candidate for the active

species in the catalytic process.223 However, other workers favour W(CO)4C12 for this The above process suffers from the disadvantage of requiring high W(CO), concentrations. Warwel and Laarz have recently reported that mixtures of W(CO), and Bu'AICI, are very active photocatalysts at concentrations of 1 part catalyst to 11 300 parts of olefin.226 The mechanism for the photochemically induced 1,6hydrogenation of 1,3-dienes in the presence of Cr(CO), is still not completely elucidated. One of the species which may be involved is the corresponding (diene)Cr(CO)4. Complexes of this type have now been prepared by low-temperature photolysis of Cr(CO), and either 1,3-butadiene or trans-,trans-2,4-he~adiene.~~~ These species are not active as hydrogenation catalysts in the dark, but are readily activated by illumination. This suggests that (diene)Cr(CO), may be the active species. M. S. Wrighton and J. L. Haverty, Z . Naturforsch., 1975, 30b, 254. J. Nasielski and 0. Denisoff, J. Organometallic Chem., 1975, 102, 65. 2 2 0 P. L. Pauson and J. A. Segal, J.C.S. Dalton, 1975,2387. 221 A. N. Nesmeyanov, M. I. Rybinskaya, V. V. Krivykh, and V. S. Kaganovich, J. Organometallic Chem., 1975, 93, CS. 222 W. S. Trahanovsky and R. A. Hall, J. Organometallic Chem., 1975, 96, 71. 223 P. Krausz, F. Garnier, and J.-E. Dubois, J. Organometallic Chem., 1976, 108, 197. 224 A. Agapiou and E. McNelis, J. Organometallic Chem., 1975, 99, C47. 226 S. Warwel and W. Laarz, Chem. Ztg., 1975, 99, 502. 226 I. Fischler, M. Budzwait, and E. A. Koerner von Gustorf, J. OrganomefaZZic Chem., 1976, 105, 325.

218

21B

201

Photochemistry of Inorganic and Organometallic Compounds

Photolysis of CpMo(CO),(NCS) (or its iron analogue) leads to photochemically induced isomerization of the complexed thiocyanate ion [equation (66)].,,' Irradiation of CpMo(CO),(PPh,)X (X = Br or I) causes both cis- to hv

CpMo(CO),NCS

hv

CpMo(CO),(SCN)

(66)

trans-isomerization and disproportionation to give CpMo(CO),X and CpMo(CO)(PPh,),X. In this case the proposed mechanism requires either (or both) photo-induced CO- or PPh,-dissociation. Last year, photochemical syntheses of alkyldiazenido-complexes from Mo(N,),(dppe), [dppe = 1,2-bis(diphenylphosphino)ethane] and alkyl bromides were reported [reaction (67)].22* The scope of this reaction is clearer after Mo(N,),(dppe),

+ RBr A

MoBr(N,R)(dppe),

+ N,

(67)

(33)

investigations of the products formed from various alkylhalides (MeBr, MeI, MeCl, iodocyclohexane, and ~x-bromo-p-xylene).~~~ The iodo-compounds reacted to produce compounds analogous to (33). With the alkyl bromides this type of product was accompanied by MoBr(N,)(dppe), and for methylchloride, MoCl(N,)(dppe), was the sole product. Although the mechanism has not been discussed in detail, it seems probable that the initial photo-process is N2-dissociation. Photolysis of the tungsten analogue in the presence of dibromomethane yields the first diazomethane complex [reaction (68)].230 It was previously proposed

hv

W(N,),(dPP4, + CHzBr, [WBr(N,CH,)(dPPe),lBr + N2 (68) that irradiation of M(N2),(dppe), (M = Mo or W) in THF in the presence of methyl bromide gave (34) after acidification. X-Ray analysis has now shown that the structure of this product is [MBr(N-N=CH(CH2)30H}(dppe),]+Br-.231 P-b

(34)

Other reports mention that the conversion of cis-W(N,),(PMezPh), into NH, in methanol solution occurs on irradiation, as well as on r e f l u ~ i n g , ~and ~ , that photolysis of Mo(dppe),(C,H,) gives a carbon dioxide adduct in the presence of this gas.233 227 228

228 230

231

asa 233

D. G. Alway and K. W. Barnett, J. Organometallic Chem., 1975,99, C52. A. A. Diamantis, J. Chatt, G. J. Leigh, and G . A. Heath, J . Organometallic Chem., 1975, 84, C1 1. V. W. Day, T. A. George, and S. D. A. Iske, J. Amer. Chem. SOC.,1975, 97, 4127. R. Ben-Shoshan, J. Chatt, W. Hussain, and G. J. Leigh, J. Organometallic Chem., 1976,112, c9. P. C. Bevan, J. Chatt, R. A. Head, P. B. Hitchcock, and G. J. Leigh, J.C.S. Chem. Comm., 1976, 509. J. Chatt, A. J. Pearman, and R. L. Richards, Nature, 1976, 259, 204. T. Ito and A. Yamamoto, J.C.S. Dalton, 1975, 1398.

Photochemistry

202

A further account of the photosubstitution of the aryl isocyanide ligand in Cr(ArNC), by ~ l e f i n s and , ~ ~a~report on the electronic structure of M(CNPh)6 complexes 235 have been published. Irradiation of a mixture of CrCl, and Pr'MgBr yields CrPrip,236while in the presence of 1,3-cyclo-octadiene and 1,3,5-cyclo-octatriene,bis(cyc1o-octadieny1)chromium(r1) is formed.237 The photodecarbonylation reaction (69) is a con[W(CO),(COR)]-

[W(CO),R]-

+ CO

(69)

venient route to the novel pentacarbonyltungsten alkyl and aryl derivatives (R = Me, Ph, or CH2Ph).238 Three primary photoprocesses should be considered for the metal-metal bonded complex [CpMo(CO),],. These are (i) homolytic cleavage (70), (ii) heterolytic cleavage (71), and (iii) CO-substitution (72). Recent publications have described [CPMo(CO),Iz [CPMo(CO),Iz [CPMo(CO)31,

-

2CPMO(CO),

(70)

[CPMo(CO),I+ Cp,Mo2(CO),

+ [CPMo(CO),I-

+ co

(71)

(72)

the light-induced reactions of [CpMo(CO),], and [CpW(C0),l2 with halogeno240 the flash photolysis of [ C ~ M O ( C O ) , ] , ,and ~ ~ ~the photochemistry Photolysis of the dimer in carbon of [CpMo(CO),], in highly polar tetrachloride gives CpMo(CO),Cl, the quantum yield varying slightly with irradiation This result is consistent with homolytic cleavage of the Mo-Mo bond [reaction (70)], followed by abstraction of a chlorine atom from the solvent by the metal centre. Further evidence for the importance of reaction (70) as an initial photo-process has been obtained from flash photolysis experiments with mixtures of [CPM~(CO),]~ (M1 = Mo or W) and M22(CO)lo(M2 = Mn or Re). In these cases good yields of the mixed products are formed as shown in equation (73).239 The reaction presumably involves combination of the [CpMl(CO),],

+ M22(CO)lo A

2CpM1(CO),M2(C0),

(73)

CpM1(CO), and M2(CO), formed by the flash. Examination of the intermediates formed on flash photolysis of [CpMo(CO),], itself reveals, however, that two processes occur on photolysis in solvents such as acetonitrile, THF, or cycloh e ~ a n e . ~From ~ l the spectra and decay kinetics of the transient species, it is deduced that both reactions (70) and (72) take place. The lack of ionic strength 234

K. Iuchi, S. Asada, T. Kinugasa, K. Kanamori, and A. Sugimori, Bull. Chem. SOC.Japan,

a36

K. R. Mann, M. Cimolino, G. L. Geoffroy, G. S. Hammond, A. A. Orio, G. Albertin, and H. B. Gray, Inorg. Chim. A d a , 1976,16, 97. J. Mueller and W. Holzinger, Angew. Chem., 1975, 87, 781 ; Angew. Chem. Internat. Edn.,

1976, 49, 577. 236

1975, 14, 760. 237

as8 230

240 a41 248

J. Mueller, W. Holzinger, and F. H. Koehler, Chem. Ber., 1976, 109, 1222. C. P. Casey, S. W. Polichnowski, and R. L. Anderson, J. Amer. Chem. SOC.,1975,97,7375. M. S. Wrighton and D. S. Ginley, J. Amer. Chem. SOC.,1975, 97, 4246. C. Giannotti and G. Merle, J . Organometallic Chem., 1976, 105, 97. J. L. Hughey, C. R. Bock, and T. J. Meyer, J. Amer. Chem. Soc., 1975, 97,4440. D. M. Allen, A. Cox, T. J. Kemp, Q. Sultana, and R. B. Pitts, J.C.S. Dalton, 1976, 1189.

Photochemistry of Inorganic and Organometallic Compounds

203

effects proves that in these solvents of low polarity, heterolytic cleavage (71) is not a primary process. Both processes (70) and (72) are induced by either U.V. or visible light, indicating that initial population of either UU* or do* (Mo-Mo) states cause these reactions to take place. A quite different picture emerges for experiments in high-donicity solvents such as DMF, DMSO, and ~ y r i d i n e . , ~ ~ Under these conditions the anion [CpMo(CO),]- has been identified after photolysis of the dimer. The reaction is very efficient, the quantum yield being higher in the polar solvents than in non-polar (for pyridine, <9436 = 0.79; for CC14, @,so = 0.35). This suggests that heterolytic cleavage may be the primary photoreaction in such solvents. These results, and those found with Mn,(CO),,, suggest that the course of reaction and probably, but not necessarily that of the initial step, is determined by solvent polarity. It is also possible that the metalcentred radicals or ions formed in steps (70) and (71) may undergo dissociation or exchange leading to overall more complicated chemistry, as has already been found for Mn2(CO)loand Re2(CO)lo. The photochemistry of various compounds of the type CpM1(CO),M2(CO), (M1 = Mo, W, or M2 = Mn, or Re) has been investigated. It has been shown that that homolytic rupture of the M1-M2 bond (to give d5 and d7 fragments) is favoured over the alternative heterolytic cleavage.243 Again in this case, a aa* excited state appears to be responsible for the reaction. Photochemical disproportionation has also been described for several R,SnMo(CO),Cp c o m p l e x e ~245 .~~~~ Manganese and Rhenium.-Wrighton and Ginley 246 have shown previously that Mn,(CO),, and Re,(CO),, undergo homolytic cleavage [reaction (74)J on irradiation in non-polar halogenocarbon solvents. It was further suggested, and confirmed by other that the M(CO), species is co-ordinatively labile M2(CO)lo

A

2M(CO),

(74)

M(CO), + L M(CO)*L + CO (75) so that substitution reactions with compounds such as phosphines may take place [reaction (75)], probably via prior dissociation. Combination of M(C0)4L with M(CO), gives Mn,(CO),L, the overall process thus appears to be substitution of CO in the M,(CO),, by L. Evidence this year demonstrates that the reaction course in polar solvents may be quite different. Cox and co-workers 242 have shown that irradiation of Mn2(CO)lo in pyridine gives [Mn(py),][Mn(CO),],. Similar results are obtained with DMF and DMSO, but not in cumene or dioxan. To rationalize this behaviour, the primary photochemical reaction is proposed to be heterolytic cleavage of the Mn-Mn bond [reaction (76)], although a ____+

Mn,(CO),,

a

[Mn(CO),]+(solvent) + [Mn(CO),]-(solvent)

(76)

scheme involving homolytic cleavage (74) and secondary reactions of M(CO), with the solvent should not be excluded. 244 246

246 247

D. S. Ginley and M. S. Wrighton, J. Amer. Chem. SOC.,1975, 97, 4908. K. Triplett and M. D. Curtis, Inorg. Chem., 1976, 15, 431. B. I. Petrov, G . S. Kalinina, and Y. A. Sorokin, Zhur. obshchei Khim., 1975, 45, 1905. M. S. Wrighton and D. S. Ginley, J. Amer. Chem. SOC.,1975, 97, 2065. B. H. Byers and T. L. Brown, J. Amer. Chem. SOC.,1975,97, 3260.

Photochemistry The nature of the e.s.r. detectable species formed on photolysis of Mn,(CO),, in THF has been the subject of some debate. Originally this species had been assigned to Mn(C0)5,248and this view was upheld in a later However, it has been convincingly argued that the species present is solvated Mn11.250s 251 It is proposed that, as in high-donicity the overall reaction is a photoinduced base-disproportionation. Most of the work reported above involves irradiation into the lowest or second lowest absorption bands, and recent MO calculations confirm that these are (T)-u* and U-U* transitions respectively.252 The photoreaction of Re,(CO),, in water is reported to give the cluster compounds (35) and (36).253Photolysis of Mn,(CO),, in the presence of KX (X =

204

H

F, C1, Br, or I) and crown ethers is a usefulpreparative route to [Mn2(CO)9X]-.254 [Et4N]+[Mn2(C0),(p-CI),I- is the final product of the photolysis of Mn,(CO),, and [Et4N]+Cl- in a variety of The use of mixtures of M,(CO),, (M = Mn or Re) and acetylenes as initiators for the photopolymerization of methyl methacrylate has been described.2s8 The long-wavelength band in the absorption spectrum of Mn,(CO),(phen), ReZ(CO)8(phen),or Re,(CO)8(biquin) (biquin = 2,2’-biquinoline) is held to be a o[M--M)-n*(L) CT transition.257 Thus it must be expected that, because of the depopulation of the binding orbital of the M-M bond, excitation of these molecules in this band will lead to homolytic cleavage. This proves to be the case. For example, irradiation of Mn,(C0)8(phen) in CH,CIZ-CC14 produces Mn(CO),Cl and ClMn(CO),(phen) with a quantum efficiency of about 0.96. From absorption and emission spectroscopic data, it has been shown that the lowest excited states of Re(CO),X (X = Cl, Br, I) are ls3E LF states.268 In agreement with observations with several other complexes of second and third 248

24B 260

252 253

S. A. Hallock and A. Wojcicki, J. Organometallic Chem., 1973, 54,C27. C. L. Kwan and J. K. Kochi, J . Organometallic Chem., 1975,101,C9. A. Hudson, M. F. Lappert, and B. K. Nicholson, J . Organometallic Chem., 1975,92,C11. A. Hudson, M. F. Lappert, J. J. McQuitty, B. K. Nicholson, H. Zainal, G. R. Luckhurst, C. Zannoni, S. W.Bratt, and M. C. R. Symons, J. OrganometaIIic Chem., 1976,110, C5. R. A. Levenson and H. B. Gray, J. Amer. Chem. SOC.,1975,97,6042. M. Herberhold and G. Suess, Angew. Chem., 1975, 87, 710; Angew. Chem. Internat. Edn.,

1975,14,700.

2b4 266

257 268

J. L. Cihonski and R. A. Levenson, Inorg. Chim. Acta, 1976, 18,215. J. L. Cihonski, M. L. Walker, and R. A. Levenson, J. Organometallic Chem., 1975,102,335. C. H.Bamford and S. U. Mullik, J.C.S. Faraday I, 1976,72,368. D. L. Morse and M. S. Wrighton, J. Amer. Chem. SOC.,1976,98,3931. M. S. Wrighton, D. L. Morse, € B. I. Gray, and D. K. Ottesen, J. Amer. Chem. SOC.,1976,

98, 1111.

205 row elements, where spin-orbit coupling is important, the luminescence yields and lifetimes decrease substantially as the temperature increases. In the absence of potential ligands, photolysis leads to (37), while in the presence of pyridine

Photochemistry of Inorganic and Organometallic Compounds

or phosphines the product is the corresponding cis-XRe(CO),L. For all Re(CO),X species the quantum yield is higher at 313 nm than at 366 nm. The authors propose that the longer wavelength irradiation, populating a d,, a-antibonding orbital, leads to axial CO dissociation, whereas shorter wavelength excitation populates a dza-y4orbital resulting in equatorial CO dissociation [equations (77) and (78) respectively]. It is presumed that the intermediate Re(CO),X

oc, oc’

X oc, I Re’

XI Re’

co ‘co + co

(77)

co oc’ co I ‘co

+ co is fluxional, rearranging prior to reaction with the ligand, so that only the cisXRe(CO),L is formed. An unusual observation with these compounds is that at 77 K, no CO dissociation can be observed, a behaviour different from that of most other metal carbonyl complexes. Photolysis of M(CO)5X (M = Mn, X = C1, Br, or I; M = Re, X = Cl or Br) in DMSO, DMF, or pyridine leads to [M(CO),]-.242 The electronic structures of group VII M(CO)5X complexes have been discussed on a basis of their p.e. Complete replacement of the CO ligands in CpMn(CO), by cycloheptatriene to give CpMn(CHT) may be induced on irradiation.22o The dicarbonyl complex is formed in the early stages of the photolysis, and in the case of the analogous CpRe(CO), complex, this is the only product isolable. The fluxional molecule (~5-cycloheptatrienyl)Mn(CO), may be synthesized by the low-temperature photodecarbonylation of (38).260 Expulsion of CO on irradiation of a variety of

0

!-Mn(C0~,

(38) 259

B. R. Higginson, D. R. Lloyd, S. Evans, and A. F. Orchard, J.C.S. Faraday ZZ, 1975, 71,

280

T. H. Whitesides and R. A. Budnik, Inorg. Chem., 1976, 15, 874.

1913.

8

206

Photochemistry

carbamoyl complexes is a convenient synthetic route to diaza-ally1 derivatives [equation (79)].261 Mn(CO),( CONR1CR2=N R3)

Mn(CO),( N R1CR2=NR3)

(79)

The photo-induced isomerization of Re(O)X,(PPh,)[P(OMe),] (X = C1 or Br) has been described.262

Iron and Ruthenium.-A clearer understanding of the quenching action of ferrocene has emerged from the investigations of two groups of workers this year.l0,263, 264 Using an approach similar to that reported last year by Kikuchi et aZ.,266they have studied the efficiency of quenching of the triplet states of a large number of organic sensitizers, particularly those of low energy (ET). For these low-energy sensitizers (with one exception, methylene blue), the rate constant for quenching ( k ~falls ) off smoothly as a function of ET (Figure 6). From this dependence on sensitizer Er and from other experiments, it has been clearly demonstrated that mechanisms such as catalysed intersystem crossing, electron transfer or charge transfer are not operative. These observations are wholly compatible with energy transfer from the organic triplet state to the 3Els state of the ferrocene being the predominant deactivation process. It is to be noted that the fall-off of k~ with ET is much slower than that predicted by an Arrhenius relationship (curve 1 in Figure 6), but the empirical relationship (80) k~ = k m { l

+ exp [-(ET - E ~ ~ ) / r n R T l } - l

is consistent with the observations (curve 2 in Figure 6) (EFc = 13 300 cm-1 (159 kJmol-l) and m = 2.9).263The explanation for this behaviour is that the 3E1,state is considerably distorted from its ground-state geometry, as has already been shown for ruthenocene from luminescence measurements. As may be seen in Figure 6, methylene blue quenches more efficiently than predicted by equation (80), and in this case it has been proved that an electron is transferred from the excited state of ferrocene to the q u e n ~ h e r . ~ ~The ~ ~deactivation of the triplet states of organic molecules by ferrocene has been reviewed (in Japanese).267 Electron transfer between 3-ferrocenylpropanoate and nitrous oxide,26Eand between polyvinylferrocene and dyes 269 have been reported. Other publications have described the photochemical preparation of ferrocene-percyanocarbon complexes,27othe light-induced reactions of ferrocene-lithium chloride comp l e x e ~ the , ~ ~ferrocene-sensitized ~ cis-trans isomerization of ole fin^,^^^ and the 261 26a 268 264 26s 266

267

2es

270

271 272

T. Inglis, M. Kilner, T. Reynoldson, and E. E. Robertson, J.C.S. Dalton, 1975, 924.

N. P. Johnson and M. E. L. Pickford, J.C.S. Dalton, 1976, 950. A. Farmilo and F. Wilkinson, Chem. Phys. Letters, 1975, 34, 575. W. G. Herkstroeter, J. Amer. Chem. SOC.,1975, 97, 4161. M. Kikuchi, K. Kikuchi, and H. Kokubun, Bull. Chem. SOC.Japan, 1974, 47, 1331. K. Kikuchi, H. Kokubun, and M. Kikuchi, Bull. Chem. SOC.Japan, 1975, 48, 1378. K. Kikuchi and H. Kokubun, Kaguku (Kyoto), 1976,31, 59 (Chem. A h . , 1976,84,179 027). E. K. Heaney and S. R. Logan, J . Organometallic Chem., 1976, 104, C31. K. Kojima, S. Iwabuchi, T. Nakahira, T. Uchiyama, and Y . Koshiyama, J. Polymer Sci., Part By Polymer Letters, 1976, 14, 143. 0. Traverso, E. Horvath, and S. Sostero, Ann. Uniu. Ferrara, Sez. 5, 1974, 3, 175. 0. Traverso, E. Horvath, and S. Sostero, Ann. Uniu. Ferrara, Sez. 5, 1974, 3, 153. J. Wojtczak and A. Jaworska-Augustyniak,Pol. Chem. Stosow, 1975, 19, 359 (Chem. Abs., 1976, 84, 73 396).

Photochemistry of Inorganic and Organornetallic Compounh 207 photoreactions of ferrocenesulphonyl a z i d e ~ . The ~ ~ ~absorption and m.c.d. spectra of f e ~ r o c e n e ,and ~ ~ ~calculations on the electronic structure of ferrocene 275--277 have been the subjects of recent papers. Fe(CO), may be used for photocatalysed alkene isomerization and for olefin h y d r ~ g e n a t i o n . ~For ~ ~ the 1-pentene to 2-pentene isomerization reaction the

I

I I' 0 I

I I

I

I

I

I

6

I

8

I

10

1

I

12

14

1

16

1

1

18 20 ET/ 1 0 3 d

I

22

I

24

I

26

Figure 6 Variation of the rate constants for quenching of the triplet states of organic compounds by ferrocene as a function of their triplet energy (ET) (Reproduced by permission from Chem. Phys. Letters, 1975, 34, 575)

quantum yield increases with concentration, reaching a value of 420 in neat 1-pentene. This indicates that light has served to produce a species which may act as a catalyst without requiring further photo-activation. The proposed mechanism, with the active species (39), is presented in Scheme 2. In the catalytic 978

274 276

a77 276

R. A. Abramovitch and W. D. Holcomb, J. Org. Chem., 1976, 41, 491. D. Nielson, M. Farmer, and H. Eyring, J. Phys. Chem., 1976, 80, 717. M.-M. Rohmer and A. Veillard, Chem. Phys., 1975, 11, 349. R. F. Kirchner, G. H. Loew, and U. T. Mueller-Westerhoff, Theor. Chim. Acfa, 1976,41, 1. P. S. Bagus, U. I. Walgren, and J. Almlof, J. Chem. Phys., 1976, 64, 2324. M. A. Schroeder and M. S. Wrighton, J. Amer. Chem. SOC.,1976,98, 551.

208

Photochemistry

Fe(CO), 4-

Fe(CO),

( y)

Scheme 2

hydrogenation processes it has been shown that the active catalyst is not Fe(C0)4H2,but probably a species of the type Fe(CO),H(alkyl). Photosubstitution of the silylnitrene complex (40; L = CO) by P(OMe), or PBu, proceeds in stages, so that the mono-, di-, or tri-substituted compounds may be readily isolated.279 Compound (41), formed from (40; L = CO) by photolysis in the presence of hydrogen, is a useful catalyst for the light-induced selective hydrogenation of dienes and of activated olefins.

Addition reactions of metal co-ordinated dienes and olefins are the subject of both practical and theoretical interest. Some authors have suggested that these reactions may be treated by the Woodward-Hoffmann rules, and have discussed the role of the metal in lifting symmetry-imposed restrictions on the course of the reaction. However, there is substantial evidence that most of these processes are not concerted. A recent example of this is the observation that the photochemically produced (b~tadiene)Fe(CO)~-hexafluorobut-2-yne adduct (42) 278

I. Fischler, R. Wagner, and E. A. Koerner von Gustorf, J. Organometallic Chern., 1976,112, 155.

Photochemistry of Inorganic and Organometallic Compounds

209

thermolyses to the 1,3-~yclohexadienederivative (43), via the corresponding 1,4-~yclohexadienespecies.28oGreen and co-workers have also isolated products of type (44) following reactions of perfluoroethylene or perfluoropropene with (diene)Fe(CO), 282 [(44) is the product from (butadiene)Fe(CO),

(CC03-*

(44)

(45)

and perfluoropropene.] Analogous products are formed from hexafluoroThe mechanism favoured is initial photochemical production of a (+diene)Fe(CO),, followed by attack of the fluorocarbon to give species such as (45). A different interpretation of the initial steps has been given by Kerber and Koerner von G u ~ t o r f . ~They * ~ have studied the closely related addition of CFz= CCI, to (diene)Fe(CO), complexes. The only product formed, for example, from 1,3-pentadiene or l-phenyl-lY3-butadieneis (46). The proposed mechanism is summarized in Scheme 3, and it may be remarked that the initial photo-

(46) R

=

Me or Ph

Scheme 3

process is CO dissociation. Supporting evidence for this has been acquired by preparing the dicarbonyl THF complex at low temperatures and allowing it to 280 281 282

283 284

R. Davis, M. Green, and R. P. Hughes, J.C.S. Chem. Comm., 1975,405. A. Bond, B. Lewis, and M. Green, J.C.S. Dalton, 1975, 1109. M. Green, B. Lewis, J. J. Daly, and F. Sanz, J.C.S. Dalton, 1975, 1118. M. Green and B. Lewis, J.C.S. Dalton, 1975, 1137. R. C. Kerber and E. A. Koerner von Gustorf, J. Organometallic Chem., 1976, 110, 345.

210

Photochemistry

react with the halogeno-olefin. It seems probable that this is also the lightinduced step in reactions involving perfluoro-olefins etc. Both photolysis and thermolysis of (cyclopentadiene)Fe(CO), yield [CpFe(CO)2]2.286 However, the initial photochemical step is CO expulsion, whereas the thermal reaction proceeds via the (v2-diene)Fe(CO),. The photochemical cycloaddition reactions of alkynes to various iron carbonyl complexes of compounds containing cis-azo groups (47), such as 2,3-diazonorbornene, 3,3-bis(methoxycarbonyl)-4-alkyl-pyrazolines~ and benzocinnoline, have been described.286Three types of products are formed [(48)-(50)]. A

detailed study of the photoreactions of the diazoferrole species (49) has been carried In particular the substitution of CO by phosphines, the addition of acetylenes to give (50), and of dienes to yield (51) have been investigated. Flash photolysis of (49) in benzene solution indicates that the principal photochemical process is expulsion of CO and creation of the corresponding Fe(CO), species. In the absence of reactants the recombination process occurs efficiently (k = 2 x lo6dm3mol-1 s-l). In the above study it was also possible to show that the addition of diphenylacetylene to (49) to give a product of type (50) proceeds via co-ordination of the acetylene to the Fe(CO), derivative and subsequent rearrangement of the adduct (k = 1.2 x lo3s-l). Other examples of photo-induced coupling of organic compounds involving iron carbonyl derivatives are the production of (52) from Et2NC=CNEt2 and Fe(C0)6,288the formation of (53) from photolysis of CpFe(CO),Me in excess CNR (R = cyclohexyl or t - b ~ t y l )and , ~ ~the ~ conversion of (54) into (55).290 a86 286

287

288

aS9 290

T. H. Whitesides and J. Shelly, J. Organometallic Chem., 1975, 92, 215. A. Albini and H. Kisch, J . Organometallic Chem., 1975, 101, 231. A. Albini and H. Kisch, J. Amer. Chem. SOC.,1976, 98, 3869. R. B. King and C. A. Harmon, Inorg. Chem., 1976, 15, 879. Y. Yamamoto and H. Yamazaki, J . Organometallic Chem., 1975, 90, 329. A. de Cian, R. Weiss, Y. Chauvin, D. Commereuc, and D. Hugo, J.C.S. Chem. Comm., 1976, 249.

''

Photochemistry of Inorganic and Organometallic Compounds NEt,

Et2N

CpFe-C

=

//

I I ,N=C,

R

Et,N Fe NEt,

(54) R

R N/

co

, ,

21 1

Me

(55 )

PhMeHC

The reversible photo-isomerization of the co-ordinated thiocyanate ligand has been described [equation (Sl)]."' CpFe(CO),NCS

CpFe(CO),SCN

hv

(8 1)

Photo-induced cleavage of the metal-metal bond in [CpFe(CO),], in halo carbon solvents leads to formation of the corresponding CpFe(CO),X comThis, on further irradiation, produces ferrocene and ferrous halide. Photochemical expulsion of SO, from CpFe(CO)2S0,S02Fe(CO),Cp yields CpFe(CO),S0,Fe(CO),Cp.291Photochemical decarbonylation of CpFe(CO),(COR) complexes provides convenient routes to Fe-C a-bonded derivatives of furan or t h i ~ p h e n ,and ~ ~ ~of adamantane.293 Extended irradiation of CpFe(CO),Me with excess P(OPh)3 produces the orthometallated derivative (56).294 (CF,),C

0

CF3

CDWCO), F C F 2 (57)

+-= CF3

F, c

c(CF, )

2

CpFe(C0) (58)

Photolysis of the a-bonded derivative (57) leads to the q3-allyl complex Photosubstitution of carbon monoxide takes place on further photolysis of (58) in the presence of phosphine. The photochemical conversion of the ally1 complex Cp($-C,H,)Fe(CO), to Cp(q3-C3H,)Fe(CO) has been (58).2953296

201 2e2 2e3

2e5

2e6

N. H. Tennent, S. R. Su, C. A. Poffenberger, and A. Wojcicki, J. Organometallic Chem., 1975,102, C46. A. N. Nesmeyanov, N. E. Kolobova, L. V. Goncharenko, and K. N . Anisimov, Izuest. Akad. Nauk S.S.S.R., Ser. khim., 1976, 153. S. Moorhouse and G. Wilkinson, J . Organornetallic Chem., 1976, 105, 349. R. P. Stewart, J, J. Benedict, L. Isbrandt, and R. S. Ampulski, Znorg. Chem., 1975, 14, 2933. A. N. Nesmeyanov, N. E. Kolobova, I. B. ZIotina, B. V. Lokshin, I. F. Leshcheva, G. K. Znobina, and K. N. Anisimov, J . Organometallic Chem., 1976, 110, 339. A. N. Nesmeyanov, G. G . Aleksandrov, N. G. Bokii, I. B. Zlotina, Y . T. Struchkov, and N. E. Kolobova, J. Organometallic Chem., 1976, 111, C9.

212

Photochemistry

reinve~tigated.~~~ It has been demonstrated that, contrary to an earlier report, both exo- and endo-+allyl compounds are produced. Irradiation of Fe(GO), with 1,2-di-iodobenzeneor cis-2,3-dibromobutene gives (59) and (60).2g8Although a detailed mechanism is not proposed in the paper, it appears that two photolysis steps are necessary.

(60)

(59)

Recent examples of light-induced reactions of Fe(CO), with strained ring compounds include those with bullvalene (61) in wet benzene, giving (62) and (63),299and with optically active vinyloxirans, e.g. (64), giving (65).300 In contrast with that induced thermally, the photochemical reaction with the vinyloxirans proceeds stereospecifically.

0 II

Me (64)

Me (65)

2e7

2os

R. W. Fish, W. P. Giering, D. Marten, and M. Rosenblum, J . Orgunometullic Chem., 1976, 105, 101. F. W. Grevels, J. Buchkremer, and E. A. Koerner von Gustorf, J. Organometallic Chem., 1976,111, 235. R. Aumann, Chem. Ber., 1975, 108, 1974. K.-N. Chen, R. M. Moriarty, B. G. de Boer, M. R. Churchill, and H. J. C. Yeh, J. Amer. Chem. SOC.,1975,97, 5602.

Photochemistry of Inorganic and Organometallic Compounds

213

Other reports have considered the photolysis of iron-germanium heterocyclic and have remarked on the photostability toward CO expulsion of some dithiolene-iron carbonyl complexes.302 RuHCl(PPh,), is a most effective hydrogenation catalyst. However being air-sensitive, it has not found wide application industrially. The discovery that it is readily prepared by photodecarbonylation of the air-stable RuHCI(C0)(PPh3)3(0= 0.06) may mean that it will be more commonly employed.303 The photochemical cis- to trans-isomerization of R U ( C O ) ~ ( E R ~ )(ER3 ~X~ = arsine or phosphine; X = halogen) has been described.304 Cobalt, Rhodium, and Iridium.-Two of the classes of reactions catalysed by co-enzyme B 12 (5'-deoxyadenosylcobalamin) (66) are those involving carbon

It'

yoI

NH

"A

CH,

I

HC/

0 " " l c

I/

P

CH,OH

H,C'

skeleton rearrangements [equation (82)] and diol dehydrations [equation (83)]. Although the actual mechanistic details of these biologicaI processes are still a matter of controversy, most researchers believe that the first step is homolytic cleavage of the Co-C bond of the co-enzyme to yield the corresponding Co" *01

802

K. Triplett and M. D. Curtis, Znorg. Chem., 1976, 14, 2284. J. S. Miller, Inorg. Chem., 1975, 14, 2011. G. L. Geoffroy and M. G. Bradley, J.C.S. Chern. Comm., 1976,20. C. F. J. Barnard, J. A. Daniels, J. Jeffery, and R. J. Mawby, J.C.S. Dalton, 1976, 953.

214

-

X I I

-c-cI

I

H

RCH(OH)CH,OH

-

Photochemistry

I

X

I

-c-cI

(82)

I

H

RCH2CHO

+ H2O

(83)

species, B12r, and the 5’-deoxyadenosyl radicaL30s As homolysis of the Co-C bond has previously been suggested as the primary photoprocess on irradiation of alkylcobalamins in solution, an aim of several research groups has been to emulate nature by carrying out reactions of types (82) and (83) in uitro. Thus Dowd et al. have recently shown that photolysis of the cobalamin compound (67) (THP = tetrahydropyranyl) produces a mixture of compounds from which

vco2(THP) substantial quantities of a-methylene glutaric acid (68) could be isolated after hydrolysis.306 Similarly other workers have found that the dimethylmalonylcobaloxime derivative (69a) gives methylsuccinyl diethyl ester [(CH,CO,Et),] as well as dimethylmalonyl diethylester MeCH(C02Et)2.307It is interesting to note

(69) a; R b; R C;

R

=

= =

(C02Et)2CMeCH2-; X Me; X = H 2 0 ROO-; X = py

=

py

that although this reaction apparently proceeds via homolytic cleavage of the Co-C bond, the radical so produced does not rearrange in the absence of the cobalt compound. An example of a reaction of type (83) is that reported by Golding et al.S08 In this case photolysis of methylaquocobaloxime (69b) in aqueous 1,Zethanediol produces a 10% yield of acetaldehyde. Essential steps in the reaction are presumed to be (84)-(86). 805 *O6

R. H. Abeles and D. Dolphin, Accounts Chem. Res., 1976, 9 , 114. P. Dowd, M. Shapiro, and K. Kang, J. Amer. Chem. SOC.,1975,97,4754. G. Bidlingmaier, H. Flohr, U. M. Kempe, T. Krebs, and J. Retey, Angew. Chem., 1975, 87, 877; Angew. Chem. Internat. Edn., 1975, 14, 822. B. T. Golding, T. J. Kemp, E. Nocchi, and W. P. Watson, Angew. Chem., 1975, 87, 841; Angew. Chem. Internat. Edn., 1975, 14, 813.

Photochemistry of Inorganic and Organometallic Compounds Me-[Col-OH, Me-

+ CH,(OH)CH,OH *CH(OH)CH,OH

-

Me* + [Col-OH,

+ -CH(OH)CH,OH *CH,CHO + H,O

CH,

215 (84) (85)

(86)

Support for an alternative mechanism of the co-enzyme B12 catalysed biological reactions has been found in some studies on the cobalamin derivative (70), a close analogue of the c o - e n ~ y m e .Although ~~~ the compound does not function as an efficient coenzyme in the dioldehydrase enzyme, it may be activated to do so by a short period of U.V. irradiation. As (70) forms the Cot cobalamin Adenine

(Hlzs) and (71) on photolysis in uitro, these observations suggest to the authors that Blzs may be an intermediate in the enzyme reactions. Other publications on the photochemistry of alkylcobalamins consider the , ~ monitoring ~~ interaction of co-enzyme B12 with lipoic acid and other t h i o l ~the of the photolysis of alkylcobalamins by n.m.r.,311 and the photolability of formylmet hylcobalamin.312 As may be seen above, most of the experiments carried out on alkylcobalamins and alkylcobaloximes are consonant with Co-C bond homolysis as the predominant photochemical process. However, more recent studies suggest that other modes of reactions are possible for alkylcobaloximes depending on such variables as the alkyl substituent R, base X, solvent, or temperature. For example, e.s.r. studies by Giannotti and co-workers 314 have revealed that after photolysis of alkyl cobaloximes in degassed chloroform solution at 77 K, a Co" species is formed in which the Co-C bond is still intact. This species is apparently produced by electron transfer from the dimethylglyoxime ligand to the metal, the ligand then rearranging with expulsion of a hydrogen atom. At 7 7 K this is the main reaction even for complexes with axial alkyl ligands possessing /3-hydrogen atoms (e.g. isopropyl). At higher temperatures, signals due to a Co"-dimethylglyoxime complex, in which the Co-C bond is broken, are recorded. It is also proposed by these authors that the absorption band at lowest energy is not due to an alkyl + C o charge-transfer transition, as pre3139

alo

813

*I4

G. N. Schrauzer, R. N. Katz, J. H. Grate, andT. M. Vickrey, Angew. Chem., 1976,88, 186; Angew. Chem. Internat. Edn., 1976, 15, 170. 1. P. Rudakova, E. G. Chauser, and A. M. Yurkevich, Bioorg. Khim., 1975, 1, 616. H. P. C. Hogenkamp, P. J. Verganiini, and N. A. Matwiyoff, J.C.S. Dalton, 1975, 2628. T. M. Vickrey, R. N. Katz, and G. N. Schrauzer, J. Amer. Chem. SOC.,1975, 97, 7248. C. Giannotti, G . Merle, and J. R. Bolton, J. Organometallic Chem., 1975, 99, 145. C. Giannotti and J. R. Bolton, J. Organometallic Chem., 1976, 110, 383.

216 Photochemistry viously postulated, but rather to a dimethylglyoxime -+ Co charge-transfer transition. In air-saturated solutions, photolysis of alkylcobaloximes leads to the insertion of oxygen and the production of the corresponding alkylperoxycobaloxime (69c). This insertion reaction is also accompanied by light-induced base exchange. After a detailed discussion of the various possible mechanisms for the oxygenation reaction, Jensen and Kiskis 315 have favoured the route given in Scheme 4.

(Co)OOR t-- (Co'') Scheme 4

+

ROO

The assumption that vibrationally excited ground-state species are important is most surprising, but it is interesting to note that in some recent theories for the photoreactions of Co"' ammine complexes, 'hot' ground-state compounds have been postulated as reactive intermediates (see ref. 90). Giannotti 313 has suggested that, at least in aprotic solvents, the Col* species, formed by electron transfer from the equatorial ligand, is an important intermediate (Scheme 5).

R

Scheme 5

One particularly controversial aspect of the oxygen insertion reaction is whether the optical activity of the alkyl group is lost during the reaction. Jensen and Kiskis316have published a full report in which the photolyses of optically active 2-butyl(pyridine) cobaloxime and 2-hydroxy-l-phenethyl(pyridine)cobaloxime have been examined. In contrast to the method of earlier workers, the optical activity of the product was measured by reducing the peroxy complex with sodium borohydride to give the alcohol. As no optical activity was detected in the alcohols, complete racemization of the alkyl group during the photochemical reaction was assumed. More recently, however, the validity of this 315

F. R. Jensen and R. C.Kiskis, J. Amer. Chem. SOC.,1975, 97, 5825.

Photochemistry of Inorganic and Organometallic Compounds

217

reduction procedure has been questioned, as it appears that the reduction of the peroxycobaloxime may yield the ketone initially and hence cause racemization of the complex.31s Clearly then further quantitative studies will have to be performed before the nature of the photochemical processes associated with the Co-C bond in alkylcobaloximesand cobalamins is understood, and its relevance to the biological processes can be assessed. Other reactions of alkylcobaloximes and related complexes include the synthesis of alkylperoxy compounds following irradiation of alkylcobaloximes in ~ ’ formation the presence of hydroperoxides R 0 2 H (R = cumyl or t - b ~ t y l ) , ~the of (RS,)(pyridine)cobaloxime and (PhS)(pyridine)cobaloxime by photolysis of alkylcobaloximes in the presence of sulphur and PhSSPh r e s p e c t i ~ e l yand , ~ ~ ~the observation of Co-C bond homolysis in alkyl derivatives of (72).31D,320 O/H* “0

0

The dinuclear complex (73) has been isolated after irradiation of C P R ~ ( C O ) , . ~ ~ ~ As mentioned last year, the analogous product is formed from CpCo(CO), on photolysis at 5 “C, and this and its subsequent reactions are the subject of a full paper.322 The light-induced reactions of CpRh(CO), with MMe,H (M = Si, Ge, or Sn) have also been described.321 With SiMe,H the niain products are CpRh(CO)(SiMe,), and CpRh(CO)H(SiMe,), and with GeMe,H the principal species formed is CpRh(CO)(GeMe,),. The reaction with SnMe,H surprisingly causes replacement of the cyclopentadienyl ligand, yielding Rh(CO),(SnMe,),. Strohmeier has discussed the photochemical activation of metal carbonyl catalysts, especially Ir(CO)C1(PPh3)2.1D7Illustrations of how irradiation can increase the rate and turnover number for various processes are presented. Further, the role of light in reactivating ‘spent’ catalysts is a particularly attractive feature, and may well be very important for expensive noble metal complexes. The selectivity of light in regenerating catalytically active species has been stressed in another publication.323 Side reactions are of negligible importance in the deoxygenation or dehydrogenation of 02-or H,-adducts of IrX(CO)(PPh,), (X = Cl or I), [Ir(dppe),]+, or [1r(cis-Ph2PCH=CHPPh,),]+[e.g. equation (87)]. 316

3J7

318

318 320

381

322 329

C. Bied-Charreton and A. Gaudemer, J. Amer. Chem. SOC.,1976, 98, 3997. C. Giannotti, C. Fontaine, A. Chiaroni, and C. Riche, J. Organometallic Chem., 1976, 113, 57. C. Giannotti and G. Merle, J. Organometallic Chem., 1976, 113, 45. M. W. Witman and J. H. Weber, Inorg. Nuclear Chem. Letters, 1975, 11, 591. V. E. Magnuson and J. H. Weber, J. Organometallic Chem., 1975, 92, 233. R. Hill and S. A. R. Knox, J.C.S. Dalton, 1975, 2622.

K. P. C. Vollhardt, J. E. Bercaw, and R. G. Bergman, J. Organometallic Chem., 1975,97,283. G. L. Geoffroy, G. S. Hammond, and H. B. Gray, J. Amer. Chem. SOC.,1975,97,3933.

218

Photochemistry 0,1rCl(CO)(PPh3)2

a IrCl(CO)(PPh,), + O2

(87)

The photoreactive state appears to be a metal-to-phosphine ligand chargetransfer species. U.V.light has been found to have no effect on the RhCI(PPh3), or Rh(CO)(PPh,),X (X = C1, Br, or I) catalysed decarbonylation of PhCOBr or PhCH2COCl,324whereas in the RhCl(PPh,),-catalysed hydrogenation of mesityl oxide to methylisobutylketone it acts as an inhibitor.S2s In the presence of (74) light induces the rearrangement of 1,5-cyclo-octadiene to 1,4-cy~lo-octadiene.~~~ The initial photochemical step seems to be the forma-

g

\Rh/C1‘Rh ‘Cl’

(74) tion of a co-ordinatively unsaturated Rh’ species by cleavage of one of the Rh-olefin bonds. Subsequently, an intramolecular [1,3]-hydrogen shift takes place via an allylrhodiumhydride complex. Platinum.-Photolysis of a large number of truns-PtCl2(olefin)(amine)complexes has been shown to cause expulsion of the olefin and formation of the halidebridged dimer [equation (88)].327,328 The dimer reacts thermally with olefin 2 trans-PtCl,(amine)(olefin)

A

+

[PtCl2(amine)l2 2 olefin

(88)

forming the cis-PtCl,(olefin)(amine) complex, and this procedure affords a convenient route to these complexes.327 Expulsion of ethylene from Pt(PPh,),(ethylene) upon photolysis has been reported.32e Irradiation of Pt(bipy)Me2 in the presence of perfluoroethylene produces Pt(bipy)(Me)(CF2CF2Me)(C,F3.530 This system is very effective for the polymerization of methylmethacrylate. Copper.-Recent reports consider the photodecomposition of certain diazoketones in the presence of cuprous and the effect of cupric acetate on the photolysis products of alkyl Gold.-The course of reaction following U.V. irradiation of MeAu(PPh,) in CDC13 has been followed by CIDNP.333 The main products formed are CIAu(PPh,) and CH3D. The reaction appears to involve the triplet state as 324 325

326 327 aZ8

32s

W. Strohmeier and P. Pfohler, J. Organometallic Chem., 1976, 108, 393. W. Strohmeier and E. Hitzel, J. Organometallic Chem., 1975, 91, 373. R. G. Salomon and N. El Sanadi, J. Amer. Chem. SOC.,1975, 97, 6214. P. Courtot, A. Peron, R. Rumin, J. C. Chottard, and D. Mansuy, J. Organometallic Chem., 1975, 99, C59. F. Pesa and M. Orchin, Inorg. Chem., 1975, 14, 994. S. Sostero, 0. Traverso, G. Carturan, and M. Graziani, Ann. Univ. Ferrara, Sez. 5, 1974, 3, 189.

330 331 331

C. H. Bamford, S. U. Mullik, and R. J. Puddephatt, J.C.S. Faraday I, 1975,71, 2213. Z. Cekovic and T. Srnic, Tetrahedron Letters, 1976, 561. S. Chakrabarty, J. K. Ray, D. Mukherjee, and U. R. Ghatak, Synth. Comm., 1975, 5, 275. P. W. N. M. Van Leeuwen, R. Kaptein, R. Huis, and C. F. Roobeek, J. Organometallic Chem., 1976,104, C44.

219

Photochemistry of Inorganic and Organometallic Compounds

-

shown in equation (89), although an alternative pathway, in which the initial step is electron transfer, cannot be excluded [equation (go)]. The polarization [MeAu(PPh,)lT MeAu(PPh,) Me-

+ CDCl, + CDCl,

+ MeAu(PPh,)

+ Me- + CDCl,*T [MeAu(PPh,)]+ + [CDCI,]MeAu(PPh,) + Me-

AuPPh,Cl

hu

---+

(89)

(90) (91)

of the signal from MeAuPPh, observed during the reaction is clear evidence for the novel S Hsubstitution ~ step (91). Mercury.-An exchange reaction analogous to (91) has been found with Hg(MMe,), (M = Si or Ge) [equation (92)] during CIDNP and e.s.r. investigations 3 3 4 ~336 The initial process on photolysis of these compounds is Hg-M

+ (Me3M),Hg + (Me,Si),Hg

Me,M* Me,%*

+ Me,M* + Me3Si* + Hg

(M%M),Hg

(Me&),

(92) (93)

bond rupture, but the lifetime of the intermediate Me,MHg’ is very short. The quantum yield for decomposition of (Me,Si),Hg is approximately 10, an indication that the S$ process (93) is important. Other authors have described the reactions of :&02Et formed on irradiation of Hg[C(N2)C02Et]2,336 and the i.r. spectra of radicals formed on the solid-state photolysis of various organomerc~rials.~~~ The importance of photodecomposition of methylmercury compounds as an essential step in the estimation of mercury in natural waters has been Table 9 Photochemical substitution reactions of metal carbonyl compounds Substrate

~CPV(CO),I [V(CO>,lM(CO), (M = Cr, Mo, or W) (M = Cr, Mo, or W)

(M = W) (M = W)

Reactant L

Products

Ref.

CNdiars or dppe

[cPv(co)2L124[V(CO)&I-

339 340

OH- or F(in crown ethers) l-allyl-3,5-dirnethyIpyrazole H,S Et,NC=CNEt,

“CO),LI-

341

M(CO),L

342

W(CO),L Et,N

+ NEt,

343 288

Et,N O (CQ.4 N E t 2 834

s36

838 837

s3a 340

341 s4a 343

F. Werner, W. P. Neumann, and H. P. Becker, J. Organometallic Chem., 1975, 97, 389. M. Lehnig, F. Werner, and W. P. Neumann, J. Organometallic Chem., 1975, 97, 375. T. B. Patrick and G . H. Kovitch, J. Org. Chem., 1975, 40,1527. A. K. Mal’tsev, N. Kagramanov, and 0. M. Nefedov, Doklady Akad. Nuuk S.S.S.R.,1975, 224, 630. A. M. Kiemeneij and J. G . Kloosterboer, Anulyt. Chem., 1976, 48, 575. D. Rehder, 2.Nuturforsch., 1976, 31b,273. J. E. Ellis and R. A. Faltynek, J. Organometallic Chem., 1975, 93, 205. J. L. Cihonski and R. A. Levenson, Znorg. Chem., 1975, 14, 1717. K. Fukushima, T. Miyamoto, and Y. Sasaki, J. Organometallic Chem., 1976, 107,265. M. Herberhold and G. Suess, Angew. Chem., 1976, 88, 375; Angew. Chem. Znternat. Edn., 1976, 15,366.

220 Table 5 (cont.) Substrate (CO), CrPMe,P Me, LM(CO), (M = Cr, Mo, or W) CpCr(CO),(NO)

Photochemistry Reactant L

cis-Y(CF,)C= C(CF,)Y

(Y = AsMe,) MeCN, pyridine, or cyclo-olefins CpMo(CO),I PhC-CPh c pw(co)zcl PhC=CPh (cycloheptatriene)Cr(CO), P(OMe), (CO),Cr[C(OMe)Me] Mn,(CO),o Mn,(CO),o Re,(CO),o Rez(CO),o Mn(CO)6{E(CF3)2) (E = P or As) Mn(CO),(SnMe,CI) CpMn(CO13 CpMn(CO),

SR2 dPPe

-

Fe(CO),

p406

a44

846 347 948

548 s60

351 3aa

363 366 *56

858

360 361

346, 347 348 349 226

R1R2CR3=CR4S

P(OPh), N 2

AsPh, or SbPh,

[(pMe2sn),Mn,(Co),J CpMn(CO),L CpMn(CO),L and CpMn(C0)L Fe(CO),L

350 351 288

352 353

354 244 355

356 357

Fe(CO),L and trans-Fe(CO), L2

358

(q4-R1R2C=CR3CR4=S)Fe(C0)3 CpFeL,( SnX,)

359

[{CPFe(dPPe))zU2+ CpFe(COMe)(CO)L

361 362

360

L. Staudacher and H. Vahrenkamp, Chem. Ber., 1976,109,218. W. R. Cullen and L. Mihichuk, Canad. J. Chem., 1975, 53, 3401. M. Herberhold and H. Alt, Annalen, 1976, 292. M. Herberhold, H. Alt, and C. G. Kreiter, Annalen, 1976, 300. J. L. Davidson and D. W. A. Sharp, J.C.S. Dalton, 1975, 2531. J. L. Davidson, M. Green, F. G. A. Stone, and A. J. Welch, J.C.S. Dalton, 1976, 738. E. 0. Fischer and K. Richter, Chem. Ber., 1976, 109, 1140. F. Mathey, J. Organometallic Chem., 1975, 93, 377. H. Alper, Inorg. Chem., 1976, 15, 962. R. Davis and I. A. 0. Ojo, J. Organometallic Chem., 1976, 110, C39. J. Grobe and R. Rau, 2. anorg. Chem., 1975,414, 19. I. S. Butler and T. Sawai, Inorg. Chem., 1975, 14, 2703. I. B. Nemirovskaya, A. G. Ginzburg, V. N. Setkina, and D. N. Kursanov, Zhur. obshchei Khim., 1975,45, 893. H. Schumann, L. Roesch, H.-J. Kroth, H. Neumann, and B. Neudert, Chem. Ber., 1975, 108, 2487.

868

CpCr(CO)(NO)L

-

ButnP(EMe&-, (E = Si, Ge, Sn)

CpFe(CO),SnX, (X = C1 or Br) [CPwCo)(dPPe)l+ CpFe(CO), Me

Ref. 344 345

CpMo(CO)(I)L CpW(CO)(Cl)L (CHT)Cr(CO),L and (CHT)Cr(CO)L, ER, (E = P, As, or Sb) (CO),LCr[C(OMe)Me] 1,3,4-substituted Mn,(CO)*L and phospholes Mn,(CO)?L EtzNCz CNEta Mn2(CO)8L (isomer of) MeOPhC(S)PhMe Re,(CO)aL Re2(C0)& and cycloheptatriene Re2(CO)7 L [Mn(CO),~E(CF,),)la

FdCO),

Fe(CO),

Products (P-P2Me,)Z Cr(CO)8 L, M(CO), and L2M(CO),

M. L. Walker and J. L. Mills, Inorg. Chem., 1974, 14, 2438. D. C. Dittmer, K. Takahashi, M. Iwanami, A. I. Tsai, P. L. Chang, B. B. Blidner, and I. K. Stamos, J. Amer. Chem. SOC.,1976, 98, 2795. B. Herber and H.Werner, Synth. React. Inorg. Met.-Org. Chem., 1975,5, 381. D. Sellmann and E. Kleinschmidt, Angew. Chem., 1975, 87, 595; Angew. Chem. Internat. Edn, 1975, 14, 571. A. C. Gingell and A. J. Rest, J. Organometallic Chem., 1975, 99, C27.

Photochemistry of Inorganic and Organornetallic Compounds

22 1

Table 5 (cont.) Reactant L

Substrate M%SnrFe(co),cPl,

(p-Sn MePh), Fe,( CO)8 Ru3(C0)12 [Os(SiM%)(CO)4I, CPCO(CO), CPC0(CO),

-

PPhs, PMePh,, or PBun3 cyclo-octatetraene c6F$sc6F,

-

R1R2CR3=CR4S

-

Me,Sn[Co(CO)& [CO(CO>,tR,PCH,),CMe]+[Co(CO),] -

C1-, Br-, NCO-, or

Products (p-CO)(p-Me, Sn)-

Ref. 244

Fe,(CO),CP, (p-SnMePh),Fe,( C0)7 363 Ru(CO),L and 3 64 t ~ n s - R uCO),L, ( Os(CO),L 365 CpCo(C0)L and 366 [CPco(s c6Fd 12 (q4-R1R2C=CR3CR4=S)- 359 cpco (pMe,Sn),Co2(CO), and 244 (p-CO)(p-Me,Sn)C0,(CO), CoL(R,PCH,)&Me 3 67

N3-

3 Metalloporphyrins and Related Compounds This section consists of a review of recent developments in the photochemistry and photophysics of metalloporphyrins, including haem and the cytochromes. Chlorophylls and bacteriochlorophylls are discussed in Part VI. Relatively few studies on the photochemistry of these compounds have been published this year, although substantial consolidation of our understanding of their photophysical properties is apparent. Reviews of the photochemistry of porphyrins and metall~porphyrins,~~~ and of the status of MO calculations on porphyrins and their complexes 369 have been published. Photochemical ejection of carbon monoxide from (OEP)Ru(CO)(py) (OEP = octaethylporphyrin) has been studied previously. The quantum yield of this reaction, which is accompanied by phosphorescence, is wavelength dependent. It has now been shown that for (TPP)Ru(CO)(piperidine) (TPP = tetraphenylporphyrin) the quantum efficiency for CO dissociation is markedly temperature dependent.370A LF state, or possibly a CT state, appears to be responsible for this reaction, and the temperature dependence of the quantum yield suggests that it is formed after back-intersystem crossing from the low-lying phosphorescent, porphyrin-localized, triplet state. An understanding of the photochemistry of simple carbonyl complexes of iron porphyrins would, of course, be helpful for discussions on the photodecarbonylation of haemoglobin and other haem-proteins (see below). Unfortunately such studies are extremely difficult because of these complexes’ instability in solution in the absence of carbon 3a3 364 365

367

868

96s

370

T. J. Marks and G. W. Grynkewich, J. Organometallic Chem., 1975, 91, C9. B. F. G. Johnson, J. Lewis, and M. V. Twigg, J.C.S. Dalton, 1975, 1876. P. J. Harris, J. A. K. Howard, S. A. R. Knox, R. P. Phillips, F. G. A. Stone, and P. Woodward, J.C.S. Dalton, 1976, 377. J. L. Davidson and D. W. A. Sharp, J.C.S. Dalton, 1975, 813. J. Ellermann and J. F. Schindler, Chem. Ber., 1976, 109, 1095. F. R. Hopf and D. G. Whitten, in ‘Porphyrins and Metalloporphyrins’, ed. K. M. Smith, Elsevier, Amsterdam, 1975, Chapter 16, p. 667. S. J. Chantrell, C. A. McAuliffe, R. W. Munn, and A. C. Pratt, Co-ordination Chem. Rev., 1975, 16, 259. A. Vogler and H. Kunkely, Ber. Bunsengesellschaftphys. Chem., 1976, 80, 426.

Photochemistry

222

monoxide. However, no phosphorescence from (TPP)Fe(CO)(pip) or (TPP)Fe(pip), can be detected. This suggests that a LF state lies below the porphyrin triplet, and that it is such a LF state, which is responsible for the photodecarbonylation of the biologically important Photocatalysis of the aerobic oxidation of Fe" porphyrin complexes in the presence of hydrazine has been demonstrated.371 Kapinus and Dilung have examined the photoreduction of metalloporphyrins of the titanium 372 and aluminium s ~ b g r o u p s .It~ has ~ ~ been demonstrated that the course of the photoreduction by hydrazine of (TPP)TiO differs from those of (TPP)Zr(acac), and (TPP)Hf(acac),. The reactions of the ZrIV and HfIV complexes proceed in two stages. Initially the protonated radical-anion is produced, and further photolysis gives the chlorin (dihydroporphyrin). This is consistent with the lowest-lying states in the original complexes being porphyrinlocalized and not CT in nature. With the TiIV complex, on the other hand, reduction at the metal centre is observed. Comparison with metallic sodiuminduced reduction indicates that this distinctive pathway for the photoreduction of (TPP)TiO is a property of the Ti=O group, probably involving cleavage of the Ti=O bond. The photoreduction by hydrazine of (TPP)MBr (M = Al, Ga, or In) eventually leads to their chlorin In the case of (TPP)InBr, it is suggested that two intermediates are present. Thus irradiation at 597 nm causes the complete reaction of the porphyrin to give its protonated radical-anion. Evidence is presented to show that the conversion of this latter species to the chlorin requires two photons. A reinvestigation of the photolysis of cyanocobalamin both in the presence and absence of oxygen confirms that the only photoprocess occurring is lowand Q6,, = 3 x This is efficiency aquation (95) (@)960 = 4 x [CofCN

+ Ha0

hv'

~

([CofOH,)+

+ CN-

(95)

consistent with the lowest-lying state being a Co"' LF state. The dicyano-Rh"' derivative of the corrin (75) exhibits quite different properties, showing strong phosphorescence and no photo-aquation. This indicates that for this complex Me Me

CN (75) s71 378

s73 s74

C. Bartocci, R. Rossi, and F. Scandola, Ann. Univ. Ferrara, Sez. 5, 1974, 3, 111. E. I. Kapinus and I. I. Dilung, Khim. uysok. Energii, 1975, 9, 492. E. I. Kapinus and I. I. Dilung, Khim. uysok. Energii, 1975, 9, 353. A. Vogler, R. Hirschmann, H. Otto, and H. Kunkely, Ber. Bunsengesellschaft Phys. Chem., 1976, 80, 420.

Photochemistry of Inorganic and Organometallic Compounds

223

the corrin triplet state must be the lowest. Interestingly, the corresponding dichloro-compound not only photo-aquates but also phosphoresces at 77 K, implying that here the LF and corrin triplet levels are of comparable energy. On photolysis in aerated non-polar solvents, zinc oxo-octaethylphlorin (76) is converted into (77).375 On the other hand, the Nil1 derivative gives initially a

one-electron oxidation product, which on further irradiation under nitrogen forms (78). Van der Waals and co-workers have studied the effect of magnetic field (075 kG) on the emission spectra of Cu"-porphin in oriented single crystals of n-octane at 1.3-4.2 K.376 The results of these experiments confirm the assignment of the lowest state as quartet, being one of the energy levels arising from the coupling of the porphyrin triplet with the d 8 Cu" centre. Substantial JahnTeller distortion of this state is observed, and this conclusion has also been reached by other authors from studies of the magnetically induced circular emission.377 Application of this technique to (0EP)Pd supplies results which suggest that the triplet state is much less perturbed by the metal ion. The phosphorescence lifetime of Pd"-porphyrin complexes, measured between 77 and 293 K, depends markedly on on cent ration.^^^ This effect is attributed to formation of triplet excimers. Phosphorescence polarization of dimeric Cu- and VO-aetioporphyrin complexes has been studied at 77 K.379 One of the few previous reports of the emission of lanthanide porphyrins dealt with ytterbium derivatives. The principal emission band for these species corresponds to a transition between metal-localized states. Lifetime measurements on this emission for the tetrabenzoporphyrin (TBP) and aetioporphyrin complexes have now been Gouterman et al. have more recently published the results of a systematic investigation of eight lanthanide coniplexes of the type (OEP)LnOH.381 It is demonstrated that closed-shell yttrium (fo) 376

J.-H. Fuhrhop, S. Besecke, J. Subramanian, C. Mengersen, and D. Riesner, J. Amer. Chem. SOC.,1975, 97, 7141.

W. G. Van Dorp, G. W. Canters, and J. H. Van der Waals, Chem. Phys. Letters, 1975, 35, 450. 377

378

380 381

R. A. Shatwell, R. Gale, A. J. McCaffery, and K. Sichel, J. Amer. Chem. SOC.,1975,97, 7015. V. V. Sapunov and M. P. Tsvirko, Doklady Akad. Nauk Belarus. S.S.R., 1976,20,208. S. S. Dvornikov and M. P. Tsvirko, Izvest. Akad. Nauk S.S.S.R.,Ser. fiz., 1975, 39, 2316. M. P. Tsvirko and T. F. Kachura, Zhur. priklad. Spektroskopii, 1975, 23, 907. M. Gouterman, C. D. Schumaker, T. S. Srivastava, and T. Yonetani, Chem. Phys. Letters, 1976,40,456.

224

Photochemistry

and lutetium (f14) compounds show both porphyrin fluorescence and phosphorescence, whereas none of the open-shell compounds fluoresce. The gadolinium (f’)complex phosphoresces strongly, and as noted above, the main emission band from the ytterbium (f13) derivative corresponds to a metallocalized transition, although a weak porphyrin phosphorescence was also detected. Kotlo et al. have extended their investigations of the emission from higher excited states of porphyrins, and have shown that such fluorescence is found not only for TBPH2, (TBP)Zn, (TBP)Cd, but also for (TBP)Mg, (TPP)Zn, and (tetrapropylp~rphyrin)Zn.~~~ The heavy-atom effect on the quantum yields and lifetimes of fluorescence and phosphorescence has been demonstrated for (mesoporphyrin IX dimethy1ester)M (M = Mg, Zn, Cd, Hg),383for octaethylporphyrin in acid media containing heavy atom c o ~ n t e r - i o n s ,and ~ ~ ~ for tetra-arylporphyrins and their zinc

Measurements of the phosphorescence spectra of various free-base-, Mg- and A l - p o r p h y r i n ~ ,and ~ ~ ~ of the phosphorescence polarization of Zn- and Cdporphyrins 387 have been carried out. Transient variations in the intensity of the fluorescence of free-base porphyrin in n-octane crystals have been induced using microwave radiation corresponding to transitions between the sublevels of the triplet From these induced signals, information about the rates of population and depopulation of the sublevels of this non-phosphorescent state has been obtained. In the same experiments it has been demonstrated that tautomerism of the porphyrin (involving photo-induced shifts of nitrogen-bound protons) occurs even at temperatures as low as 1.3 K. Similarly other authors have attributed changes in the quasilinear spectra of TBP on irradiation in n-octane matrices at 77 K to this t a u t o m e r i ~ m . ~ ~ ~ No analogous conversion could be found for the triplet state of chlorin or tetraphenylchl~rin.~~~ Other experiments with Shpolskii matrices include polarization studies of deuteriated and non-deuteriated free-base p ~ r p h y r i n s392 ,~~~~ and a determination by e.s.r. of the zero-field splitting in free-base p o r p h y r i n ~ . ~ ~ ~ 38a

V. N. Kotlo, K. N. Solov’ev, and S. F. Shkirman, Izvest. Akad. Naiik S.S.S.R., Ser. $z., 1975, 39, 1972.

3a3 884

38s

387

888

B. M. Dzhagarov and E. I. Sagun, Zhur. priklad. Spektroskopii, 1975, 23, 285. A. T. Gradyushko, V. N. Knyukshto, K. N. Solov’ev, and M. P. Tsvirko, Zhur. priklad. Spektroskopii, 1975, 23, 444. D. J. Quimby and F. R. Longo, J. Amer. Chem. Soc., 1975,97, 5111. M. P. Tsvirko, K. N. Solov’ev, A. T. Gradyushko, and S. S. Dvornikov, Optika i Spekrroskopiya, 1975, 38, 705. G. A. Zagusta, V. N. Kotlo, K. N. Solov’ev, and S. F. Shkirman, Zhur. priklad. Spektroskopii, 1976, 24, 352. W. G. Van Dorp, - . W. H. Schoemaker, M. Soma, and J. H. Van der Waals, Mol. Phys., 1975, 30, 1701.

88s

880

s81

I. E. Zalesskii, V. N. Kotlo, K. N. Solov’ev, and S. F. Shkirman, Optika i Spektroskopiya, 1975, 38, 917. S. Van der Bent and T. J. Schaafsma, Chem. Phys. Letters, 1975, 35, 45. A. T. Gradyushko, K. N. Solov’ev, and A. S. Starukhin, Optika i Spektroskopiya, 1976, 40, 469.

3sg s88

A. T. Gradyushko, K. N. Solov’ev, A. S. Starukhin, and A. M. Shul‘ga, Izvest. Akad. Nauk S.S.S.R., Ser. fiz., 1975, 39, 1938. A. Scherz, N. Orbach, and H. Levanon, Israel J. Chem., 1974, 12, 1037.

Photochemistry of Inorganic and Organornetatlie Compounds

225 Some particularly interesting publications on the spectra and electronic structure of metalloporphyrins are those on the resonance Raman effect in Mnrrlp o r p h y r i n ~396 , ~ and ~ ~ ~Fell1 p o r p h y r i n ~linear , ~ ~ ~ dichroism spectra of Fellr p o r p h y r i n ~m , ~. ~ ~ . d . ~and O ~ electronic spectra 3g9 of Fell1 porphyrins, p.e. spectra of various metallo-derivatives of TPP,400 and calculations on the d-orbital energies for a variety of Co" porphyrin~.~~' The magnesium phthalocyanine-sensitized photo-oxidation of pinene has been reported to proceed via singlet oxygen Other authors have described the effect of irradiation on the electrocatalytic reduction of oxygen on metallophthalocyanines,403and the influence of magnetic field on the rate of triplet-triplet annihilation in platinum phthalocyanine c r y ~ t a l s405 .~~~~

Haems and Cytochromes.-Flash photolysis experiments with carbonyl derivatives of haem models (79) have provided useful information on the rates and sequence of binding of carbon monoxide and the imidazole base (L) to the iron a t o ~ n407. ~ ~ ~

I

I

CHz I C02Me

CHz

I

R

P N

CONH- (CH,), - N d

(79) R

=

H or Me

It has been demonstrated that the rate of reformation of the carbonyl derivative is a function of the pH (because of protonation of the imidazole) and of the CO concentration. Thus at pH = 3 and at high CO concentration, the ratedetermining step is the rate of base binding (99), while at lower concentrations of CO the slowest process is (98). 3g4 395

S. Asher and K. Sauer, J . Chcm. Phys., 1976, 64,4115. J. A. Shelnutt, D. C. O'Shea, N.-T. Yu, L. D. Cheung, and R. H. Felton, J. Chem. Phys., 1976, 64, 1156.

396 387

388 3B8

400 401

402

(03 404 406 406

407

F. Adar and T. S. Srivastava, Proc. h'at. Acad. Sci. U.S.A., 1975, 72, 4419. R. Gale, R. D. Peacock, and B. Samori, Chem. Phys. Letters, 1976, 37, 430. H. Kobayashi, T. Higuchi, and K. Eguchi, Bull. Chem. SOC.Japan, 1976,49,457. H. Kobayashi, T. Higuchi, Y. Kaizu, H. Osada, and M. Aoki, Bull. Chern. SOC.Japan, 1975, 48, 3137. S. C. Khandelwal and J. L. Roebber, Chem. Phys. Letters, 1975, 34, 355. W. C. Lin, Inorg. Chenz., 1976, 15, 1114. H. Kropf and B. Kaspar, Annalen, 1975, 2232. G . A. Alferov and V. I. Sevast'yanov, Elektrokhimiya, 1975, 11, 827. K. Kaneto, K. Yoshino, and Y. Inuishi, Chem. Phys. Letters, 1976, 40, 505. K. Kaneto, Y. Ido, K. Yoshino, and Y. Inuishi, J . Phys. SOC.Japan, 1975,38, 1042. J. Geibel, C. K. Chang, and T. G.Traylor, J . Amer. Chem. SOC.,1975, 97, 5924. J. Cannon, J. Geibel, M. Whipple, and T. G. Traylor, J. Amer. Chem. SOC.,1976, 98, 3395.

226 (L]Fe(por)(CO) -

hv

(L]FeJpor) Fe(por) (L)+

co

U

(CO)Fe(por]

y)

-

-

Photochemistry (L)Fe(por) 1

1

+ CO

(96)

Fe(por] I’L)

(97)

(CO)Fe(por)p

(98)

(L)E(por)(CO)

(99)

-

The diphasic kinetics of the recombination of the fragments formed on carbon monoxide expulsion from flash-photolysed carbonylhaemoglobin has been attributed to change-over from the ‘T-state’ to the ‘R-state’ of the haemoglobin. The decay kinetics and relative concentrations of the two species have been monitored as a function of pH,408of carbon monoxide c o n c e n t r a t i ~ nand , ~ ~ of ~ temperature in the ranges 173-323 K,410 and 218-293 K.411 Potential energy surfaces for the binding of carbon monoxide to ferrous porphyrin in the lAl and 6Bz states have been derived by a semi-empirical method.412 M.c.d. studies on haemoglobin and myoglobin have been r e p ~ r t e d414 .~~~~ The influence of pH on the photoreduction of cytochrome C1,416the reduction of haem,416and of ferriperoxidase417by photoreduced NADf, and the photoconversion of horseradish peroxidase compound I to compound I1 at low temperatures 418 have been the subject of recent reports.

4 Water, Hydrogen Peroxide, and Anions Recent advances in the photosensitized photolysis of water are discussed in Part V. The technique of phase-sensitive modulation spectroscopy has been used to examine the emission of the gas-phase Hg-H20 e ~ c i p l e x . ~The ~ @lifetime of the s, and as well as radiative and exciplex has been found to be G3.3 x non-radiative processes, evidence is found for reaction (100). *(Hg H,O)

HgH

+ OH*

(100)

Emission (at ca. 380 nm) observed following X-ray or electron irradiation of ice has been assigned to that from the 3B1state of water. However, this matter is one of some dispute, and despite recent ~ t ~ d i ethe ~ nature , ~ ~of~the- emitting ~ ~ ~ R. P. May and A. Adalbert, European J . Biochem., 1975,52, 589. Y . A. Ermakov, V. I. Pasechnik, and S. V. Tul’skii, Biofizika, 1975, 20, 591. l r o V. N. Kulakov, A. L. Lyubarskii, E.E. Fesenko, and M.V. Vol’kenshtein, Mol. Biol. (Moscow), 1975, 9, 246. 411 M. Bernard, C. Balny, R. Banerjee, and P. DOUZOU, Biochim. Biophys. Acta, 1975, 393, 389. 41a K. L. Yip and D. R. Franceschetti, Chem. Pliys. Letters, 1975, 36, 580. 418 J. I. Treu and J. J. Hopfield, J. Chem. Phys., 1975, 63, 613. 414 L. Vickery, T. Nozawa, and K. Sauer, J. Amer. Chem. SOC.,1976,98, 343. 41b C.-A. Yu, Y.-L. Chiang, L. Yu, and T. E. King, J. Biol. Chem., 1975, 250, 6218. 416 K. Kano, T. Shibata, M. Kajiyara, and T. Matsuo, Tetrahedron Letters, 1975, 3693. 417 F. I. Ataullakhanov and A. M. Zhabotinskii, Biofizika, 1975, 20, 596. 418 J. S. Stillman, M. J. Stillman, and H. B. Dunford, Biochemistry, 1975, 14, 3183. 4l9 A. B. Harker and C. S. Burton, J. Chem. Phys., 1975, 63, 885. 4111 R. H. Prince, G. N. Sears, and F. J. Morgan, J. Chem. Phys., 1976, 64, 3978. 4s1 H. B. Steen and J. A. Holteng, J . Chem. Phys., 1975, 63, 2690. 422 H. B. Steen, Chem. Phys. Letters, 1975, 35, 508. 408

409

Photochemistry of Inorganic and Organometallic Compounds

227 species is not resolved. For example, Steen 422 has examined the radiofluorescence (i.e. emission during X-radiation) and thermoluminescence (i.e. radiation from the sample on warming after irradiation) of H 2 0 and D20.As a result of these studies, he believes that the emission at 335 nm, and not that at 380 nm, is more likely to be that from the triplet state of water. Recent electron-impact studies in the gas phase reveal that the vertical excitation energy to the 3BI state of water is 7.0 eV.423 The photoreduction of hydrogen peroxide by hydrogen has been studied in solution under a hydrogen pressure of 100 atm.424The quantum efficiency is a function of hydrogen peroxide concentration, because of the effective competition of the chain termination step (101) with the propagating reaction (102) at higher hydrogen peroxide concentrations. A complete analysis of the kinetics of this system requires the participation of 21 reactions.

It has previously been demonstrated that the quenching of the fluorescence of aromatic hydrocarbons by inorganic ions is due to catalysed inter-system crossing involving CT states of the organic compound and the anion.425 A similar mechanism has now been proposed for the quenching of the triplet states of organic carbonyl compounds (M) by anions.4226The order of the quenching efficiency is NCSe- > I- > SZO3'- > N3-> Br- > CI-, NO3-, and this correlates with the calculated energy of the (M-• X-) CT state. With the exception of no products of chemical reactions such as electron transfer accompany the quenching process. The above anions do not quench the triplet state of naphthalene-2-sulphonateor other aromatic hydrocarbons (k, = lo4 dm3mol-1 s-l), an observation which is consistent with the high calculated values for the energies of the intermolecular CT states. NO,- was found to be an effective quencher, and in this case it is postulated that energy transfer takes place, the value of ET being determined to be 223 kJ mol-l. With Br-, I-, NCS-, or SO,*and naphthalene-2-sulphonate,transient absorption due to oxidized species can be detected. It has been proposed, however, that this arises from reaction of the anion with the aromatic compound cation, formed by self-ionization of the naphthalene triplet state, and not by direct electron transfer. In another publication it is reported that the quenching of both the fluorescence and phosphorescence of biacetyl by anions parallels the oxidation potential of the anion, and it appears that in this case, too, a CT mechanism is Other authors have examined the deactivation by anions of the singlet states of quinoline-land of singlet oxygen.43o of eosin triplet 423 424

426 426

427 428 429 430

K. N. Klump and E. N. Lassettre, Canad. J. Phys., 1975, 53, 1825. R. J. Field, R. M. Noyes, and D. Postlethwaite, J. Phys. Chem., 1976, 80, 223. A. R. Watkins, J. Phys. Chem., 1974, 78, 1885, 2555. A. Treinin and E. Hayon, J. Amer. Chem. Sac., 1976, 98, 3884. P. Bortolus and S. Dellonte, J.C.S.Faraa'ay ZI, 1975, 71, 1338. G. G. Aloisi and G. Favaro, J.C.S. Perkin ZZ, 1976, 456. T. Akiyama, M. Kamiya, and Y. Akahori, Bull. Chem. Sac. Japan, 1975, 48, 1033. I. Rosenthal and A. Frimer, Phatachem. and Photobiol., 1976, 23, 209.

Photochemistry The kinetics of disappearance of Bri- formed after flash photolysis of Brhave been It is shown that the equilibrium (103) is established

228

Br-

-

+ Br-

Bri-

(103)

shortly after photolysis, and that the decay of the Br;- is due both to bimolecular reactions of Br2*- and to those of Br atoms. The spectroscopic properties of species of the type M+X2- have been examined following synthesis of the compounds by the codeposition of alkali metal atoms and F,, Cl,, Br,, and I2 in argon matrices at 17 K.432 Two reports on the photolysis of nitrate ion [reaction (104)] have been 434 The quantum yield for nitrite ion formation is wavelength dependent (at pH = 11.7, @,, = 0.23, @313 = 0.021), and this has been attributed to reactions occurring from the T-T* and n-n* states respectively.4s3 It has been shown that the primary process is (105) and not (106). Isomerization of the 2N03NO3-

NO,-

+ 0, NO2- + 0 A NO, + o-

A

2N02-

hv

( 104)

(105)

nitrate ion to give peroxynitrite ion is also observed, and this reaction originates directly from the triplet w-n* state. The quantum efficiency for nitrite formation drops substantially at low pH, a phenomenon ascribed to the protonation of the excited state of the nitrate ion. Photolysis of S2OS2-is a convenient route to SO4’-, and this method has been used to study the reactivity of this radical species.43s The generation of SO;from SzOs2-by pulse radiolysis and the results of radiolysis studies of P,0s4have also been reported.43s 5 Main-group Elements Magnesium.-The photoisomerization of ally1 Grignard reagents to their cyclopropyl analogues has been Boron.-The mercury-sensitized photolysis of carboranes and boranes leads to B-B coupled products 438 [equations (107) and (lOS)]. A mechanism involving hydrogen atom abstraction by excited mercury atoms is consistent with the

433 434

435 436 437

4313

D. Wong and B. di Bartolo, J . Photochem., 1975, 4, 249. L. Andrews, J . Amer. Chem. SOC.,1976, 98, 2147, 2152. N. S. Bayliss and R. B. Bucat, Austral. J. Chem., 1975, 28, 1865. E. A. Podzorova, V. D. Orekhov, and I. V. Vereshchinskii, Khim. vysok. Energii, 1975, 9, 446. 0. P. Chawla and R. W. Fessenden, J. Phys. Chem., 1975,79,2693. G . Levey and E. J. Hart, J. Phys. Chem., 1975, 79, 1642. S. Cohen and A. Yogev, J. Amer. Chem. SOC.,1976,98,2013. J. S. Plotkin and L. G. Sneddon, J.C.S. Chem. Comm., 1976, 95.

Photochemistry of Inorganic and Organornetallic Compoundy

229 observed results. 2-CI-1 ,6-C2B4H5may be synthesized either by irradiation of 4-CI-2,3-C2B4H7or by the photochemical chlorination of 1,6-C2B4H6.43Q Reaction (109) occurs by excitation of a complex of the two reactants.440The reaction may also be sensitized by dienes. (But),C=NH

+ BEt3

(But)#-NHBEt2

+ Et-

(109)

This year has seen further developments in the study of the dissociation of boron-containing compounds, following multi-photon absorption of i.r. radiation from carbon dioxide lasers. With BC13 and Hzthis leads exclusively to BHClz and HC1,441while laser-induced cleavage of the B-Cl bond in the presence of acetylene initiates the reaction (1 Similarly, selective photolysis of BC13 or BCl,

+ HC=CH

-

HC=CBCI,

+ HCl

(1 10)

SF6 leads to isotope enrichment in these species.443BMe,Br and BMeBr are the products of laser irradiation of BMe, in the presence of HBr, and in this case it can be demonstrated that the reactions are not merely thermally acti~ a t e d Other . ~ ~ ~publications mention the luminescence of molecules formed by the reaction of various compounds with the fragments from the laser-induced dissociation of BC13.445-448

Aluminium and Thallium.-BuiAIC1, is an especially active catalyst for the cis-trans photo-isomerization of 4 - 0 c t e n e . ~ The ~ ~ reaction proceeds cleanly, no other products, such as those from double-bond migration, being detectable. The photolysis of solid-state AlH3 has been i n ~ e s t i g a t e d451 .~~~~ An investigation of the temperature dependence of the decay time of the luminescence of TI+ in lithium chloride solid solutions indicates that several emitting species are present.452 Silicon, Germanium, and Tin.-Evidence for the production of compounds with unsaturated Si-C linkages has been obtained from a study of the vacuum U.V. photolysis of Me&-SiMe, and of Si,Me6.453Even in the presence of oxygen, under 439

440 441

442

443

444

446 446

447 448

449 450

40-1 454

463

J. R. Spielman, R. G . Warren, D. A. Bergquist, J. K. Allen, D. Marynick, and T. Onak, Synth. React. Inorg. Met.-Org. Chem., 1975, 5, 347. J. C. Scaiano and K. U. Ingold, J.C.S. Chem. Comm., 1975, 878. S. D. Rockwood and J. W. Hudson, Chem. Phys. Letters, 1975, 34, 542. N. V. Karlova, G. P. Kuz’min, A. M. Mikheev, V. N . Panfilov, A. K. Petrov, and V. N . Sidel’nikov, Kratk. Soobshch. Fiz., 1973, 35. R. V. Ambartsumyan, N. V. Chekalin, Y. A. Gorokhov, V. S. Letokhov, G. N . Makarov, and E. A. Ryabov, Lecture Nores Phys., 1975, 43, 121. H. R. Bachmann, H. Noeth, R. Rinck, and K. L. Kompa, Chem. Phys. Letters, 1975, 33, 261. S. D. Rockwood, Chem. Phys., 1975,10,453. R. V. Ambartsumyan, V. S. Dolzhikov, V. S. Letokhov, E. A. Ryabov, and N. V. Chekalin, Zhur. eksp. teor. Fiz., 1975, 69, 72. V. N. Burimov, V. S. Letokhov, and E. A. Ryabov, J. Phorochem., 1976,5, 49. R. V. Ambartsumyan, N. V. Chekalin, V. S. Letokhov, and E. A. Ryabov, Chem. Phys. Letters, 1975, 36, 301. S. Warwel and C. Von Fragstein, Chem.-Ztg., 1975, 99, 465. Y . I. Mikhailov, Y. G. Galitsin, V. V. Boldyrev, and Y . D. Pimenov, Optika i Spektroskopiya, 1975,39, 1136. A. P. Bobrovskii and Y. D. Pimenov, Optika i Spektroskopiya, 1975, 39, 989. M. U. Belyi, N . G. Musienko, and B. A. Okhrimenko, Ukrain.fiz. Zhur., 1975, 20, 1909. P. Boudjouk and R. D. Koob, J. Amer. Chem. Soc., 1975,97, 6595.

230

Photochemistry

which conditions radical pathways are excluded, a high yield of SiMe,H is obtained from Si,Me,. This indicates that reaction (111) is occurring. For Me,C-SiMe,, two such pathways are possible; and from product analyses it has been concluded that process (113) is twice as important as (112). These Me,Si-SiMe,CH,-H Me,Si-CMe,CH,-H H-CH,Me,Si-CMe,

hv

-

+ Me,Si=CH, Me,SiH + Me,C=CH, Me,CH + Me,Si=CH, Me,SiH

(1 11) (112) (113)

studies therefore suggest that the failure to isolate compounds with Si=C double bonds is due to their extreme reactivity and not to difficulties in their formation. Photolysis of n-tetrasilane in solution causes disproportionation as shown in equation (114).454 2H3Si-(SiH,),-SiH,

hv

HSi(SiH,)(Si,H,),

+ Si,H,

(1 14)

Cleavage of the 0-0 bond is the only reaction in the photolysis of bis(trimethylsilyl)peroxide.4s5 However, e.s.r. studies show that alkyl radicals are produced on irradiation of higher bis(trialkylsily1)peroxides. While definite evidence is lacking, a possible mechanism for this is (115). (R,Si-O)2

hv

R,Si-0

I

I

+ 2R*

0-SiR,

The highly reactive compounds, silaimines, have been generated photochemically from the corresponding silylazides [equation (1 16)], and trapped by hexamethylcyclotrisiloxan.456 R,Si-N3

A

R2Si=NR

+ N,

(116)

Other reports consider the photochemical addition of HSiCl, to fluorinated ole fin^,^^^ and the light-induced formation of pinacol derivatives { [R1R2(0MMe,)C ] , )from the corresponding aromatic ketone or aldehyde and bis(trimethylsily1)mercury or he~amethyldistannane.~~~ The mercury-sensitized reactions of germane with nitric and the photochemical synthesis of O,+[GeF,]- from O,, F2, and GeF4460have been reported. The gas-phase photolyses of tetraethyltin and tetravinyltin have been investigated.461 It is proposed that these compounds decompose to give radicals, and also undergo molecular dissociation processes, e.g. (1 17) and (1 18). 54 455

456 457

458 45s

a6O

461

F. Feher and I. Fischer, 2. anorg. Chem., 1976, 421, 9. P. G. Cookson, A. G . Davies, N. A. Fazal, and B. P. Roberts, J. Amer. Chem. SOL,1976,98, 616. D. R. Parker and L. H. Sommer, J. Amer. Chem. Soc., 1976,98,618. R. N. Haszeldine, C. R. Pool, and A. E. Tipping, J.C.S. Dalton, 1975, 2292. W. P. Neumann, B. Schroeder, and M. Ziebarth, Annalen, 1975, 2279, R. Varma, K. R. Ramaprasad, A. J. Signorelli, and B. K. Sahay, J. Inorg. Nuclear Chem., 1975, 37, 563. K. 0. Christie, R. D. Wilson, and I. B. Goldberg, Inorg. Chem., 1976, 15, 271. M. Christianson, D. Price, and R. Whitehead, J. Organometallic Chem., 1975, 102, 273.

Photochemistry of Inorganic and Organometallic Compounds

Sn(CH=CH,),

a

Sn(CH=CH&

+ CzH2 + C2H4

23 1

(118)

A l19Sn CIDNP study of the photo-induced reactions of di-t-butylperoxide and dibenzyl ketone with SnMe,H has yielded useful information about the SnMe, radical.462The quenching of singlet excited benzene by Me,SnH proceeds via catalysed intersystem crossing.463 However, some low efficiency chemical processes are also observed. On excitation of the carbonyl band of ketoalkyltrimethylstannanes the principal reaction is that represented in equation (119) (n = 2-4).464 With the MeCO(CH,),SnMe,

MeCO(CH,),-

+ mSnMe,

(1 19)

compound having n = 2, this is the exclusive reaction, whereas for those with n = 3 or 4, the quantum yield for Sn-C rupture is lower (@ = 0.29 and 0.18 respectively), and products from Norrish Type I1 reactions are also formed. Oxidative addition of either iodomethane or di-iodomethane to Cp,Sn or (acac),Sn is catalysed by light.466 Lead.-The photochemical decomposition of PbC14 in D M F has been Recent reports have described the photoprocesses in solid-state lead(@ halides 467-46g and a~ides.~~O Phosphorus, Arsenic, Antimony, and Bismuth.-It has been proposed that the species previously identified as PH, after photolysis of phosphine in krypton matrices is more probably [ P Z & ] * + . ~1~,ZAddition ~ of PzF4 to fluorinated olefins has been induced by initial photocleavage of the P-P bond.472 The quenching of the singlet state of substituted anthracenes by triphenylphosphine has been investigated.473Two distinct quenching processes appear to be operative. In the case of compounds with electron-withdrawing substituents, charge-transfer quenching is important, whereas with those having electron-donating substituents, exciplex formation is indicated. Compounds of the type PhsM (M = N, P, As, Sb, or Bi) are effective quenchers of fluorenone The quenching rate for Ph,Bi is slightly lower than that for 482

M. Lehnig, Chem. Phys., 1975, 8, 419.

463

A. Delaby, D.Rondelez, and S. Boue, J. Photochem., 1975,4, 399. H.G. Kuivila, P. L. Mafield, K.-H. Tsai, and J. E. Dixon, J. Amer. Chem. SOC.,1976, 98,

464

104. 466

466

467

46 8

469 470

471 4711 413

414

K.D. BOS,E. J. Bulten, and J. G . Noltes, J. Organometallic Chem., 1975, 99, 397. J. Szychlinski, J. Biedrzycki, J. Blazejowski, and M. Sobieralska, Rocznicki Chem., 1975, 49, 1465. M. T. Kostyshin, E. V. Mikhailovskaya, and V. M. Sharyi, Zhur. nauch. priklad. Fotograf. Kinernat., 1975,20,213 (Chem. Abs., 1975, 83, 139 758). V. M. Sharyi, V. G . Plekhanov, and E. V. Mikhailovskaya, Zhur.priklad. Spektroskopii, 1975, 22, 551. A. B. Buckman, N . H. Hong, and D , Wilson, J. Opt. SOC.Amer., 1975, 65, 914. J. Schanda, B. Baron, and F. Williams, Acta Tech. Acad. Sci. Hung., 1975, 80, 185. T.A. Claxton, B. W.Fullam, E. Platt, and M. C. R. Syrnons, J.C.S. Dalton, 1975, 1395. W.K.Glanville, K. W. Morse, and J. G . Morse, J. Fluorine Chem., 1976,7, 153. M.E. R. Marcondes, V. G . Toscano, and R. G. Weiss, J . Amer. Chem. SOC.,1975,97,4485. R. H. Lema and J. C. Scaiano, Tetrahedron Letters, 1975, 49, 4361.

232

Photochemistry

Ph,Sb, and it seems that the quenching efficiency is controlled by the availability of the lone pair of electrons on the M atom. Sulphur.-Invest igations of the transient species formed from elemental sulphur either by flash photolysis or on irradiation in rigid solutions reveal that rupture Evidence of an S-S bond to give linear s8 molecules is the principal of cleavage to give S, and S has also been obtained. Photochemical addition of CF3SF4Cl to X-CN (X = C1 or CF,) gives CF3SF4N=CCIX.47s The radical species SF6* has been identified spectroscopically following photolysis of SF6X (X = CI, Br, F) in argon ~ of SF4 in Photolysis of SF$r is also a convenient route to B T F . ~ ,Photolysis argon matrices produces SFs- and SF,.470 These are suggested to be formed in primary photochemical reactions. The first spectroscopic characterization of the species SO4 is reported following irradiation of ozone co-deposited with sulphur t r i o ~ i d e .The ~ ~ ~photochemistry of matrix-isolated cis-thionylimide is strongly ~avelength-dependent.~~~ Thus with long wavelength light (A > 300 nm) the initial process is photo-isomerization to give trans-HNSO, whereas for h > 200 nm, the principal reaction is formation of HOSN. Excitation in the vacuum U.V. ( h = 1216 nm) causes cleavage of the N-H bond, and gives NSO as the main product. Selenium and Tellurium.-On photolysis 2-phenylselenophen (80) yields 3-phenylselenophen, together with PhC=C-CH=CH, and elemental selenium.482 In

the case of 2-phenyltellurophenYonly the decomposition products have been isolated. With both compounds the initial effect of the radiation is to cause rupture of the M-C bond. Photolysis of TeMe, and RfI (Rf = CF, or C,F,) produces the corresponding MeTeRf and (Rf),Te Photocleavage of the Se-Se bond is the first step in the deselenation reaction (120).484Other Et,Se,

+ Ph,MeP

hv

Et,Se

+ Ph,MePSe

reports include the addition of photochemically generated singlet oxygen to dialkyl- and alkylaryl-~elenides,~~~ and the photo-induced transformation of (8 1) to (82).48s 47b 476

477

478 47g 480

483 484

405

486

C. Nishijima, N. Kanamaru, and K. Kimura, Bull. Chem. SOC.Japan, 1976, 49, 1151. S. L. Yu and J. M. Shreeve, Znorg. Chem., 1976, 15, 14. R. R. Smardzewski and W. B. Fox, J . Fluorine Chem., 1976, 7, 456. R. R. Smardzewski and W. B. Fox, J. Fluorine Chem., 1976, 7, 453. R. R. Smardzewski and W. B. Fox, J. Fluorine Chem., 1976,7, 353. R. Kugel and H. Taube, J. Phys. Chem., 1975,79,2130. P. 0. Tchir and R. D. Spratley, Canud. J. Chem., 1975, 53, 2318, 2331. T. J. Barton, C. R. Tully, and R. W. Roth, J. Orgunometullic Chem., 1976, 108, 183. M. L. Denniston and D . R. Martin, J. Znorg. Nitclear Chem., 1975, 37, 1871. R. J. Cross and D. Millington, J.C.S. Chem. Comm., 1975, 455. L. Hevesi and A. Krief, Angew. Chem., 1976,88,413; Angew. Chem. Znternat. Edn, 1976, 15, 381. A. G . Schultz, J. Org. Chem., 1975, 40, 3466.

Photochemistry of Inorganic and Organometallic Compounds

233

Halogens.-Addition of iodide ion to concentrated solutions of iodine (e.g. 6 x mol dm-*) in t-butanol gives I2 and Is- as the major species.487Laser photolysis (530nm) of such solutions produces 1;- and I;-, the former by reaction of iodine atoms with 16-, and the latter by photolytic decomposition of Is-. The spectra of iodine atoms formed on photolysis of iodine in alkanes and halogenocarbon solvents have been recorded following laser-flash p h o t o l y ~ i s . The ~~~ results of experiments in mixed solvents indicate that the interaction between the solvent and the iodine atom is one of contact charge-transfer and not the formation of a thermodynamically stable complex. The pho t odissociation dynamics of ICl in neon and argon matrices have been U.V. irradiation of mixtures of fluorine and oxygen at 77 K provides a convenient route to Fa02.490 Under similar low-temperature conditions, NF4+salts may be synthesized from nitrogen trifluoride and fluorine in the presence of a suitable Lewis acid (e.g. BF3).491The vibrational spectra of XeC12,4B2 XeF, and KrF 493 have been recorded in low-temperature matrices following photochemical synthesis from the elements. 4as

P. Fornier de Violet, Chem. Phys. Letters, 1976, 37, 478. S. R. Logan, R. Bonneau, J. Joussot-Dubien, and P. Fornier de Violet, J.C.S. Faraday I , 1975,71,2148.

4n0 4p0 481

492 493

V. E. Bondybey and L. E. Brus, J. Chem. Phys., 1976,64,3724. A. Smalc, K. Lutar, and J. Slivnik, J. Fluorine Chem., 1975, 6, 287. K. 0. Christie, C. J. Schack, and R. D. Wilson, Inorg. Chem., 1976, 15, 1275. I. R. Beattie, A. German, H. E. Blayden, and S. B. Brumbach, J.C.S. Dalton, 1975, 1659. B. S. Ault and L. Andrews, J. Chem. Phys., 1976, 64, 3075.

Part III ORGANIC ASPECTS OF PHOTOCHEMISTRY

I Photolysis of Carbonyl Compounds BY W. M. HORSPOOL

1 Introduction Photochemistry of organic systems is going through a period when, as in recent years, the photochemistry of carbonyl compounds is of less interest than previously. However the subject as a whole continues to expand although the rate of growth has been demonstrably less over the past few years. Several review articles have been published during the past year. Scharf and Fleischhauer have reviewed the chemical reactivity of the ground and excited states of organic molecules. The photochemistry of compounds in the crystalline state vas been reviewed by Cohen2 and by Nakanishi and N a k a n i ~ h i .Other ~ reviews have focused attention on the synthetic value of photochemical reactians4 and on the use of photochemical methods in the synthesis of natural products.6 The involvement of metal catalysis in photochemical reactions has also been surveyed.6 A review lecture by Chapman’ has dealt with the elegant low-temperature studies carried out by his group. The third edition of Molecular Photochemistry has been published.8 During the past few years several expressions of the Stern-Volmer treatment of the kinetics of excited-state behaviour have appeared in print. A further generalized treatment for handling both singlet and triplet states has been published.g The photochemical reactivity of the nn* singlet states of alkanals has been studied,1° and the photochemical and photophysical properties of acetophenone have received specific attention.ll During the past few years interest has been turned towards the properties of compounds adsorbed on solids, and in this respect the influence of oxygen upon the photochemical reactions of simple ketones on a Vycor glass surface has been examined.12 Amrein and Schaffner l3 have studied the influence of conformation upon intramolecular energy transfer in the ketones (1) and (2). The phosphorescent behaviour of such molecules has a

ti

* lo

l1 la

lS

H. D. Scharf and J. Fleischhauer, Method Chim., 1974, lB, 650 (Chem. Abs., 1975,83,68 990). M. D. Cohen, Angew. Chem. Internat. Edn., 1975, 14, 386. H. Nakanishi and F. Nakanishi, Yuki Gosei Kagaku Kyokai Shi, 1975, 33, 661 (Chem. Abs., 1976, 84, 3881). N. J. Turro and G. Schuster, Science, 1975, 187, 303. S. Isoe, Yuki Gosei Kagaku Kyokai Shi, 1975,33,460 (Chem. Abs., 1976.84, 3835). T. Sato, Yuki Gosei Kagaku Kyokai Shi, 1974,32,989 (Chem. Abs., 1975, 82, 169 523). 0. L. Chapman, Pure Appl. Chem., 1974,40, 511. N. J. Turro, ‘Molecular Photochemistry’, Benjamin, Menlo Park, California, 1974. J. C. Dalton and J. J. Snyder, Mol. Photochem., 1974, 6, 291. J. C. Dalton, M. W. Geiger, and J. J. Snyder, J. Amer. Chem. SOC.,1976, 98, 398. M. Berger and C. Steel, J. Amer. Chem. SOC.,1975,97, 4817. Y. Kubokawa and M. Anpo, J . Phys. Chem., 1975,79,2225. W. Amrein and K. Schaffner, Heh. Chim. Acta, 1975, 58, 397.

9

237

Photochemistry

238

OEt

S

T

R2CO(CH,”),NHR1 R1= Pri, cyclohexyl, adamantyl R2

= Ph, p-anisyl, /3-naphthyl

also been described.14 Intermolecular energy transfer processes encountered in the quenching of excited singlet states of aliphatic ketones by CCl, have been rationalized in terms of an exciplex mechanism.16 The thioester (3) has been suggested as a useful triplet sensitizer with a triplet energy of 55 kcalmol-l (230.5 kJ mol-l). The intersystem crossing efficiency in this compound is 100%.la The photoreactivity of Michler’s ketone in several solvents l7 and the photopinacolization of the aminoketones (4) have been investigated.18

2 Norrish Type I Reactions Acyl radicals ( 5 ) are formed upon the photolysis of cyclopropane carboxaldehyde in inert ~olvents:~@ these either dimerize or add to unreacted aldehyde. Addition of acyl radical ( 5 ) to the dimer, dicyclopropylethan-l,2-dione,also occurs. A

0

(5)

R\“

R2

(6) a ; R1 = H, R2 = Me b;R1 = R2 = Me C; R1 = H, R2 = ~ydopropyl

OH

R1

l8

K. Schaffner, W. Amrein, and I.-M. Larsson, Israel J. Chem., 1975, 14, 48. R. 0. Loutfy and A. C. Somersall, Cunud.J . Chem., 1976, 54, 760. M. Gisin and J. Wirz, Helu. Chim. Acta, 1975, 58, 1768. P. Suppan, J.C.S. Faruday I, 1975, 539. H. J. Roth, A. Abdul-Baki, andT. Schrauth, Arch. Pharm., 1976,309,2 (Chem. Abs., 1976,84,

l9

C. W. Funke and H. Cerfontain, Tetrahedron Letters, 1975,4061.

l4 l6 l6

l7

121 346).

239 study of the reactivity of the nn* states of the cyclopropyl ketones ( 6 ) has been published.20 The acetoins (7) are also photolabile and undergo Norrish Type I fission to afford the acetylpyrroles (8). The reaction occurs upon either direct or 1,2-diketone-sensitizedirradiation.21 CIDNP studies are still of considerable interest, and the photochemical fission of di-t-butyl ketone into pivaloyl and t-butyl radicals and the resultant recombination process have been studied by the 13C-CIDNP technique.22 The photochemistry of benzoyl cyanide in cyclohexane has been shown to be dominated by the fission into benzoyl and cyano radicals.23 The photochemical ring-expansion reactions of cyclobutanones have been a fertile area of study. One review 24 has dealt specifically with the ring-expansion and fragmentations reactions of these molecules and another has dealt with the photochemical ring-expansion reactions of cyclic ketones in Yates and Hagens 26 originally proposed that photo-ring-expansion of a cyclic ketone to an oxacarbene was dependent in certain cases on the absence of other competing pathways, e.g. enal formation. To test this postulate the bicyclic ketone (9) was irradiated in methanol (with added NaHCO,) to afford the epimeric acetals (10, (D = 0.03). The low quantum yield in this experiment 27 is thought to be reasonable evidence for the structural features which prevent enal formation

PhotoZysis of Carbonyl Compounds

. .

a ; R1 = H, R2 = Me b; R1 = Me, R2 = H

(13) a ; R1 = H, RZ = Me b; R' = Me, R2 = H

and favour an oxacarbene intermediate. A trace of the ketene-derived product (11) was also isolated. A low yield of a ketene-derived product (12a) was also obtained in the irradiation of ketone (13a), but the major product was the ketal 2o 22

a4 26

2a

27

J. C. Dalton, J. F. Williams, and J. J. Snyder, Tetrahedron Letters, 1975, 1823. H A . Ryang and H. Sakurai, J.C.S. Perkin I, 1975, 1590. W. B. Moniz, C. F. Paranski, jun., and S. A. Sojka, J. Org. Chem., 1975,40,2946. J. Kooi and J. H. Boyer, J.C.S. Perkin I, 1975, 2374. D. R. Morton and N. J. Turro, Ado. Photochem., 1974, 9, 197. P. Yates and R. 0. Loutfy, Accounts Chem. Res., 1975, 8, 209. P. Yates and R. Hagens, Tetrahedron Letters, 1969, 3623. P. Yates and J. C. L. Tam, J.C.S. Chem. Comm., 1975, 737.

240

Photochemistry

(14). However ketone (13b) gave only ketene-derived product (12b), thereby further demonstrating the influence of structural features on the reaction path.28 Brook and his co-workers have studied the photochemical ring-expansion reactions of silacyclohexanones [e.g. (15)], and the earlier work in this area has been the subject of a review.2B A further report on the photochemistry of the silacyclohexanone (15) has shown that the dimers (16) and (17) are The structures of the products have been verified by X-ray analysis. The dimers are the principal products of the photolysis, but they are also accompanied by

(20) a ; RI-RZ = 0 b; R1 = H, 1 x 2 = Me

the decarbonylation product (18). In line with the previous photochemistry of the ketone (15), the products are thought to arise by the photochemical conversion of the ketone into an oxacarbene (19) which is then trapped by attack on a second molecule of ketone.31,32 The presence of carbene intermediates is substantiated by the photolysis of the ketone (15) in oxygen when the lactone (20a) is formed, or in methanol whenzthe:ketall(20b) is Although there has been some discussion concerning the concertedness (or lack of it) in the ring-expansion process with the weight of evidence falling in favour of a concerted there is no doubt that the Norrish Type I fission of ketones involves the formation of a radical pair. Typical of this is the Norrish Type I photochemical cleavage of the keto-ester (21) which results in the formation of reasonable yields of the esters (22).34 Together with ring-opened material the cyclopentenol [(23), 25x1 was also formed, but the detailed mechanism for its formation is uncertain. Further work on the photochemistry of the triterpene Friedelin (24) has shown that the aldehyde (25) is one of the many z8

2B 31 32

s3 34

36

J. C. L. Tam and P. Yates, J.C.S. Chem. Comm., 1975, 739. A. G . Brook, Zntra-Science Chern. Rep., 1973, 7 , 131 (Chem. Abs., 1975, 82, 124 228). A. G. Brook, J. B. Pierce, and J. M. Duff, Canad. J. Chem., 1975,53, 2874. A. G . Brook, H. W. Kucera, and R. Pearce, Canad. J . Chem., 1971, 49, 1618. A. G . Brook, R. Pearce, and J. B. Pierce, Canad. J. Chem., 1971,49, 1622. G. Quinkert, P. Jacobs, and W.-D. Stohrer, Angew. Chern. Internat. Edn., 1974, 13, 197; G . Quinkert and P. Jacobs, Chem. Ber., 1974, 107, 2473. J. P. Morizur, G. Bidan, and J. Kossanyi, Tetrahedron Letters, 1975, 4167. H. Shirasaki, R. Aoyagi, T. Tsuyuki, T. Takahashi, and R. Stevenson, Bull. Chem. SOC.Japan, 1975,48, 1073.

241

Photolysis of Carbonyl Compounds 0’W

R

2

(22:) a; R1 = CO,Et, R2 = Me, 30% b; R1 = Me, R2 = CO,Et, 47%

(21)

(23)

Another product has been isolated and identified as (26) which is produced when (24) is irradiated in ether-acetone. The product is formed by the addition of the ketyl radical of acetone to the ketene (27) formed by Norrish Type I fission of Friedelin.36 The photochemical Norrish Type I reaction of the gibberellin (28) yields the ring-opened compound (29).37

0

Fallis 38 has re-examined the photochemistry of cis-verbanone (30a) and has found that the product originally reported by Matsui 3B was correctly assigned as (31a). However another aldehyde (32a) was detected in trace amounts arising from the alternative mode of Norrish Type I cleavage of the starting material. Fallis3* reasoned that steric interaction between the bridge and the C-4 methyl 36

37 38

38

H. Shirasaki, T. Tsuyuki, T. Takahashi, and R. Stevenson, Tetrahedron Letters, 1975, 2271. G. Adam and T. V. Sung, Tetrahedron Letters, 1976, 247. A. G. Fallis, Canad. J. Chem., 1975, 53, 1657. T. Matsui, Tetrahedron Letters, 1967, 3761.

Photochemistry

242

group might be, in part, responsible for the preferential formation of the cyclobutane aldehyde (31a). Nopinone (30b) provides a case where this interaction is eliminated 40 and gives the aldehydes (31b and 32b), but again the preference lies

(30) a ; R1

b;R1

=

= C ; R1 = d ; R1 =

Me, R2 = H R? = H H, R3 = Me' R2 = Me

(31) a; R

=

b;R

=

Me H

(32) a; R = Me

b;R=H

heavily towards the cyclobutane aldehyde (31b). The author 38 suggests that there is a close similarity between a-cyclopropyl ketones and a-cyclobutyl ketones and that photochemical bond fission will follow the path whereby the bond remote from the cyclobutyl group is broken. This is further exemplified by a study of trans-verbenone (30c) and the dimethyl derivative (30d). The Norrish Type I ring cleavage of the tricyclic ketones [(33) and (34)] in benzene solution affords the enals [(35a), 54x1 and [(35b), 58x1 and the bicyclic enones [(36a), 29x1 and [(36b), 17%], re~pectively.~~ The aldehydes and the bicyclic ketones presumably arise from a common biradical intermediate [(37) for ketone (33) and

b; R1 = CH3CH0, R3 = H

&O

R

(36) a ; R = a-H b; R = /3-H

40

41

G. W. Shaffer, A. B. Doerr, and K. L. Purzychi, J. Org. Chem., 1972, 37, 25. A. B. Smith, tert., L. Brodsky, S. Wolff, and W. C. Agosta, J.C.S. Chem. Comm., 1975,509.

Photolysis of Carbonyl Compounds 243 (38) for ketone (34)]. This biradical intermediate partitions between the normal Norrish Type I aldehyde formation and ring-opening of the cyclopropyl moiety in a controlled fashion to afford the new biradicals (39) and (40) respectively. Ring-closure of these affords the observed ketones (36). a-Bond fission to a biradical intermediate is the result of irradiation at 307 nm of 7-ketonorbornane (41) in pentane The biradical undergoes a variety of processes : hydrogen abstraction, to afford cyclohex-l-en-4-carboxaldehyde, decarbonylation, and ring-opening to hexa-l,5-diene or ring-closure to bicycl0(2,2,O)hexane.~~Ring-opening by a-fission and hydrogen abstraction is also found in the bicycloundecanone (42) which yields the ester (43) when the reaction is carried out in methan~l-CCI,.~~ Norrish Type I fission is also found CO,Me

CO,Me

as the primary photochemical pathway in the reaction of the bicyclic ketone (44) to afford the aldehyde (45).44 The aldehyde (45) is still the main product when the reaction is carried out in methanol but a small fraction of the biradical (46), the intermediate in the process, follows the ketene-forming pathway and is trapped as the ester (47). Norrish Type I fission is also the dominant reaction path in the photochemistry of the ketones (48).46 The intramolecularity of the disproportionation reactions was demonstrated by the use of labelled compounds. The formation of ketenes in the reactions resulted, when the hydroxy(48) a; R1 = RZ = R3 = b; R1 = R2 = R3 =

H, R’ = OH H, RS = OD C; R1 = R2 = R3 = R4 = H, R j = O-tetrahydropyranyl d; Rt = RZ = R3 = D, R4 = H, Rj = O H e ; R1 = R2 = R3 = D, R4 = H, Rj = OD f; R1 = D, R2 R3 = R4 = H, R5 == OH g; R1 = R4 = H, R2 = .R3= D, R’ = OH h ; R1 = R = Rj = H, R3 = D, R4 = O-tetrahydropyranyl i; R1 = R3 K j = H, RS = D, R4 = O-tetrahydropyranyl j * R1 = R? == R3 == Rj = H, Ra = OH k; R1 = = R3 = Rj = H, R4 = OD

6;;

R1 0

ra 43

44

45

R.4

=

R4 =

T. F. Thomas, B. Matsuzewski, and R. S. Givens, J. Phys. Chem., 1974,78,2637. S . Durani, R. S. Kapil, and N. Anand, Indian J. Chem., 1975,13,946 (Chem. Abs., 1976,84, 59 031). R. 0. Duthaler, R. S. Stingelin-Schmid,and C. Ganter, Helv. Chim. A m , 1976,59, 307. R. 0. Duthaler and C. Ganter, Helv. Chim. Acfa, 1976, 59, 415.

244

Photochemistry

substituent was unprotected, in the formation of lactones by an intramolecular trapping pathway. A representative sample of the reactions described is given in Scheme 1.44

aH

0

(48a,b,c)

5

___,

R = H P /R=D

O 4 3 i R ,

"OR

R

=

R

= tetrahydropyranyl

H, D, or tctrahydropyranyl

Scheme 1

3 Norrish Type II Reactions

A study has been made of singlet reactivity of acyclic ketones (pentan-3-one, hexan-3-one, heptan4-oneYoctan-4-one, nonan-5-one, 2-methylnonan-5-one, and 2,8-dimethylnonan-5-one)which both fluoresce and undergo Norrish Type I1 hydrogen-abstraction reaction^.^^ The fluorescence quantum yields for each of the ketones decreased with increasing chain length of the alkyl substituents. This is a result of a change in the nature of the abstractable y-hydrogen and also to an increase in the number of available y-hydrogens. The temperature dependence of the triplet quantum yield for the Norrish Type I1 elimination reactions of heptan-Zone, heptan-3-oneYand hexan-2-oneY4'and of 4-methylpentan-2one 48 has been measured. Arrhenius parameters for the singlet processes were then derived. The triplet state Norrish Type I1 elimination reaction of butyrophenone can be quenched by various thiophens in rates which correlate with the ionization potentials of the q u e n c h e r ~ . ~ ~ Two reactions, elimination and cyclobutanol formation, are important in the Norrish Type I1 reactions of carbonyl compounds. The cyclobutanol process is often of considerable synthetic value, as in the conversion of the aminoketones (49) into the azetidinols (50).s0 Wagner and Thomas61 have investigated the photochemistry of the fluorinated ketone (51a) and have found that this ketone undergoes exclusive ring-closure in the biradical formed by Norrish Type I1 hydrogen abstraction to yield the cyclobutanol (52). Ring-closure is also prominent in the difluoroketone (51b) but in this instance elimination is equally

a

J. C. Dalton and R. J. Sternfels, Mol. Photochem., 1974, 6, 307. M. V. Encina and E. A. Lissi, J. Photochem., 1975, 4, 321 (Chem. Abs., 1976, 84, 16 514). M. V. Encina, A. Nogales, and E. A. Lissi, J. Photochem., 1975, 4, 75 (Chem. Abs., 1975,83,

4s

V. Avila, S. E. Braslavsky, and J. C. Scaiano, J. Photochem., 1975, 4, 375 (Chem. Abs., 1976,

M

''

42 565). 84, 16515). 61

E. H. Gold, U.S.P., 3 898 142, 204, 158 (Chem. Abs., 1975, 83, 178 795). P. J. Wagner and M. J. Thomas, J. Amer. Chem. SOC., 1976, 98, 241.

245

Photolysis of Carbonyl Compounds OH P11fl-R~

PhCOCH,N K T H , R2

NR1

(49) R' = p-MeC,H,SO,, PhCO; R2 = H or Me

F I Ph CO CCH,CH,CH, I R (51) a; R b;R

= =

'.'re (50)

F

H F

R

(52)

important. The authors 51 consider a variety of reasons for the influence of fluorine substitution and suggest that the most likely one is hyperconjugation by the fluorine. This hyperconjugative interaction is such that the fluorine atom will be antiperiplanar to the radical centre adjacent to the phenyl group. Such a conformation does not allow overlap between the two radical sites (such overlap is necessary for fragmentation). The problem of why some ketones undergo facile fission in Norrish Type I1 processes rather than cyclobutanol formation has been examined in some detail. Originally s 2 it was proposed that the fragmentation process was only efficient when the bonds and the radical centres were properly aligned for the bond fission. This hypothesis was formulated from a study of l-adamantylacetone which yielded the two cyclobutanols (53a) and (53b). Mechanistic studies have estab-

(53) a; R1 = Me, R9 = OH b; R1 = OH, RZ = Me

lished63that both singlet and triplet states are involved in the process. There is no solvent effect upon the singlet excited-state reaction and the quantum yields for formation are virtually the same in benzene (QSS = 0.0083, @53b = 0.0017) as in methanol (Q6% = 0.0084,@53b = 0.0016). However the triplet state reaction exhibits a solvent dependency (benzene @ 5 S = 0.0017,0 6 3 b = 0.0016; methanol @53a = 0.0186, @53b = 0.0104)with the methanol reaction being more efficient. Such an effect of alcohols on triplet states has been reported p r e v i o ~ s l y . ~ ~ Regardless of the explanation of exclusive cyclobutanol formation in the above example, Fleming et aLS5have studied the photochemistry of a series of ketones 6z

6s

Kb

R. B. Gagosian, J. C. Dalton, and N. J. Turro, J. Amer. Chem. SOC.,1975, 97, 5189. R. B. Gagosian, J. C. Dalton, and N. J. Turro, J. Amer. Chem. SOC.,1970,92,4752. e.g. P. J. Wagner, Tetrahedron Letters, 1967, 1753; ibid., 1968, 5385. I. Fleming, A. V. Kemp-Jones, W. E. Long, and E. J. Thomas, J.C.S. Perkin 11, 1976, 7.

246

Photochemistry [(54)-(57) J and measured the ratio of cyclobutanol formation to fragmentation (values obtained are given in parentheses). The results measured show that there is a difference between the ratios for the singlet and the triplet processes. The influence of the unsuitably oriented bond is small and indeed the principal effect of this is to reduce the reactivity of the ketone for both fragmentation and

3"

(54) (singlet, cyclobut: frag. = 13:87 triplet; 73:27)

Ir

( 5 5 ) a ; R1 = a-H, R2 = OAc(singlet, 24:76; triplet, 30:70) b; RI = /3-H, R" = C,HI7 (singlet, 11:89; triplet, 70:30)

QAc

(57) (singlet, 64:36; triplet, 80:20)

(56) (singlet, 4654)

cyclobutanol formation. It is interesting that ketone (54) gave the same ratio for the two paths when the reaction was either carried out in acetone as solvent or was deliberately sensitized. It was concluded that, at least in this instance, acetone can be used as a triplet sensitizer for a cycl~alkanone.~~ Jeger and his co-workers 57 have examined the photochemistry of the dihydroionone (58), which, together with other reactions discussed in the following chapter, undergoes elimination of the side-chain to afford (59). Elimination of the side-chain is also encountered in the conversion of (60) into (61).58 This type of side-chain fission is the basis of a new method for the degradation of the lanosterol (62) into (63).69 Other ketones (64), based on the steroidal skeleton, are also photochemically reactive. However in this instance Norrish Type I1 hydrogenabstraction by the carbonyl group affords a biradical which ring-closes to l6 67

1. Fleming and W. E. Long,J.C.S. Perkin 11, 1976, 14.

M. P. Zink, H. R. Wolf, E. P. Muller, W. B. Schweizer, and 0. Jeger, Helv. Chim. Acra, 1976, 59, 32.

68

G. Ohloff, C. Vial, H. R. Wolf, and 0. Jeger, Helv. Chim. Acta, 1976, 59, 75. J.-M. Bernassau and M. Fetizon, Synthesis, 1975, 795.

247

Photolysis of Carbonyl Compounds

ci-s' @

HO

OAc

@

R

Jf? OH

(65)

R

cyclobutanols (65).s0 Norrish Type I fission also competes in this reaction sequence, and in benzene (64a) affords an intermediate product (66) which undergoes a Norrish Type I1 elimination reaction to yield the fission product (67). When ButOH is used as solvent for the reaction of (64a), the ester (68) is formed together with the alcohol (65). An analogous range of products is formed from (64 b) .6o

Irradiation of the epoxyketone (69a) results in the formation of fourteen products, three of which have been identified.61 The main products (24%) were 61

D. Guenard and R. Beugelmans, Bull. SOC.chim. France, 1975, 2202. E. P. Muller and 0. Jeger, HeZv. Chim. Acta, 1975, 58, 2173.

248

Photochemistry

the isomeric cyclobutanols (70a, b) formed by Norrish Type I1 reactions with the proximate methyl groups. The minor product (71) is the result of 1,5-hydrogen transfer from a ring carbon yielding the biradical (72) which undergoes bond fission to (71). The epoxyketone (69b) gave the cyclobutanols (~OC, d).sl

Photochemical excitation of the phenacylcyclopropane (73) in benzene leads to trans-cis-isomerization,62Further irradiation of the mixture affords a new product identified as (74) (an oxidation step is required). The formation of this product and the trans-cis-isomerization can be accommodated within a reaction scheme which involves a Norrish Type I1 hydrogen-abstraction reaction yielding the biradical (75). Surprisingly, the biradical prefers this route (isomerization) for decay rather than the more normal fission pathway usually encountered in Norrish Type I1 reactions. The ketones (73b, c) are photochemically inert.

An earlier study of the photochemistry of the three pyrimidyl ketones (76a-c) had demonstrated that both C=N and C=O systems could undergo hydrogenabstraction p r o c e s ~ e s .A ~ ~kinetic study of the reactions of these ketones has shown that the triplet state is In the case of the ketone (76a) only a y-hydrogen can be abstracted by the C=N system, and this reaction gives rise to the cyclopropanol (77a). The butyl compound (76b) gives the cyclopropanol (77b) together with the elimination product (76d) which is presumed to arise by M. J. Perkins, N. B. Peynircioglu, and B. V. Smith, J.C.S. Chem. Comm., 1976,222. E. C. Alexander and R. J. Jackson, J. Amer. Chem. Soc., 1974,96, 5663. I* E. C. Aiexander and R. J. Jackson, J. Amer. Chem. Soc., 1976, 98, 1609.

62

(*

249

Photolysis of Carbonyl Compounds

carbonyl group hydrogen-abstraction. The valeryl compound (76c) affords only the fission product (76d) indicating that in this instance only the carbonyl excited state is reactive. The analysis of the system has shown that the C=O and C=N systems show equal reactivity towards primary hydrogen atoms but the C=O is twelve times more reactive to secondary hydrogens than the C=N.

OAR =

Et

K

= =

d;R

=

Pr BlI Mt:

(76) a ; R b; R C;

(77) a ; R = H b ; R = Me

(78) . , a ; R1 -= b ; R1 = C; R' = d ; R1 = ~

R4

R4 = H, R2 = PhCH,CO, R3 = R3 = H, R2 = PhCH,CO, R4 = PhCH,CO, R' = R4 = H, R3 = PhCH,CO, R2 = R3 = H, R' =

D

D D D

Esters are also photoreactive in Norrish Type I1 reactions. Thus the elimination of phenylacetic acid from (78) by photochemical excitation is thought to follow this general reaction type. The process has been suggested to be a synintramolecular elimination process,66 and has been re-examined using the deuterium-labelled compounds (78).66 The results of the irradiation are shown in the Table and indicate that a trans-elimination occurs. The authors 66 reason

Table Isotopic composition of products from the irradiation of (78) 66 Compound

(7W (7%) (78c)

(7W

HID Loss 3.9 8.4 1.6 50

that steric factors play a large part in determining the trans-elimination pathway, and the cis-decalin transition state (79) for cis-elimination has an unfavourable 1,3-diaxial interaction which destabilizes this transition state in comparison with the trans-decalin transition state (80) for trans-elimination. The thiobenzoate (81) undergoes Norrish Type I1 photo-fragmentation relatively efficiently (a = 0.49).67 A biradical mechanism for the fission is extremely Iikely although evidence from the photolysis of the (+)-(S)-ester (81) indicates txi

eo O7

M. L. Yarchak, J. C. Dalton, and W. H. Saunders, jun., J. Amer. Chem. SOC.,1973, 95, 5224, 5228; J. G. Pacifici and J. A. Hyatt, Mol. Photochem., 1971,3,267,271; J. E. Gano, ibid., p. 79; R. Brainard and H. Morrison, J. Amer. Chem. SOC.,1971, 93, 2685; J. E. Gano, Tetrahedron Letters, 1969, 2549. G. Eadon, E. Bacon, and P. Gold, J. Org. Chem., 1976,41, 171. Y. Ogata, K. Takagi, and S. Ihda, J.C.S. Perkin I, 1975, 1725.

250

Photochemistry

I " " " ' ( C H J n~ (83) a; R = H, n = 1 b ; R = D,n = 1 c;R=H,n=2

o

L

2

-m

H (84)

that the biradical undergoes fission extremely readily and there is no evidence for back transfer of hydrogen in biradical (82) (recovered starting material had not racemized). The regiospecificity of the remote functionalization of steroids by benzophenone units has been studied using the derivative (83a, b). Irradiation of this compound affords the unsaturated product (84) where the C-15 deuterium has ended up on the benzylic carbon of the reduced benzophenone.68 Thus the mechanism of the remote oxidation follows the path whereby the excited benzophenone oxygen abstracts the C-14 hydrogen. This substantiates the mechanism proposed originally.6BThe problem of whether the oxidation-reduction process was also stereospecific at the benzylic carbon was examined. Irradiation of the ester (83a) in benzene gave a mixture of epimers of (84) in a ratio of 55 : 45. The ratio changed depending on the type of solvent used for the irradiation; thus, in t-butanol the ratio was 47 : 53 while in acetonitrile the ratio was 45 : 55. Clearly the overall effect is small and the results do not indicate a strong preference for a transition state for hydrogen transfer where the phenyl or alternatively the hydroxyl is under the steroid ring within the biradical intermediate. Small asymmetric induction is also found in the photolysis of the ester (83c). Other products (85) and (86) are obtained from the irradiation of (83a). These R. L. Wife, D. Prezant. and R. Breslow, Tetrahedron Letters, 1976, 517. 6B

R. Breslow, S. Baldwin, T. Flechtner, P. Kalicky, S. Liu, and W. Washburn, J. Amer. Chem. SOC.,1973, 95, 3251.

25 1

Photolysis of Carbonyl Compounds

I

P 11 (87)

I

OH

(88) a; R b; R C; R

p-MeC,H,SO, p-EtCO,C,H,NHCO, = p-NO,C,H,CO, =

=

d;R=

0

products are each formed as a single diastereoisomer, so the reaction evidently exhibits a reasonable degree of stereochemical control. The photochemically induced enolization of o-alkylaryl ketones and related compounds has been the subject of a recent review.'O The o-alkylbenzophenones (87) undergo photochemical elimination reactions to afford o-vinylbenzophenone.?l The mechanism of the elimination is thought to involve the enol(88), but attempts to trap such a compound were unsuccessful. Evidence for the generation of such an intermediate was obtained from the low-temperature irradiation of the ketone (87c). The generation of a transient having an absorption maximum at 400 nm is thought to be good evidence for the formation of (88c). A study of the conformational effects operative on the photochemical enolization of 2-methylphenylalkyl ketones has been In an earlier report on the photochemistry of 2-methylacetophenone it was suggested 7g that 80% of the photoenolization occurs from the singlet state. Wagner and Chen72now suggest that this interpretation of the results is incorrect. They do, however, agree that only 80% of the photoenolization can be quenched. In addition to this they have determined that the quantum yield for the production of a longlived triplet state is only 21%. Stern-Volmer treatment of the kinetics suggests that two triplet states are formed from the acetophenone with decay rates of 3 x lo7 and 5 x lo0s-l. Wagner and Chen 73 interpret this result in terms of two triplets derived from the anti and syn conformers, respectively, of the ketone. They deduce that the short-lived triplet is derived from the syn conformer and that the syn conformer shows substantial enol formation from the singlet state. They have substantiated this claim by a study of the photochemistry of the ketone (89) which can either abstract hydrogen from the methyl in the syn conformer (79% in ground state) or abstract hydrogen from the side-chain in a Norrish

72

78

P. G. Sammes, Tetrahedron, 1976,32, 405. S.-S. Tseng and E. F. Ullman, J . Amer. Chem. SOC.,1976, 98, 541. P. J. Wagner and C.-P. Chen, J. Amer. Chem. SOC.,1976, 98, 239. H. Lutz, E. Breheret, and L. Linqvist, J.C.S. Faraday I, 1973, 2096.

252

Photochemistry

p" singlet-.yw

triplet-syn

lhv 60

O'

A

enol

singlet-on ti

k = I", 10's-1 p

triplet-anti Scheme 2

-i

Norrish Type 11 elimination

Type I1 process in the anti-conformer (21% in ground state). The reaction (Scheme 2) is suggested by them to account for the various processes encountered, indicating that there is a 33.5 kJ mol-1 energy barrier for the interconversion of the anti- and cis-forms of the excited state prior to enol formation. The terephthalate (90) undergoes photochemical enolization in MeOH in the presence of oxygen and subsequently affords the phthalide (91).74 A reasonable H OMe

Meo:2Me Me0,C \

(94) a ; R1 = Me, RZ = H b; R1 = H, R2 = Me c; R1 = R2 = Me l4

(95) a ; R = H b ; R = Me

M. Julliard and M. Pfau, J.C.S. Chem. Comm., 1976, 184.

Photolysis of Carbonyl Compounds 253 explanation of this reaction is that the intermediate enol(92) is trapped by oxygen as (93) which is thermally transformed into the phthalide. The aroylchromone (94a) does not undergo a (96) photochemical However the isomeric species (94b) does and yields the xanthenone (95a) via hydrogen abstraction by the benzoyl group from the methyl group and thermal cyclization of the resultant quinomethide intermediate. Cyclization of (94c) to (95b) also occurs. This reaction follows the same path as for (94b), but the quinomethide intermediate undergoes a 1,7-hydrogen shift yielding (96) prior to cyclization to (95b).76 4 Rearrangement Reactions It has been shown that irradiation of esters (97) results in a previously undetected scrambling of the l80label.76 This indicates that radical recombination occurs in competition with decarboxylation 77 in ester photolysis. The irradiation of the optically active ester {(97b), [a]:;; - 121.8') showed that although the scrambling

4 Ph/rCO,Me OMe (97) a; R = H b ; R = Me

(98)

reaction still took place the group migration occurred with considerable retention of stereochemical integrity ([a]4335;- 118.0°).76Fission of a C-0 bond is also the route for the decomposition of the singlet state of methyl (+)-O-methylmandelate (98).78 Racemization also occurs but arises via an enolization route. Irradiation of the compound (99) in benzene at 300 nm affords a single photoproduct [(loo), 27%].79The authors 79 suggest that absorption of light is followed by bond fission to yield the biradical (101). This biradical is capable of closing to the observed product, but the alternative path involving formation of 2-benzoylnorbornadiene (which could not be detected) followed by photochemical dimerization cannot be discounted. However 2-benzoylnorbornene was isolated from the irradiation of the tetrahydrodimer corresponding to (99) indicating the feasibility of the cycloreversion pathway. There is no evidence that the tetrahydrodimer yields a cyclobutane product corresponding to (loo), although this compound, obtained by reduction of (loo), undergoes Norrish Type I1 hydrogen abstraction to yield the cyclobutanol (102). Examples of the photoisomerization of pyrazolidinones [e.g. (103)] into azetidinones (104) have been reported.80-82 These results are complementary to those reported earlier.83 7a 78 70

81 8a

P. G. Sammes and T. W. Wallace, J.C.S. Perkin I., 1975, 1845. R. S. Givens and B. Matuszewski, J. Amer. Chem. SOC., 1975, 97, 5617. R. S. Givens, B. Matuszewski, and C. V. Neywick, J. Amer. Chem. SOC.,1974, 96, 5547. M. Yoshida and R. G. Weiss, Tetrahedron, 1975, 31, 1801. P. S. Venkataramani, S. Chandrasekaran, and S. Swaminathan, J.C.S. Perkin I, 1975, 730. P. Y. Johnson, C. E. Hatch, and N. R. Schmuff, J.C.S. Chem. Comm., 1975, 725. P. Y . Johnson and C. E. Hatch, J. Org. Chem., 1975,40, 3502. P. Y . Johnson and C. E. Hatch, J. Org. Chem., 1975,40, 3510. P. Y . Johnson and C. E. Hatch, J . Org. Chem., 1975,40,909; Tetrahedron Letters, 1974,2719.

254

Photochemistry

HO Ph

@ The problems associated with the photolysis of acid-sensitive compounds in methanol with 253.7 nm light 84 have been further exemplified by the irradiation of the glycidate (105) in This reaction affords the ionic addition product (106) which is also produced under thermal conditions. Indeed the product (106) is not formed when basified methanol is used for the photolysis. In ether, however, a different photochemical reaction ensues to give a j3-keto-ester (107) and products of fission involving a carbene (108) (Scheme 3). The j3-ketoester is itself photochemically labile and undergoes reduction of the ketonic group and addition of ether to yield (109). The carbene-fission pathway has been thoroughly studied by Griffin et aLss but the present example adds to the small group of compounds which fragment even although they do not have a 1,ldiphenyl grouping. Padwa et aZ.87have observed that the aziridines (110) do not undergo photochemical ring-opening (disrotatory) to yield pyrroles. Instead, 84

8E

87

S. J. Cristol, G. A. Lee,and A. L. Noreen, Tetrahedron Letters, 1971,4175; G. Roussi and R. Beugelmans, ibid., 1972, 1333; C. Baker and W. M. Horspool, ibid., 1974, 3533. V. V. Chung, M. Tokuda, A. Suzuki, and M. Itoh, Bull. Chem. SOC.Japan, 1976,49, 341. N. R. Bertoniere and G. W. Griffin, ‘Organic Photochemistry’, Vol. 3, ed. 0. L. Chapman, Dekker, New York, 1973, p. 115. A. Padwa, D. Dean, and T. Oine, J. Amer. Chem. Sac., 1975, 97, 2822.

255

Photolysis of Carbonyl Compounds Me

C0,Et

Ph%%H 0

eGr>

phwoEt

+ MeCOPh + MeC0,Et

I

Me

(105)

(107)

+

Ph Me MeWC0,Et HO H

+

MeCHOEt

I

MeCHOEt

(109)

Scheme 3

irradiation of either isomer (110a or b) affords the enone [(llla), 27 and 37% respectively]. Aziridine (110a) also affords [(112), 16x1which arises by a complex oxidative mechanism, and aziridine (1lob) yields [(ll 1b), 16x1. 0

But

UR2

Ph

H R1

Ph

. ,

(1 12)

R1 = H,R2 = Me b; RI = Me,R2 = H

(110) a;

But

(111) a; R1 = H,R2 = Me b; R1 = Me, R2 = NHBut

5 Oxetan Formation Four products [(113)-(116)] were obtained from the photochemical addition of benzophenone to diketene.88 Product (113) is the primary result of the addition of benzophenone to the diketene. The bisoxetan (114) presumably arises by a further photoaddition of benzophenone to (117) the decarboxylation product of

q;

; +T 0

(113)

PhsCOH

Ph Ph

P11

Ph

(114)

0 0 (116)

(115)

(133). There are, of course, two possible modes of addition of benzophenone to (117). The second mode must yield an unstable intermediate which eliminates formaldehyde to afford (115). Photoenolization of O-alkyl aromatic carbonyl compounds is now a well investigated reaction and is discussed in Section 3 (p. 251). Carless and Trivedi89 have, however, studied the reaction of the T. Kato, M. Sato, and Y. Kitagawa, Chem. Pharm. Bull. Japan, 1975, 23, 365 (Chem. Abs., 1975, 83, 8762).

H. A. J. Carless and H. S. Trivedi, J.C.S. Chem. Comm., 1975, 581.

o-methylbenzaldehydes (118) with 2,3-dimethylbut-2-ene and have observed the formation of the oxetans (119) in high chemical and quantum yields. 6 Fragmentation Reactions A full report of the photochemical decarboxylation of the cyclic carbonates (120), originally reported in note formysohas been published.@l The reaction leads to the formation of arylcarbenes.86

The oxiran [(121), 36%] is formed by decarbonylation when the oxetanone (122) is irradiated (350 nm) in benzene Benzophenone ( 5 5 7 3 , diphenylketene, and tetraphenylethylene (10%) are also formed. The tetraphenylethylene is formed only when benzophenone and diphenylketene are together in the reaction mixture. The benzophenone, under these conditions, acts as a sensitizer for the dimerization of diphenylketene to 2,2,4,4-tetraphenylcyclobutan-l,3-dione.

Mc

R. L. Smith, A. Manmade, and G. W. Griffin, J. Heterocyclic Chem., 1969,6,443; Tetrahedron Letters, 1970, 663. G. W. Griffin, R. L. Smith, and A. Manmade, J. Org. Chem., 1976,41, 338. J. P. Wasacz, M. M. Joullie, U. Mende, I. Fuss, and G. W. Griffin, J. Org. Chem., 1976,41,572.

Photolysis of Carbonyl Compounds 257 This dione then undergoes double decarbonylation to afford the ethylene. When the irradiation of the oxetanone (122) is carried out in propan-2-ol-benzene the first two reaction paths, uiz. fission to benzophenone and diphenylketene and decarbonylation to afford the oxiran, are still followed, but a third path, that of The tetracyclic reduction of the carbonyl group to afford (123), is also Ph

Ph

M C

ketone (124) undergoes photodecarbonylation to yield the hydrocarbon (125).93 A remarkably efficient synthesis of barrelene (24% from cyclo-octatetraene) has been reported.94 One of the key steps in the overall transformation is the photochemical decarbonylation and retro Diels-Alder reaction of the adduct (126) which yields barrelene (in 50% yield) and (127). In a like manner the adduct (128) decarbonylates and eliminates (127) to yield (129). Photochemical decarbonylation of the isomeric esters (130a, b) yields the bicyclo-octanes (131a) and (131 b), respecti~ely.~~ Interestingly, the decarbonylation and ring-closure result in inversion of configuration so that the trans-ketone (130a) affords a cis-product (131a), and vice uersa. The authorsg5suggest that Norrish Type I fission of the starting material affords a biradical (132) which inverts to (133) to permit a backside attack of the radical site on the C-atom as the CO departs. Although the authorsg5 have no substantiation of this they suggest that such attack may be a heretofore unrecognized feature of decarbonylation reactions. In the presence of methanol, formation of ester (134) competes with decarbonylation. Again the biradical (133) is a likely intermediate since g3

94 96

K. Hayakawa, H. Schmid, and G . Y. Frater, Chimia (Swiz.), 1975,29, 530 (Chem. Abs., 1976, 84, 104719). W. G. Dauben, G . T. Rivers, R. J. Twieg, and W. T. Zimmerman, J. Org. Chem., 1976,41,887. D. S. Weiss, M. Haslanger, and R. G . Lawton, J. Amer. Chem. SOC.,1976, 98, 1050.

Photochemistry

258

(131) a; R' = CO,Me, R2 = H b; R1 = H, RZ = C0,Me

= H, RZ = COiMe b; R1 = CO,Me, RZ = H

(130) a; R'

(132)

(134) a; R1 = H, R2 = C0,Me b; R1 = CO,Me, R2 = H

this puts the hydrogen in the correct spatial environment for intramolecular disproportionation and ketene formation. The bicyclic product (13la) undergoes photochemical conversion into the isomer (131b) when irradiated at 200 "C. This presumably involves fission of the central bond to reform a biradical. An additional product formed in this high temperature reaction is tentatively assigned as the trans-bicy~lo-octane.~~ Evidence has been collected which suggests that the ketone (135) decarbonylates (irradiation at 313 nm) to afford the o-quinodimethane (136).9s Photoreduction of esters [e.g. (137a)l to the corresponding alkanes [e.g. (137b)l can be readily effected in high yield by irradiation (254 nm) of the ester in hexa-

R1 = H, R2 = CO,Me b; R1 = CO,Me, R2 = H

(135) a ;

R

PhCH,CO, N=CPh,

(138) (137) a; R = OAc b;R = H D. S. Weiss, J. Amer. Chem. SOC.,1975, 97, 2550.

Photolysis of Carbonyl Compounds 259 methylphosphoramide-H,O systems.07 Fission of a C-0 bond is also involved in the formation of iminyl and acyloxy radicals on photolysis of acylketoximes 1e.g. (138)]? Alkoxy radicals produced by the photolysis of a-peroxynitriles [e.g. (139)] are involved in a method for the functionalization of unactivated sites in an alkyl chain.OO The reactions in Scheme 4 are representative of the systems examined. NC OH

NC 0'

--+ HO /

CN

(139) AcOZ CN

Ph

+

-hP N C 0,Ac

Ph

-

P

PIl&

O

0

Scheme 4

Low temperature (6 K) irradiation of acetyl benzoyl peroxide in an argon matrix has been studied.loO A full account of the photochemical transformations of some a-chloroketones [e.g. (140)], originally published in note form.lol has appeared.lo2 Other workers have studied similar systems. Thus the cyclobutanone (141) undergoes photochemical conversion in methanol into three products (142a), (143a), and (144a) in the ratio 6 :2 : 1 (quantum yields are also given), The saturated analogue of H. Deshayes, J.-P. Pete, C. Portella, and D. Scholler, J.C.S. Chem. Comm., 1975,439. M. Yoshida, H. Sakuragi, T. Nishimura, S. Ishikawa, and K. Tokamaru, Chem. Letters, 1975, 1125 (Chem. A h . , 1976, 84, 89 177). O9 D. S. Watt, J. Amer. Chem. SOC.,1976, 98, 271. l o o J. Pacansky and J. Bargon, J. Amer. Chem. SOC.,1975,97, 6896. lol R. S. Givens, L. Strekowski, and R. Devonshire, J. Amer. Chem. SOC.,1974, 96, 1633. l o a R. S. Givens and L. Strekowski, J . Amer. Chem. SOC., 1975,97,5867. O7

260

&: 0

(1 40)

Photochemistry Me

H (141)

o

@e

f $ C 0 2 M e

H

'CO, Me

(142) a ; U)

=

b;(D

=

0.59 0.19

H

Me

(143) a ; 0 b; CD

= =

0.14 0.04

this compound gave the same products.lo3 The ring-contraction reaction was dependent on the stereochemistry of the chloro-substituent. Thus the cyclobutanone (145) gave the same two cyclopropyl esters (142b) and (143b) as did (141), but with less efficiency. The principal product from this reaction was an isomer of the ester (144). The reaction is thought to involve a singlet excited state where ionization of the C-C bond affords a carbocation (146) which ringcontracts to the ion (147): this is then trapped as the ester by attack of solvent. Irradiation of the ketostannane (148a) leads to fission of the Sn-C bond and the formation of radicals. With the analogues of greater chain length (148b and c), the Norrish Type I1 process, yielding acetone, becomes more important. In

the norbornyl system (149), fission of the Sn-C bond is much less important and Norrish Type I reactions dominate to yield the aldehyde (150). A singlet excited state has been imp1i~ated.l~~ Photolysis of the thiaketone (151a) results in the formation of the thialactone (152a, 62%) as the main product.lo5 The formation of this product is thought to involve the fragmentation intermediate (153), a type which had been proposed earlier in the fragmentation of 3,3,6,6-tetramethyl-l-thiacycloheptan-4,5loS

lo' lo(

G. Jones, jun. and L. P. McDonnell, J.C.S. Chem. Comm., 1976, 18. H. G. Kuivla, P. L. Maxfield, K.-H. Tsai, and J. E. Dixon, J. Amer. Chem. SOC.,1976,98, 104. P. Y. Johnson and M. Berman, J. Org. Chem., 1975,40, 3046.

261

Plzotolysis of Carbonyl Compounds

(152) a; R = OAc (151) a; R = OAc b; R = OC0,CH,CCf3 b; R = OCO2CH2CCI3

R = OH d;R = H C;

c;R=OH

dione.lo6 The authors lo5reason that their mechanism of an intramolecular electron-transfer best accommodates the product formation and they discount an alternative biradical path.lo7 Several other minor products are also obtained which arise from Norrish Type I fission of the ketone. Thialactones (152b, c) are also the main products from the photolysis of the thiaketones (151b, c). The thiaketone (151d) did not afford a thialactone product. Io6

P. Y . Johnson, Tetrahedron Letters, 1972, 1991. J. Kooi, H. Wynberg, and R. M. Kellogg, Tetrahedron, 1973, 29, 2135.

3 Enone Cycloadditions and Rearrangements : Photo reactions of Cyclo hexadieno nes and Q uino nes BY W. M. HORSPOOL

1 Cycloaddition Reactions Intramolecular.-The synthesis of the tetracyclic ketone (1) originally reported by Hart and Love1 has been described in greater detaiLa The compound (1) is prepared by a photochemical [2 + 21 reaction of the bicyclic enone mixture (2a) and (2b) obtained from treatment of the enone (3) in trifluoroacetic acid. The quantum yield for the photochemical cyclization of (4) into (5) has been measured as 0.374.40in a variety of solvents.s Excitation with 330-380 nm light gives quantitative conversion of (4) into the cage isomer (5). The cyclization was originally reported by Cookson et aL4 several years ago. Closely related to this cyclization is the report of the [2 + 21 closure of cyclopentadienone dimer in methanol to afford the cage compound (6) in 85% yield.5 Other cage compounds 0

Ph (2) a; R1 = Ph, R2 = Me b; R1 = Me, R2 = Ph

0

(4)

(3)

(5) X = H (6) XX = 0

H. Hart and G. M. Love, J. Amer. Chem. SOC.,1971, 93, 6266. H. Hart and M. Kuzuya, J. Amer. Chem. SOC.,1975,97, 2450. G. Jones, jun. and B. R. Ramachandran, J. Org. Chem., 1976,41, 798. R. C. Cookson, J. Hudec, and R. 0. Williams, J. Chem. SOC.( C ) , 1967, 1382. U. C. Chong, S. H. Chang, and K. V. Scherer, jun., Taihan Hwahak Hoechi, 1974, 18, 437 (Chem. Abs., 1975, 82, 124 869).

262

263

Enone Cy cloadditions and Rearrangements Ph

4

R

Ph

0

0

Me (10) a; X = N,

R

Me0

OEt b:X=N.R=Ph C: X = CH, R = OEt =

(11) a; R

b; R

OMe OMe = =

CH,CH=CH, Prn

(7) and (8) have been prepared by irradiation at 350 nm, in methanol, of the enones (9) and (lo), respectively.6 Isoasatone (lla) is formed by a [2 21 cycloaddition of (12a) induced by U.V.irradiation (0.02%aqueous K,CrO, as the filter).' A similar sequence was observed for the conversion of (12b) into (llb).

+

X

(13) a; X = 0

b; X = CH,

(14) a; X = 0 b; X = CH2

T. Mukai, Y. Yamashita, H. Sukawa, and T. Tezuka, Chem. Letters, 1975, 423. S . Yamamura and Y. Terada, Tetrahedron Letters, 1975, 1903.

264

Photochemistry

The problems associated with charge distribution in cycloadditions to cyclohexenones have been examined further.* Thus the irradiation of the ketenesubstituted cyclohexenones (13a) and (14a) results in addition to yield 1,4diketones (15) and (16) respectively. However, the allenes (13b) and (14b) follow a different route yielding the cycloadducts (1 7) and (18) respectively. It is obvious from these results that if polar association of the excited state enone and the addend is important, the opposite charge distribution in the ground states of allene and ketene is dominant. A full account of the work, originally reported in 21 photochemical cyclization note form,@dealing with the intramolecular [2 of e.g. (19a) and (19b) (Scheme 1) has been published.1° The products (20) from

+

= NMe b;X=O

(19) a; X

Scheme 1

this reaction are formed by a crossed [2 + 21 addition. Crossed addition is also found in the triplet-sensitized (benzil or benzanthraquinone) irradiation of (21).11 This affords (22) as the main product although the alternative head-to-head cycloaddition product (23) is also formed. cis-trans-Isomerization of the double bonds in the starting material precedes the cycloadditions.

(21)

(22) a; R2 = CO,Me, R1 = H b; R1 = CO,Me, R2 = H

(23)

A study of the photochemistry of the keto-olefins (Scheme 2) has shown that the inefficiency in product formation is a result of inefficient formation of the biradical which is an intermediate common to the formation of both photoproducts, v i a Cope rearrangement and uia cycloaddition.12 The inefficiency of the biradical formation is due to exciplex formation between the singlet nrr* state and the olefin. Oxetan (24) and (25) formation has also been reported in irradiation (medium-pressure Hg arc) of cyclohexane solutions of the cyclo-

lo l1 la

D. Becker, Z . Harel, and D . Birnbaum, J.C.S. Chem. Comm.,1975, 377. Y. Tamura. Y. Kita. H. Ishibashi, and M. Ikeda, Chem. Comm., 1971, 1167; Tetrahedron Letters, 1972, 1977. ' Y. Tamura, H. Ishibashi, M. Hirai, Y . Kita, and M. Ikeda, J. Org. Chem., 1975, 40, 2702. K. Honda, A. Yabe, and H. Tanaka, Bull. Chem. SOC.Japan, 1975,48,2062. J. C. Dalton and S. J. Tremont, J . Amer. Chem. SOC.,1975, 97, 6916.

Enone Cycloadditions and Rearrangements

265 @ = 0.04

I

CP = 0.12, 0.13

Scheme 2

alkanones (26) at temperatures below 25 "C.13 At temperatures above 25 "C the reaction was complicated by thermal decomposition of the primary product (24). In the case of (24a), irradiation at temperatures above 25 "C also gave rise to Norrish Type I fission of starting material, yielding (27). The quantum yield for the formation of oxetans (28a-g) from the photochemical cyclization of the norbornene derivatives (29a-g) is dependent to a certain extent on the bulk of the alkyl substituent.l* Interestingly, a singlet excited state is proposed as the reactive state in this cyclization and also in the conversion of (30) into (31).16

(24) a; 5% b; 2% c ; 2%

(25) a; 6% b; 3% c; 3%

(28) a; @ = 0.1 b; @ = 0.14 C; @ = 0.17 d ; @ = 0.16 e; @ = 0.22 f; @ = 0.17 g ; @ = 0.18 l3 l4

lb

(26) a ; n ' b; n c; n

= = =

1, R = H 2;R = H 2, R = H

(29) a; R b; R C;

R

d; R e; R f; R

= H = Me = Et

Pri But = isoamyl g; R = cyclopropyl = =

B. Furth, G. Daccord, and J. Kossanyi, Tetrahedron Letters, 1975, 4259. R. R. Sauers, A. D. Rousseau, and B. Byrne, J . Amer. Chem. SOC.,1975, 97, 4947. A. B. Smith, tert. and R. K. Dieter, Tetrahedron Letters, 1976, 327.

266

Photochemistry

COMe (30) a ; R = H b;R=Me

&

0 M e

(31)

Intermolecular.-A report (in Japanese) of the photochemical addition of cyclohexa-173-dione,as its enol, to acrolein and acrylonitrile yielding cyclooctane-1,5-diones has been published.ls The a-formyl ketones (32) exist in two tautomeric forms [i.e.one where the aldehyde is enolized, (A), and the other, (B), where the ketone is enolized] the amounts of which are insensitive to substitution patterns.17 Irradiation of these enol mixtures in the presence of an olefin (e.g. 2,3-dimethylbut-2-ene and cyclohexene) affords products [e.g. (33) from ketone

R1

(32) a; R1 = R2 = H b; R1 = H,R2 = Pri

(33)

c; R1 = R2 = Me

(34)

(35) a; R1 = H, Re = Pri b; R1 = Pri, R2 = H

(32a)l formed by [2 + 21 addition to the enolic double bond and subsequent ring-opening. Surprisingly the photo-addition seems only to involve the enol A, and no evidence for the formation of products from enol B could be obtained. The ketoaldehydes (32a, b) were also added successfully to 1,2-dirnethylcyclohexene and gave high yields of the keto-aldehydes (34).le These adducts (34) could be readily converted into valerane (35a) or isovalerane (35b). Cyclopentenone undergoes photochemical addition to hept-1-yne via a quenchable triplet state to afford the adducts (36a, b; 0 = O.3).lsa Additions of pent-l-yne and hex-l-yne have also been reported,lsb yielding adducts (36c-f) of which the photochemically stable 7-alkyl product predominated. The products with a 6-alkyl group photochemically rearrange into (36g) via a Norrish l6

I. Agata, K. Kawashima, and T. Aono, Yakugaku Zasshi, 1975, 95, 1013 (Chem. Abs., 1975, 83, 178401).

l7

S . W. Baldwin, R. E. Gawley, R. J. Doll, and K. H. Leung, J. Org. Chem., 1975, 40,1865. S. W. Baldwin and R. E. Gawley, Tetrahedron Letters, 1975, 3969. (a) E. P. Serebryakov, S. D. Kulomzina, and W. F. Kucherov, Izvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 12, 2739 (Chem. Abs., 1976, 84, 73 354); (b) S. D. Kulomzina, E. P. Serebryakov, and V. F. Kucherov, Zzvest. Akad. Nauk. S.S.S.R.,Ser. khim., 1974,9,2055 (Chem. Abs., 1975, 82, 97 734).

267

Enone Cycloadditions and Rearrangements

(36) a; R1 = n-pentyl, R2 = R3 = H b; R1 = R3 = €1, R2 = n-pentyl c; R1 = Pr, R2 = R3 = H d; R1 = H, R2 = Pr, R3 = H e; R1 = Bu, R2 = R3 = H f ; R1 = H, R2 = Bu, R3 = H g; R1 = R2 = H, R3 = Pr, Bu, or n-pentyl

Type I process. The cycloaddition (sensitized by Michler's ketone) of methyl acrylate, acrylonitrile, and methyl vinyl ketone to silyl ethers (37) affords [2 + 21 adducts (38).20 The cycloaddition of ethylene at 0 "C to the cyclopentenone (39) by irradiation in n-hexane solution gave the two adducts (40a) and (40b).21 A full account,22following an earlier note,23of the photochemical additions of olefins to the isoindolone (41) has been published. Details of the [2 + 21 additions to 25 the oxazolinone (42) have also been

R

(37) a; n = 2 b;n=1 R

(38)

O

0 d\ @ N

ng g Me R K

(40) a; R b; R

= =

N

..j /3-H; 8% a-H; 75%

OEt (41) . ,

The photochemical addition of ethylene and acetylene to the lactones (43a-e) has been reported to give high yields of the [2 21 products (44) and (45) respectively when the solvent is acetone (triplet sensitization).2s In one case (42e) the yield of adduct was small since the formation of a head-to-head dimer competed with the [2 + 21 addition, Other olefins such as 1,l-dimethoxyethylene and cyclohexene were also used as addends. The application of the addition

+

2o

21 22

28 24 26 28

K. Mizuno, H. Okamoto, C. Pac, H. Sakurai, S. Murai, and N. Sonoda, Chem. Letters, 1975, 237. Y. Ohfune, H. Shirahama, and T. Matsumoto, Tetrahedron Letters, 1975, 4377. K. A. Howard and T. H. Koch, J . Amer. Chem. SOC.,1975,97, 7288. T. H. Koch and K. A. Howard, Tetrahedron Letters, 1972, 4035. R. M. Rodehorst and T. H. Koch, J. Amer. Chem. SOC.,1975,97,7298. T. H. Koch and R. M. Rodehorst, Tetrahedron Letters, 1972, 403. H. Kosugi, S. Sekiguchi, R. Sekita, and H. Uda, Bull. Chem. SOC.Japan, 1976, 49, 520.

268

Photochemistry

R3

R3

R2Q R'

0

(43) a; R1 = R2 = R3 = H

b; R1 = C; R1 = d; R1 = e; R1 = f; R1 =

R3 = H,R2 = Me

(45)

R3 = H, R2 = Ph R2 = H,R3 = Me

Ph, R2 = R3 = H R2 = H, R3 = Bun

process to the synthesis of natural products was also made and syntheses of grandisol (46) and of canadenosilide (47) were reported.26 This latter synthesis involved the photoaddition of 1,l-dimethoxyethylene to the lactone (42f) followed by chemical transformation of the adduct (48). Cant~e11,~~ in a study of photochemical additions to methyl cyclohexene-lcarboxylate, has concurred with an earlier report by Kropp and Krauss28that irradiation of the ester in methanol yields methyl 2-methoxycyclohexanecarboxylate. Methyl cyclohex-l-ene-l-carboxylatealso undergoes [4 + 21 addition when irradiated in excess of furan giving the adduct (49), possibly via a transient strained trans-cyclohexene. [2 21 Adducts (50) are formed when l-cyanocyclohex-1-ene and the corresponding acetoxy-compound are irradiated in 2,3-dimethylb~t-2-ene.~~ The photochemical addition of 2,3-dimethylbut-2-ene to 3-phenylcyclohex-2-ene-l-oneaffords the single photoproduct (51) as well as the dimer of the cyclohexenone.2g Kinetic analysis of the reaction shows that the triplet state of the enone is involved in both the direct and the sensitized (Michler's ketone) irradiations. The influence of other substituents on the photochemical additions of cyclohexenones (52) to the same olefin manifests itself in a preference for either addition to the double bond (53) or to the carbonyl group (54).50,31 The relative proportions of products seem to depend on a variety of factors: at the extreme ends of the scale cyclohexenone forms no oxetan while 6-fluoro-4,4-dimethylcyclohexenone(52d) affords oxetan and cyclobutane but in a ratio of 9 : 1. A previous report stated 32 that the naphthalenone (55a) does not undergo addition when irradiated in the presence of Z-1,2-dichloroethylene although the addition of ethylene could be effected. A reinvestigation of this

+

T. S. Cantrell, J . Org. Chem., 1975, 40, 1447. P. J. Kropp and H. J. Krauss, J . Org. Chem., 1967,32,3222. J. J. McCullough, B. R. Ramachandran, F. F. Snyder, and G. N. Taylor, J. Amer. Chem. Soc., 1975,97, 6767. a1

aa

V. Desorby and P. Margaretha, Helv. Chim. Acta, 1975, 58, 2161. P. J. Nelson, D. Ostrem, J. D . Lassila, and 0. L. Chapman, J. Org. Chem., 1969, 34, 811. N. P. P e t , R. L. Cargill, and D. F. Bushey, J . Org. Chem., 1973,38, 1218.

269

Enone Cycloadditions and Rearrangements

(50) a; b;

(49)

(52) a ; R1 = R2 = H b; R1 = Me, R2 = H C; R1 = R2 = Me d; R1 = Me, R2 = F data for 52b *from ref. 31

R

C0,Me R = CN

Ph

=

(53) a ; 100 b; 48 c; 70 d; 10

(51)

(54) a; b; 5 2

c; 30 d; 90

reaction has shown that the ethylene addition could not be repeated, whereas photoaddition of the dichloroethylene gave the adduct (56a) in 20% yield when benzophenone was used as the ~ e n s i t i z e r .The ~ ~ naphthalenone (55b) gives an analogous product (56b). A review of photochemical additions to 2,3-unsaturated sugars has been [2 + 21 Cycloaddition of 4-hydroxycoumarin (57a) and the quinolone (57b) to cyclohexene can be effected by U.V. irradiation through The adducts

(55) a; R = H b; R = Pri

(56) a; R = H b; R = Pri

?H

(57) a; X = 0 b; X = NMe as s4

(58) a; X = 0,n = 2 b; X = NMe,n = 2 c; = 0,n = 1

x

I. D. Rae and B. N. Umbrasas, Austral. J. Chem., 1975,28, 2669. B. Fraser-Reid, Accounts Chem. Res., 1975, 8, 193. R. G. Hunt, C. J. Potter, S. T. Reid, and M. L. Roantree, Tetrahedron Letters, 1975, 2327. 10

270 Photochemistry formed have been assigned the structures (58). An analogous adduct (58c) is formed between coumarin (57a) and cyclopentene. The coumarin (59) undergoes hydrogen abstraction reactions when irradiated in propan-2-01.~~This leads to the formation of the dihydrodimer (60) via a radical pathway. The coumarin also undergoes [2 + 21 additions when it is irradiated in the presence of ethylenes (2- and 4-vinylpyridine, 1,l-dichloroethylene, vinylene carbonate, and cyclohexene) or 2,3-dimethylbutadiene to afford the [2 + 21 adducts (61). The vinylpyridines were also successfully added to 3-carbomethoxycoumarin to give the cyclobutanes (61f, g).sg

(61) a; R1 = 2-pyridyl, R2 = R3 = H, R4 = Ac b; R1 = 4-pyridyl, R2 = R3 = €3, R4 = Ac C; R1 = R2 = C1, R3 = H, R4 = Ac d ; R1 = H, R2-R3 = OCO,, R4 = Ac e ; R1 = H, R2-R3 = (CHd,, R4 = Ac f; R' = 2-pyridyl, R2 = R3 = H, R4 = C0,Me g; R1 = 4-pyridyl, R2 = R3 = H, R4 = C0,Me

Ethylene can be added photochemically (A > 300 nm in CH2ClBsolution) to the gibberellin (62) to afford the adducts (63).37 The photochemical addition of the steroidal dienone (64) to 2,5-dimethylhex2,4-diene and 4-methylpenta-lY3-dienegave four adducts [(65)-(68)] in each case.38 A rationalization of the stereochemical control in the photochemical addition of @unsaturated ketones to allenes has been made.39 One of the assumptions in this empirical approach is that the /%carbon of the enone is pyramidal in the excited state. 3e 57

s8 39

K.-H. Pfoertner, Helu. Chim. Acta, 1976, 59, 834. B. Voigt and G. Adam, Tetrahedron Letters, 1975, 1937. G. R. Lenz, Tetrahedron, 1975, 31, 1587. K. Wiesner, Tetrahedron, 1975, 31, 1655.

27 1

Enone Cycloadditions and Rearrangements

(65) a; R b; R

= =

(66) a; R

Me; 3.3% H; 2.7"/0

R

g @ b;

= =

Me; 26% H; 14%

0

0

R

k

(68) a; R = Me; 10% b; R = H; 44%

(67) a; R = Me; 25% b; R = H; 4.5%

n

0 % 0

(69)

0

(71) a; R = 3-cyclohexenyl b; R = 3-cyclopentenyl c ; R = l-cyclopentenyl

+

The bicyclic enone (69) has previously been shown to yield [2 21 cycloaddition products with cyclic olefins (C, and C8).40Cycloaddition has also been reported with cyclohexene yielding (70). However, a coupling product (71a) was also isolated in this instance.41 When cyclopentene was used no cycloaddition was detected and coupling products (71b, c) were dominant. The authors 41 consider that the cycloaddition, where it occurs, takes place in a twostep fashion involving the triplet state of the enone. When cyclohexene and cyclopentene are used a hydrogen-abstraction step to the p-carbon competes favourably with formation of the second bond of the cyclobutane. Dimerimtion.-A theoretical treatment of the photodimerization (w + w* singlet) of acrolein has been published.4a The photochemical dimerization of conjugated fatty acid esters has been reported.43 Interest in dimerization in the crystalline phase has continued, and a report of the low-temperature (4.2 K) 40 41 42

"

A. Kunai, T. Omori, T. Miyata, K. Kimura, and Y . Odaira, Tetrahedron Letters, 1974,2517. A. Kunia, T. Omori, K. Kimura, and Y . Odaira, Bull. Chem. SOC.Japan, 1975,48, 731. J. Bertran, V. Forero, F. Mora, and J. I. Fernandez-Alonso, Anales de Quim., 1974, 70, 195 (Chem. Abs., 1974, 81,24 849). 0. Suzuki and T. Hashimoto, Yukagaku, 1975,24,216 (Chem. Abs., 1975,83, 95 978).

272

Photochemistry

(72)

Ph

OMe OMe

(73) (74) Ph R6

R1.R2 R4 (75) a; R1 = Ph,R4 = R6 = CO,Me, R2

=

b; R2 = Ph, R3 = R5 = CO,Me, R1 C; R2 = Ph,R3 = R6 = CO,Me, R1

= =

R3 R4 R4

= = =

R5 R6 R5

= = =

H H H

(76) R1 = R2 = cinnamate R1 = PhCH2; R2 = cinnamate R1 = cinnamate, R2 = Me

R20"

R'O

~f H ~

5

0

(77) R1 = R2 = R5 = R6 = cinnamate; R3-R4 = CMe, Rl-R2 = R5-RB = CMe,, R3 = R4 = cinnamate .-OR' R1 = R6 = cinnamate, R2-R3 = R4-R5 = CH,

H R'O OAc

(78)

(79) a ; X b; X c; X d; X

= = =

=

0, R1 = H, R2 = Ac NH, R1 = Br, R2 = Ac 0, R1 = H, R2 = Me O,R1 = R2 = H

(80)

Enone Cycloadditions and Rearrangements

273

photochemical dimerization of crystalline diethyl p-benzene diacrylate has been made.44 A review of recent work in asymmetric synthesis via topochemically controlled solid-state dimerization has been published.45 The solid-phase dimerization of (72) affords the two dimers (73) and (74).46 Green et aZ.45 have utilized a [2 + 21 photocycloaddition where asymmetric induction in the resultant dimer is achieved by using mannitol as the template. Thus irradiation of mannitol hexa-trans-cinnamate (C,H,, Pyrex) followed by ester exchange gave cis- and trans-methylcinnamate, ( )-dimethyl 8-truxinate [(75a), 25-40%, 38-46% optical yield], ( f )-dimethylneotruxinate [(75b), lo%], and dimethyl p-truxinate [(75a), 20x1. The authors *' suggest that the optical, induction results solely from the predominance of ground-state conformers in the mannitol esters. Other D-mannitol derivatives (76) and (77) have also been used in this The cycloaddition reactions gave products of the head-tohead type from which the carbohydrate residue could be removed easily. Dimers (78) are obtained by Pyrex-filtered irradiation of the furo- and pyrrolopyridones (79a-c) in methanol solution, but it is not known whether these products have head : head or head : tail s t r ~ c t u r e s .A~ ~dimer (78d) is obtained from (79d) in benzene solution, but ring-opening occurs in methanol. Irradiation of the enone (80) gave a d i ~ e r . ~ O

+

2 Enone Rearrangements The azetidinones (81) can be prepared in high yield by benzene-sensitized irradiation of the afl-unsaturated amides (82).61 These products arise via 1,5-hydrogen abstraction by the /%carbon of the enone system to afford the biradical(83) which subsequently closes to the product. The ability of the enone system to undergo hydrogen-abstraction reactions has been reported frequently in recent years, but note that hydrogen abstraction by the carbonyl oxygen of the enone moiety is more common. Thus the alkylaminoenone (84a) undergoes deconjugation to (85) as a result of Norrish Type I1 hydrogen abstraction from

(81) a; R1 = R2 = H b; R1 = Me, R2 = H

(83)

c ; R1 = H, R2 = Me

so

G. N. Gerasimov, 0. B. Mikova, E. B. Kotin, N. S. Nekhoroshev, and A. B. Abkin, Doklady Akad. Nauk S.S.S.R., 1974,216, 1051 (Chem. Abs., 1975, 83, 79 646). B. S. Green, M. Lahav, and G. M. J. Schmidt, Mol. Cryst. Liq. Cryst., 1975, 29, 187 (Chem. Abs., 1975, 83, 27 033). 0. R. Gottlieb, D. P. Veloso, and M. 0. da S. Pereira, Rev. Latinoam. Quim., 1975, 6, 188 (Chem. Abs., 1976,84, 121 592). B. S. Green, Y. Rabinsohn, and M. Rejto, J.C.S. Chem. Comm., 1975, 313. B. S. Green, Y. Rabinsohn, and M. Rejto, Carbohydrate Res., 1975, 45, 115. G. Jones and J. R. Phipps, J.C.S. Perkin I, 1975, 458. J. J. Bonet, I. Portabella, and F. Servera, Afinidad., 1975, 32, 172 (Chem. Abs., 1975, 83.

b1

T. Hasegawa, H. Aoyama, and Y. Omote, Tetrahedron Letters, 1975, 1901.

44 45

I6 47 48

4B

131 820).

274

Photochemistry

(84) a ; R3 = R4 = R5 = Me, Rl-R2 = (CH,), b; R3 = R5 = Ph, R4 = H, R1-R2 = (CH,),, or R1

C; R3 = R4 = Ph, R5 = H, R1-R2

=

= Et,R2 = Me (CH,),, or R1 = Et, R2 = Me

the terminal methyl group.62 Norrish Type I1 hydrogen abstraction is also encountered in the alkylaminoenones (84b or c). Either of these enones yields a photostationary mixture of both when irradiated briefly. At longer irradiation times hydrogen abstraction by the excited carbony1 group from the N-alkyl group affords a biradical (86) which either fragments to chalcone via a hydroxyallene or else cyclizes to the dihydroisoquinoline (87).62 A Norrish Type I1 hydrogen abstraction is also encountered in the conversion of (88a) into (89a) although the closely related derivatives (88b) and (88c) are inactive.63 The N-tosyl derivatives (88d, e) are reactive, affording (89b, c) as the products. The alternative mode of cyclization in the biradical formed by 1&hydrogen abstraction is found with (89e) when the hydroxyazetidine (91a) is formed. The steric influence of p-substitution of the double bond of the enone is demonstrated by the photolysis of (90) when only the hydroxyazetidine (91b) is formed. A further report 5 4 a of the cyclization of the acrylophenones (92) to the pyrroles (93) by a 1,6-hydrogen abstraction pathway has been reported. This work is very similar to that reported earlier.s4b

PR2 R1

I

(88) a ; R1-R2 = (CHJ5 b; R1-R2 = (CH2)4 c; R1 = €I, R2 = alkyl d ; R1 = tosyl, R2 = Et or CH2Ph e; R1 = tosyl, R2 = ally1 6z 63 64

(89) a ; R1-R2 = (CH,), b; R1 = tosyl, R2 = Me C; R’ = tosyl, R2 = Ph d ; R1 = tosyl, R2 = vinyl

J. C. Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2459. J. C. Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2463. (a) H. Aoyama, T . Hasegawa, T. Nishio, and Y. Omote, Bull. Chem. SOC.Japan, 1975, 48, 1671 ; (b) H. Aoyama, T. Nishio, Y. Hirabayashi, T. Hasegawa, H. Noda, and N. Sugiyama, Chem. Comm., 1972, 775; J.C.S. Perkin I, 1975, 298.

h”

275

Enone Cycloadditions and Rearrangements

Tosyl

R R

U H , R ) ,

R

Ph

/’

(91) a; R = H b;R=Me

N(CH2R)z

(92)

CH2R (93) a; R = H; trace b; R = Et; 25% c; R = Pr”; 8%

Irradiation of the a-methylene ketone (94) affords the single photoproduct (95) in high ~ i e l d . ~ Surprisingly, 5 in view of the preceding studies of Agosta and ~ o - w o r k e r s there , ~ ~ was no evidence for the formation of a cyclobutyl ketonethe normal product from the photolysis of such ketones. The authorss6reason that the failure of the biradical (96) to cyclize to the cyclobutyl ketone is due to

~m OR

(97) R1and R2 = alkyl

(100)

(101) a; R b; R C;

55

56

J. J. Hemperly, S. Wolff, and W. C. Agosta, J . Org. Chem., 1975, 40,3315. e.g. R. A. Cormier and W. C. Agosta, J. Amer. Chem. SOC.,1974, 96, 1867.

R

= = =

Me Pri But

276 Photochemistry the hindered rotation about the Ca-CP bond. The enedione (97) is converted into the enonol (98) upon irradiation (A > 360nm) in benzene.57 Photochemical decarboxylation of the lactone (99) has been reported to yield the aromatized product Decarbonylation of the diarylcyclopropenones (101) gives high yields of diarylacetylenes.6B Direct or sensitized (acetophenone) irradiation of the epoxylathyrol (102) yields the trans-isomer (103).60 This trans-isomer is formed in only a small percentage in the direct irradiation since direct irradiation converts (102) into the furan (104) uia a carbene route. cis-trans-Isomerization is not uncommon in P

h

C

e

,

P

h

C

e

,

AcO

AcO

0

r:

R1 0 (105) R = Me, OMe (106) E ; R1 = Me, R2 = H 2; R1 = H, Ra = Me

Ph

(107) a; X = N

b;X=O

large unstrained ring systems. However, this type of isomerization is more frequently encountered in the isomerization of acyclic systems, as in the triplet isomerization of chalcone.sl Isomerization (Z-E) is also encountered in the Pyrex-filtered irradiation of the esters (105).62 Isopropyl (2E,4E)-1l-methoxy3,7,1 l-trimethyldodeca-2,4-dienoate,the insect growth regulator ‘methoprene’, undergoes cis-trans-isomerization of the 2E-bond and photodecomposition to 7-methoxycitronellalwhen irradiated as a thin film on glass, in aqueous emulsion, or under sensitization conditions in rnethan01.~~The photochemical E-2 67 68

eo

Oa

A. Mosterd and H. J. T. Bos, Rec. Trav. chim., 1975, 94, 220. B. Voigt, G. Adam, E. P. Serebryakov, and N. S. Kobrina, Z . Chem., 1975,15, 103 (Chem. A h . , 1975, 83, 97 615). R. West, D. C. Zecher, S. K. Koster, and D. Eggerding, J . Org. Chem., 1975, 40, 2295. A. Balmain, J.C.S. Perkin ZZ, 1975, 1253. V. G. Mitina, B. A. Zadorozhnyi, and V. F. Lavrushin, Zhur. obschei Khim., 1975, 45, 2713 (Chem. Abs., 1976, 84, 58 324). K. Hartke, R. Matusch, and D. Krampitz, Annalen, 1975, 1237. G . B. Quistad, L. E. Staiger, and D. A. Schooley, J. Agric. Food Chem., 1975,23, 299 (Chem. Abs., 1975, 8% 134014).

Enone Cycloadditions and Rearrangements

277

isomerization of 3-methylpent-3-en-2-one (106) has been studied. The photostationary state contains 44.9% of the Z-isomer (OE+z= 0.42; Oz+E= 0.40). As can be seen C O does not equal unity and thus, in the absence of fluorescence, non-radiative decay is competitive with double-bond Several years ago Ullman and Baumann65in their investigation of the photochemistry of the lactones (107) interpreted the results obtained from sensitization experiments in terms of the orbital symmetry requirements needed for good interaction between the substrate and the sensitizer. Sakuragi et a1.66have reinterpreted these earlier results and have shown that the oxazolone (107a) efficiently quenches the sensitizer regardless of Ullman’s 6s classification. The fluorescent emission of the sensitizers is reabsorbed by the oxazolone and it is this light energy which governs the course of the reaction. A review of the geometric isomerization of oxazolones [e.g. (107b)l has been p~blished.~’ A kinetic study of the photochemical reactions of the furanone (108) in methanol has shown that the trans-cis-isomerization occurs from the S1or Tl state and that this process is much faster than the second photochemical reaction,

dH

a 0 H 2 ) 2 0 H

0

the cyclization to the coumarin This second process involving a simple trans-esterification, arises from the T2 state. Addition of an OH function to the /3-carbon of the enone system can also be photochemically induced as in the 64 66

6’

H. Morrison and 0. Rodriguez, J . Photochem., 1975, 3, 471. E. F. Ullman and N. Baumann, J. Amer. Chem. SOC.,1970,92, 5892. H. Sakuragi, I. Ono, N. Hata, and K. Tokumaru, Bull. Chem. SOC.Japan, 1967, 49, 270. Y. S. Rao and R. Filler, Synthesis, 1975, 749. I. R. Bellobono, L. Zanderighi, S. Omarini, B. Marcandalli, and C. Parini, J.C.S. Perkin ZZ, 1975, 1529.

Photochemistry

278

conversion of (110) into (11l y 9 The enone (1 12) undergoes cis-trans-isomerization to afford the isomer (113).70 This compound is also photolabile and is converted into the hemiacetal (114) upon further irradiation. A full account of the photochemistry of enones [e.g. (115)] in the presence of TiCl, has been 7 2 Two types of reactions were encountered; incorporation of solvent into the molecule followed by cyclization and dehydration (Scheme 3), or incorporation of solvent followed by formation of an acetal or ketone (Scheme 4).

Ph

Ph

Me Scheme 3

hU

TiCI,/MeOH

Scheme 4

The cyclobutenone (1 16a) undergoes photochemical ring-opening to afford methyl 4-phenylbut-3-ene-oateand methyl 2-phenylbut-3-ene-oatewhen irradiated in methan01.~~ Irradiation of (116b) in benzene gives the furan (117). Padwa and Dehm 74 have reported that the furanones (118) undergo phenyl migration to yield (119) when irradiated (triplet-state reaction) in benzene solution. The interpretation of the reaction suggests that n-r* excitation affords a biradical intermediate with odd-electron density on the carbonyl-oxygen and on C-2 of the enone system. Phenyl migration to the terminus of the enone system results. [The odd-electron nature of the process is suggested by the migratory preference for p-CNC,H, and p-MeOC,H4 in compounds (118c, d).] The resultant biradical (120) then undergoes electron demotion to yield a 69

‘O

71 72

78

F. R. Stermitz, J. A. Adamovics, and J. Geigert, Tetrahedron, 1975, 31, 1593. B. R. von Wartburg, H. R. Wolf, and 0. Jeger, Helo. Chim. Acta, 1976,59, 727. T. Sato, G. Izumi, and T. Imamura, J.C.S. Perkin I, 1976, 788. T. Sato, G. Izumi, and T. Imamura, Tetrahedron Letters, 1975, 2191. T. Nishio, H. Aoyama, and Y . Omote, Heterocycles, 1975, 3, 703 (Chern. A h . , 1975, 83, 192 970). A. Padwa and D. Dehm, J. Amer. Chem. Soc., 1975,96,4779.

Enone Cycloadditions and Rearrangements

279

OPh

Ph

0

Ph'

R"NE2t (116) a; R = H b; R = Ph

."do

'NEt,

R1 Ph

R

R 2 Q o

(118) a; R b; R C;

R

d; R

= = = =

H Ph p-NCC& p-MeOC,H,

(119) a ; R1 = Ph, R2 = H b; R1 = R2 = Ph

zwitterion which undergoes hydride migration to product (119) in benzene or else is trapped as (121) in ButOH. {The furanone (118a) is also reactive in Zwitterionic [2 21 cycloaddition reactions affording cyclobutane intermediates are also involved in the rearrangement of the aryloxyenones (122) into the dihydrofurans (123). Irradiation in benzene-methanol-acetic acid (1 : 1 : 1) converts the enone into the ylid (124) which undergoes deprotonationprotonation to yield the furan (123) which is photolabile and can be converted into the phenol (125).

+

8 pl--Jo & R2

ButO"

0'

DR2

(121)

(124)

(122) a; R1 = R2 = I3 b; R1 = MeO, R2 = CH(OMe),

(125)

h1 / \

-

R2 (123)

(126)

A 1,3-group migration takes place via a biradical intermediate (126) when the enone (127) is converted into the ether (128, 46%).76 Another product (129, 76

76

A. G. Schultz and R. D. Lucci, J. Org. Chern., 1975,40, 1371. A. P. Alder and H. R. Wolf, Helu. Chim.Acra, 1975, 58, 1048.

280

Photochemistry

36%) can also be accounted for by the biradical mechanism. Group migration in the rearrangement of cyclohexenones has, of course, been an area of considerable activity for many years. Further work on the photochemistry of optically active cyclohexenones of greater optical purity than the material (130a) used in the previous report 77 has been published.78 This more recent work with the enone (130b) has shown that the rearrangement involving 1,2-phenyl migration and ring-closure occurs without loss of chirality in the formation of (131a). The reaction also affords (13lb) and (131~)with ca. 99% stereospecificity where rearrangement occurs without group migration. 1,2-Vinyl migration (in preference to phenyl migration) is the dominant reaction in the irradiation of the

b

Me 'R

(130) a; R b; R

Q

= =

H

Prn Ph

(131) a;

R1= Me, R2 = Ph, R3 = H

b; R1 = H, R2 = Ph, R3 = Me c; R1 = H, R2 = Me, R3 = Ph

Ph

(132)

(133) a ; R1 = vinyl, R2 = H b; R2 = vinyl, R1 = H

(134)

enone (132) which affords (133a, 51%) as the main product accompanied by a small amount of (133b, 3%).7B3-Cyano-4,4-dimethylcyclohex-2-en-l -one fails to undergo molecular rearrangement when the compound is irradiated in t-butanol, but gave the methylation product (134).80 The methyl radical must arise from the decomposition of t-butoxy radicals from t-butanol. This suggests that, in this instance, t-butanol, a solvent frequently used in photochemistry, must have undergone a hydrogen abstraction reaction. Differentiation between the two possible routes, viz. either concerted or radical pair, for the 1,3-allylic-migration in the enone (135) has been achieved 77 78

7s

D. I. Schuster and B. M. Resnick, J. Amer. Chem. SOC.,1974, 96, 6223. D. I. Schuster and R. H. Brown, J.C.S. Chem. Comm., 1976,28. J. S. Swenton, R. M. Blankenship, and R. Sanitra, J. Amer. Chem. SOC.,1975, 97, 4941. E. W. Kuemmerle, jun., T. A. Rettig, and J. K. Stille, J . Org. Chem., 1975, 40,3665.

28 1

Enone Cycloadditions and Rearrangements

by the use of the optically active forms of the enone.81s82 The conversion of the compound enriched with either the R or S form results in racemization of product (136). This establishes that the rearrangement involves a situation where the migratory carbon must be capable of configurational inversion. Thus a biradical rearrangement pathway predominates. 1,3-Allylic migration is also encountered in the conversion of enone (137) (using 254 nm light and nn* excitation) into

(138)

(139)

(140)

(1 38).83 The resultant P,y-enone (138) undergoes triplet-sensitized conversion yielding (139) and (140) by an oxa-di-n-methane reaction.83 Reactions of this type have been reported in many cases, and two reviews 84* 85 have dealt with the general subject of the photochemical rearrangements of Py-enones. A third review in this area has dealt with the photochemical reactions of l-acyl-2cyclopentenones.8s The photochemical conversion of (138) into (139) and (140) is typical of the 1,Zmigrations encountered in the triplet-sensitized reactions. The products obtained were not interconvertible although the two ketones undergo decomposition. The products (139) and (140) from the optically enriched ketone (138) showed that the rearrangement had taken place without loss of optical activity. These results indicate that rearrangement by a-fission is unlikely and that the major path for the rearrangement is via a concerted cycloaddition. However, if this were the sole reaction path the di-wmethane products would have opposite C-1 configuration. This is not the case in this experiment where the configuration at C-1 is the same, and thus the authors 83 suggest that a stepwise mechanism (Scheme 5 ) is in operation. An oxa-di-n-methane reaction is also found in the triplet-state reaction of (141) to afford (142).87 This product undergoes decarbonylation to yield (143). Direct irradiation of (141) yields (144), the product of 1,3-acyl migration (a singlet-state process).87 The photoproduct is photolabile and undergoes a novel 1,4-acyl migration reaction to 8a

83 84

86

*'

J. Gioor and K. Schaffner, J . Amer. Chem. SOC.,1975,97,4776. J. Gloor and K. Schaffner, Chimiu (Swifz.), 1975, 29, 529. B. Winter and K. Schaffner, J . Amer. Chem. SOC.,1976,98,2022. K. N. Houk, Chem. Rev., 1976, 76, 1. W. G. Dauben, G. Lodder, and J. Ipaktschi, Topics Current Chem., 1975,54,73 (Chem. Abs., 1975, 83, 68 993). K. Schaffner, Tetrahedron, 1976, 32, 641. K. Hayakawa, H. Schmid, and G. Y. Frater, Chimiu (Swifz.), 1975, 29, 530 (Chem. A h . , 1976, 84, 104 719).

Photochemistry

QJ

0

“CH(OMe),

r-13

t)

7.

CH(0Me) Scheme 5 Me

OMe

BMe

M@e- Me (143)

(142)

Me (147)

283

Enone Cycloadditioiis and Rearrangements

(1 50)

(151)

(1 52)

yield (145) when sensitized by acetone. The diketone (146) is also photolabile and affords (147) by a 1,3-acyl shift. (This product decarbonylates on further irradiation.) The acetone-sensitized reaction of (146) affords (148) by a 1,2-acyl shift. Similar rearrangements are observed for the enone (149).87 A lY3-acyl migration is also found upon direct irradiation of ketone (150) to yield (151) plus ten hydrocarbon products.88 The acetone-sensitized reaction of (150) yields the cyclopentenone (152) by a [2 21 cycloaddition followed by ring-opening of the cyclobutene. The details of the photochemistry of 3,3-dimethylpent-4-en2-one have been published.88-B0 Irradiation (Vycor filter) of solutions of the keto-epoxide (153) affords (1 54), the product of 1,3-migrationYwhich itself was rapidly consumed to yield (155) formed by decarbonylation and dehydration.*l Hexamethylcyclopentadiene and biacetyl were also formed presumably via (156), a thermally unstable intermediate. The isomer (157) is formed by the direct (A > 300 nm) or acetonesensitized rearrangement of (158).92 The reaction is related to the lY3-sigmatropic shift reactions which are to be found in the singlet-state reactions of &-enones.

+

0

(154)

0 R-R=

0

R (157) 88

91 B2

M. A. Schexnayder and P. S. Engel, J. Amer. Chem. SOC.,1975,97,4825. P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1972,94,9252. P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1972,94,4357. H. Hart and S.-M. Chen, Tetrahedron Letters, 1975, 2363. B. Fuchs, M. Pasternak, and G. Scharf, J.C.S. Chem. Comm., 1976, 53.

I

284

Photochemistry

(159) a;

b;

R1

R2

R3 = H

= Ph, = R1 = R2 = R3 =

H

R1 = Ph, R2 = Me, R3 = H d; R1 = Ph, R2 = H, RS = Me C;

However, this photoreaction arises from the triplet state, and it is thought that a biradical intermediate is involved in the isomerization. It should also be noted that in recent years examples of sensitized lY3-migrationshave been reported.03 The enones (159) do not afford photoproducts by either direct or sensitized irradiati011.O~ The authors O 4 suggest that the absence of photoproduct in the case of (159a) is the result of an energy dissipation path involving a ‘free rotor’ effect.06 The feasibility of this path was demonstrated with (159c) and (159d) which undergo cis-trans-isomerization of the exocyclic methylene group when irradiated into the T -+ T* band.04 The dienone (160) gave three products (161), (162), and (163) when irradiated in dioxan-acetic acid.Qu A detailed account of the photochemical rearrangements of the hydroxysantonene (164) originally reported in note formg7has been published.Q8Rearrangement of the acetate (165a) takes place upon photolysis in the presence of a triplet quencher to afford the acetoxy-compounds (166a).O9

8@ /

0

(165) a; R = S-MeCO, b; R = a-MeCO,

0

(166) a; R = 01- Or P-MeCO, b;R=Me

c; R = Me

@’ O6 O7 O8

P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1975,97, 145. C. Lam and J. M. Mellor, J.C.S. Perkin 11, 1975, 519. H. E. Zimmerman and G. A. Epling, J. Amer. Chem. SOC.,1972,94, 8749. L. J. Dolby and M. Tuttle, J. Org. Chem., 1975,40, 3786. D. S. R. East, T. B. H. McMurry, and R. R. Talekar, J.C.S. Chem. Comm., 1974,450. D. S. R. East, T. B. H. McMurry, and R. R. Talekar, J.C.S.Perkin I, 1976, 433. T. B. H. McMurry and R. R. Talekar, J.C.S. Perkin I, 1976, 442.

285

Enone Cycloadditions and Rearrangements

(167)

(168) a; R

b; R c; R

= = =

a-MeCO, /I-MeCO, a-Me

R

=

01-

or p-Me

(169)

Direct irradiation in the absence of quencher leads to decarboxylated products (165c, 166b), and the loss of CO, is thought to involve a triplet process. Thus, the decarboxylation reaction presumably occurs by acetate 0-C bond fission, decarboxylation of the resultant acetoxy radical, and recombination of the methyl radical with the santonenyl radical. The decarboxylation also takes place with the epimer (165b), but in this instance the recombination takes place at C-11 (167). No evidence was obtained for recombination at C-6. Recombination at C-6 is however encountered in the photolysis of the dihydrosantonenes (168) which give rise to (169). Further study has suggested that the reaction in the dihydro-series is intermolecular in ~ h a r a c t e r . ~ ~ Irradiation (254nm) of the epoxide (170) in pentane solution affords the bicyclic compound (171).loo This reaction is thought to take place by fission of the C-C bond of the epoxide to afford the biradical(l72; perhaps this could be a carbonyl ylid). The biradical ring-closes to the furan derivative (173) which

qo Q

(172)

* = *or+,-

(173)

itself is photolabile and rearranges to the isolated product (171). The ketoepoxide (170) undergoes different reactions upon nn* excitation (C-0 fission) by irradiation at h 3 280 nm. The products from this process are the enones (174), (175), the keto-aldehyde (176), and the cyclobutanone (177), which is a photoproduct of (171).loo A study of the photochemistry of the two keto-olefins (178a) and (178b) has been published.lOl The U.V. spectra of the two compounds show no evidence for intramolecular interaction between the carbonyl and the olefinic chromophore. loo

lol

B. Frei and H. R. Wolf, Helu. Chim. A d a , 1976, 59, 82. H. Morrison, V. Tisdale, P. J. Wagner, and K. Liu, J. Amer. Chem. SOC.,1975, 97, 7189.

286

Photochemistry CHMe

p

h

v

R

3

Ph+H

0

R1

(179)

(178) a; R1 = H = R2,R3 = Me b; R1 = Et, R2 = R3 = H c ; R1 = H, R2 = Me, R3 = H

Ph (180) a; R1 = H, R2 = OH b; R1 = OH, R2 = H

However, the irradiation of (178a) brings about rapid isomerization to (178c) (the photostationary state contains 82% of the trans-isomer) by an intramolecular energy transfer process. Continued irradiation affords three products [(179), (180a, b)], which are thought to arise from the rearrangement of an intermediate bicyclo-oxetan (181). The other keto-olefin (178b) affords six products (Scheme 6) either by Norrish Type I1 processes or by rearrangement of an unstable bicyclic oxetan (see ref. 102 for an earlier account of the photochemistry of this ketone). This second keto-olefin also shows enhanced deactivation of the tripletexcited state of the carbonyl function in comparison with butyrophenone. The authors lol infer from the results that charge-transfer complexation is important in the photochemistry of the two ketones.

a)

(178b)

+

a)

= 0.001

= o.Ooo1

Ph HO Q, =

Et 0.0019

a

= 0.0042

Et
Scheme 6

287

Enone Cycloadditions and Rearrangements

Jeger and his co-workers lo3 have examined the photochemistry of several dihydroionone derivatives (182). Several reaction modes are operative and a representative reaction scheme is shown below (Scheme 7). The simplest reaction encountered is reduction of the carbonyl group. The ability of the excited carbonyl group to abstract hydrogen intramolecularly is also seen in the formation of (183) as a result of Norrish Type I1 fission of the side-chain. Intramolecular oxetan formation is also found yielding (184). However, the tetrahydrofuran derivative (185) could also arise from the oxetan-forming path using a common intermediate (186) which can ring-close or else undergo hydrogen abstraction to afford (185). Together with the more common lY5-hydrogentransfer, 1,4-transfer occurs, producing the biradical (187) and subsequently the alcohol (188). Another reaction mode affords the tricyclic ether (189). This is thought to be produced by the route shown in Scheme 7. X-Ray determination of the structure of (189b) has OH

182a

iw

(189) a; b;

R R

= =

Me Ph

Scheme 7

been carried 0 ~ t . lSimilar ~~ reaction paths are encountered in the photochemistry (Scheme 8) of the’enone (190) formed by photoisomerization of enone (191).lo4 The Norrish Type I1 elimination reaction of (191) was discussed previously (Section 1, p. 246), as was Norrish Type I fragmentation of the bicyclic ketone (192) (p. 243). The resultant aldehyde (193) is subject to further photolysis loS

P. J. Wagner and K. Liu, J. Amer. Chem. Soc., 1974, 96, 5952. M.P. Zink, H. R. Wolf, E. P. Miiller, W. B. Schweizer, and 0. Jeger, Helv. Chim. Acra,

lo*

G . Ohloff, C . Vial, H. R. Wolf, and 0. Jeger, Helv. Chim. Acta, 1976, 59, 75.

log

1976, 59, 32.

288

Photochemistry

I

H- abst raction

Scheme 8

OHC?

s""

Ho6

and yields several products of fission and intramolecular addition.lo6 Among these are the two alcohols (194) and (195). 3 Photoreactions of Thymines etc. Review articles dealing with the photochemistry of pyrimidines and pyrimidine dimers have been published.lo6 A theoretical treatment of the photochemical dimerization of thymine and related compounds has been described,lo7and the possible involvement of excimers in the dimerization has been discussed.lo8 The lo6 loo lo'

lo8

R. 0. Duthaler, R. S. Stingelin-Schmid, and C. Ganter, Helu. Chim.Acta, 1976,59, 307. 'Photochemistry and Photobiology of Nucleic Acids', ed. S. Y. Wang, Academic Press, New York, 1976. V. I. Danilov, Dopou. Akad. Nauk Ukraine R.S.R. Ser. A., 1975, 1021 (Chem. Abs., 1976, 84, 58 388); ibid., 1976,61 (Chem. Abs., 1976, 84, 104 854). S. M. Shul'ga and V. I. Danilov, Kuant. Khimiya, 1975, SH-241 (Chem. A h . , 1976, 84, 89 434).

Enone Cycloadditions and Rearrangements

289

photodimerization of l-cyclohexylthymine (196a) can be quenched by cispiperylene, so evidently it arises from the triplet state.loQ The dimers of 4,6dimethyl-2-thiopyrimidine undergo photochemical monomerization.ll0 The quantum yield for the photodimerization of the alkylsubstituted 1 ,l'-trimethylene bis(3-alkyluracils) (197) in water to yield the cis-syn cyclobutane photodimers

(196) a; b; C;

(197) R = Me or Et

R1 = C6Hl1,R2 = Me R2 = F, R1 = H R2 = CFB,R1 = H

d; R1

= R2 =

H

V (198) R

=

H, Me, Et, Prn, or Bun

(199)

was dependent on the bulk of the alkyl group on the N-3 atoms.111 Thus, as the groups were increased in size from methyl to ethyl to propyl the quantum yield of dimer formation decreased. The pyrimidine-purine dinucleotide analogues (198) undergo photochemical addition in aqueous solutions to afford intramolecular adducts. In the cases examined the principal product has been isolated in each case and has been identified as (199).l12 Complete regioselectivity has been reported in the acetone-sensitized additions of isobutylene, methylenecyclopentane, and methylenecyclohexane to 5-fluorouracil (196b).l13 These additions yield the [2 21 adducts (200). Poorer regioselectivity is found in the addition of methylcyclopentene to the same uracil when four adducts are produced, the main ones (91%) being (201a) and (201b). Regioselectivity is also encountered in the formation of (200d, 95%) from the uracil (196c) and isobutylene. Although no definite mechanistic conclusions were reached in this study, it is suggested that the powerful electronegativity of the fluorine plays a major part in the regioselective reactions encountered.

+

log llfl

ll1 112

113

T. Nakata, M. Yamato, M. Tasumi, and T. Miyazawa, Photochem. and Photobiol., 1975,22, 97 (Chem. Abs., 1976, 84,42 897). M. Wrona, J. Giziewicz, and D. Shugar, Nucleic Acids Res., 1975, 2, 2209 (Chem. Abs., 1976, 84, 73 351). K. Golankiewicz and A. Zasada-Parzynska, Bull. Acad. polon. Sci. Ser. Sci. chim., 1974, 22, 945 (Chem. Abs., 1975, 82, 166052). S. Paszyc, B. Skalski, and G. Wenska, Tetrahedron Letters, 1976, 449. A. Wexler and J. S. Swenton, J. Amer. Chem. SOC.,1976,98, 1602.

290

Photochemistry

H H Me (200) a; R1 = F, R2 = Me

b; R1 = F, R2-R2 = (CH,), C ; R1 = F, R2-R2 = (CH,), d; R1 = CF3, R2 = Me

(202) a; R1-R2 = (CH,), b; R1-R2 = (CHJ, c; R1 = Me, R2 = H

(203)

a - H and Me b; ,8-H and Me

(201) a;

(204) a; b; c; d;

R1-R2 = (CH,),,R3 = I-I R1-R2 = (CH,),, R3 = H R2 = R3 = H, R1 = Me RS = R1 = Me, R2 = H

Enol acetates (202) have also been added photochemically (acetone sensitization) 21 adducts of gross to 5-fluorouracil (196b).l14 This procedure affords [2 structure (203) which on treatment with base yield the adducts (204). A similar reaction (Scheme 9) has been used in the synthesis of the adducts (205) following the [2 + 21 addition of the enol acetates (202b, c) to the 6-azauracil (2O6).ll5

+

(206) a; R = H b;R=Me Scheme 9

The triplet state of uracil (196d), which results from acetone sensitization, is responsible for the formation of products (207) when uracil is irradiated in aqueous solution with cysteine.lls Under direct irradiation two additional products (208a) and (208b) are formed. These are the sole products when the irradiation is carried out under air. Irradiation of the thiouracil derivatives (209) in the presence of l-aminobutane leads to the adducts (210) as diastereois~mers.~~~ A photochemical reaction occurs also between 1,3-dimethyl-4-thiouracil(209a) and triethylamine in water or methanol. This reaction leads to the formation of dimeric products (211) by a free radical reaction.l18 The formation of free radicals 114 115 116

117 118

A. Wexler, R. J. Balchunis, and J. S. Swenton, J.C.S. Chem. Comm., 1975, 601. J. S. Swenton and R. J. Balchunis, J. Heterocyclic Chem., 1974, 11, 917. A. J. Varghese, Biochim. Biophys. A m , 1974, 374, 109. J.-L. Fourrey, Tetrahedron Letters, 1976, 297. J.-L. Fourrey and J. Moron, Tetrahedron Letters, 1976, 301.

29 1

Enone Cycloadditions and Rearrangements

in this instance is thought to arise by way of electron transfer from the amine to the enone followed by proton transfer. Conrotatory cyclization is suggested to occur in the non-oxidative photochemical conversion of the uracil derivative (212) into (213) by way of the ylid

R1 = SR, R2 = H b; R1 = H, R2 = SR

(207) a; R = cysteine b; R.= S cysteine R 1 . S3

OAN R2

(209) a; b;

R

'

R1 = R2 = Me R1 = C8Hll, R2 = H

O

(208) a;

S

I

L

M

N

R2

O NH,

I

k

ST

e

N MeHHMe

(2 10)

(214) (see ref. 75) and 1,4-hydrogen rnigration.ll@ Oxidative cyclizations have also been described. 1,3-Dirnethyluracilabstracts hydrogen from THF when the solution is irradiated with light of wavelength greater than 260 nm.120 This reaction affords a 1 : 1 mixture (90% total yield) of the two adducts (215) and (216). It is obvious from this that hydrogen abstraction by the excited enone occurs equally at the 01- and the p-sites. Hydrogen abstraction by the 4-keto-function in 1,3-diethyluracil (217a) results in the formation of the 3-N-dealkylated product (217b, 2073, together with cyclobutane dimers which made up the bulk of photoproduct (75%).121 The dealkylation appears to follow the Norrish Type I1 path since

Me

lle lao

lZ1

S. Senda, K. Hirota, and M. Takahashi, J.C.S.Perkin I, 1975, 503. M. D Shetlar, J.C.S. Chem. Comm., 1975, 653. M. D. Shetlar and P. J. S. Koo, Tetrahedron Letters, 1975, 2015.

292

Photochemistry

tx Et

A

O

N H'

(217) a; R = Et b;R=H

y-hydrogens are essential for the process (1,3-dimethyluracil does not photochemically dealkylate). The primary photochemical process in the irradiation of 5-bromouraciI (218) is fission of the C-Br bond (0= 1.8 x 10-3).122 4 Photochemistry of Dienones Linearly Conjugated Dienones.-The low-temperature photochemical reactions of the linearly conjugated dienone (219) in the presence and absence of nucleophiles have been described.123Dienones of this type undergo facile ring-opening to afford a ketene intermediate [e.g. (220) formed from hexamethylcyclohexa2,4-dien-l-one] which is readily trapped by nucleophiles. Thus (221) is formed

from (220) in the presence of meth~1amine.l~~ The transcript of a lecture on this reaction has been pub1i~hed.l~~ Another review has dealt with the low-temperature generation of reactive interrnediates.lz6 Direct irradiation of the enones (222a, b) in furan leads to formation of the strained trans-isomer (223) which can be trapped as the furan adducts (224, 225).12'9 128 The authors lZ7reason that the trans-enone (223a) is the precursor to the two dimers which were previously reported.12@Dimers [(226), (227)] are also formed from enones (222b), and again the intervention of the trans-isomer (222b) is implicated.lZ8 Enones (222c, d) do not undergo dimerization.128 Irradiation of the cyclo-octadienone (228) in ether affords the trans-isomer (229).130 The trans-double bond in this compound is very reactive, and on silica gel chromatography gives the water-addition product (230a). A similar product (230b) is formed when the irradiation of the dienone (228) is carried out in J. M. Campbell, C. Von Sonntag, and D. Schulte-Frohlinde, 2. Naturforsch., 1974,29b, 750. G. Quinkert, B. Bronstert, D. Egert, P. Michaelis, P. Jiirges, G . Prescher, A. Syldark, and H.-H. Perkampus, Chem. Ber., 1976,109, 1332. lZ4 H. Hart, D. A. Dickinson, and W. Y. Li, Tetrahedron Letters, 1975, 2253. lZ6 G. Quinkert, Angew. Chem. Znternat. Edn., 1975, 14, 790. lZ* 0. L. Chapman, Pure Appl. Chem., 1974,40, 511. lZ7 H. Hart and M. Suzuki, Tetrahedron Letters, 1975, 3447. lZ8 H. Hart and M. Suzuki, Tetrahedron Letters, 1975, 3451. 129 H. Hart, T. Miyashi, D. N. Buchanan, and S. Sasson, J. Amer. Chem. SOC.,1974, 96,4857. 1w H. Hart and M. Suzuki, Tetrahedron Letters, 1975, 4327. 113

Enone Cycloadditions and Rearrangements

R4 (222) a; R1 = R2 = R3 = R4 = H b; R2 = R3 = (CH=CH),, R1 = R4 = H C ; R1 = R2 = (CHzCH),, R3 = R4 = H d ; R3 = R4 = (CHZCH)~,R1 = R2 = H

293

R ' (223)

R2

R3

0 0

(227) 23% (226) a; R1 = P-H, R2 = a-H, 35% b; R1 = a-H, R2 = 8-H, 15% C; R1 = R2 = p-H, 7%

(230) a; R = H b;R=Me

methanol, presumably via the trans-compound (229). The formation of enones with trans-double bonds has been suggested for some time, but usually the reactivity of the double bond has been too great to permit is01ation.l~~This is the first report of the isolation of such a species. The pyran (231) is formed upon triplet-sensitized photolysis of the ionone (232).132 The same product (231) is accompanied by the y-ionone (233) when lS2

G . L. Lange and E. Niedert, Canad. J. Chem., 1973, 51,2207,2215. A. van Wageningen, H. Cerfontain, and J. A. J. Geenevasen, J.C.S. Perkin ZI, 1975, 1283.

Photochemistry

294

direct irradiation of (232) is employed. The pyran (211) is also photolabile and yields both ionones (232) and (233). Earlier work 1 3 3 ~134 had indicated that the direct irradiation of the fl-ionone (232) gave the same product. Kurata et ~ 1 1 . l ~ ~ have re-examined the direct irradiation reaction and have found that the use of a Pyrex filter affords a high yield of the pyran (231).

Cross-conjugated Dienones.-The major photo-product from the extended U.V. irradiation of dienone (234) has been identified by spectroscopic techniques as (235), although the exact mode of cyclization is uncertain.13s The product (235)

R1 Bf

R2 i ; ,H

(237) a; R1 = Ph, R2 = H b; R1 = H, K2 = Ph

Ph

(241) 133

lS4 13’

G. Buchi and N. C. Yang, J . Amer. Chem. SOC.,1957,79, 2318. P. de Mayo, J. B. Stothers, and R. W. Yip, Canad. J. Chem., 1961, 39,2135. S. Kurata, T.Kusumi, Y. Inouye, and H. Kakisawa, J.C.S. Perkin I, 1976, 532. C. W. Shoppee and Y. Wang, J.C.S. Perkin I, 1975, 1595.

Eizone Cycloadditions and Rearrangements

295

is itself photolabile and undergoes a series of 1,3-hydrogen migrations to yield (236) amongst other products. trans-trans-2,4-Di bromo- 1,5-diphenylpenta1,4-dien-3-one (237a) affords a low yield of the cis-trans-isomer (237b) when irradiated in toluene at 300 nm.13' This product is accompanied by another monomeric product (238). This compound could in principle arise by a di-nmethane pathway, but no previous examples of a di-n-methane reaction of a dienone have been recorded. Three dimeric products (239), (240), and (241) were identified by spectral analysis. In an earlier study of the cyclohexadienone (242), Schuster et aZ.13*interpreted the results from quenching (cyclohexa-l,3-diene) studies in terms of two excited states; the nr* state was thought to abstract hydrogen from the solvent yielding p-cresal while the nn* state formed the lumiketone. However, the more recent recognition of a free-radical chain process casts considerable doubt on the original Indeed it is likely that the lumiketone product is formed ' 0

Ph. (243) a; R b; R c;

R

Ph = = =

Ph rn-MeOC,H, p-naphthyl

(244)

from the nn* state. Another result which points to that conclusion concerns the thermal decomposition of the dioxetans (243) to 4,4-diphenylcyclohexa-2,5dienone, the rearrangement product (244), and the corresponding aryl ketone (acetophenone, rn-methoxyacetophenone, or /?-acet~naphthone).l~~ The relative efficiency of formation of the rearrangement product can be as high as 17%. The authors reason that dioxetan decomposition must yield the nn* triplet of 4,4-diphenylcyclohexa-2,5-diene,and that this excited state affords the rearranged product (244). It is also evident that there is no equilibration between nn* and nn* excited states in this system. Thus when 8-acetonaphthone (ET = 59 kcal mol-l, 247 kJmol-l, nn* state) is formed from the dioxetan (243c) no excited state acetonaphthone is generated even though there is an energy gap of 9 kcal mo1-1 (37.7 kJ mol-l) between it and the nn* triplet of 4,4-diphenylcyclohexad ienone . The ketene acetal (245a) is the primary photochemical product of the irradiation of 4,4-dimethoxycyclohexa-2,5-dienone(246a) in benzene.141 The acetal (245a) is itself photolabile yielding polymeric material and the esters (247a) and (247b). The author suggests that this compound could be formed from the (unisolated) 6,6-dimethoxybicyclo(3,1,0) hex-3-en-2-0ne.l~~The formation of 13' 138

140

141

C. W. Shoppee and Y. Wang, J.C.S. Perkin I, 1976, 695. D. I. Schuster, G . C. Barile, and K. Liu, J. Amer. Chem. SOC.,1975, 97, 4441. D. 1. Schuster and K. V. Prabhu, J. Amer. Chem. SOC., 1974, 96, 3511. H. E. Zimmermann and G. E. Keck, J. Amer. Chem. Sac., 1975,97, 3527. P. Margaretha, Helu. Chim. Acta, 1976, 59, 661.

296

Photochemistry

OR (246) a; R = Me b; R-R = CH2CHz

(245) a; R = Me b; R-R = CH,CH2

0

(247) a; R1 = C02Me, R2 = H b; R1 = H, R2 = C02Me

such a bicyclic compound might be expected by analogy with the photochemistry of other cross-conjugated cyclohexadienones. A ketal (245b) is also formed when the cyclohexadienone (246b) is irradiated under the same conditions. Prolonged irradiation of this compound (245b) failed to yield isolable Photo-chemical rearrangement of the cyclohexadienone (248) in petroleum ether affords the enedione (249) in 30% yield and the quinone (250, 3%). Hydrogen abstraction by the carbonyl oxygen of the dienone (251) yields the biradical intermediate (252).143 The photochemical transformation of the dienone (253) in acetic acid into the enone (254) has been used as the basis of a route for the synthesis of grayanotoxin II.14* The spirodienone (255) is photochemically labile and when irradiated in ethanol-NaOH isomerizes in low yield to the norboldine (256). A much higher yield is obtained when sodium acetate is used as the base.14s 14a

14a

14’ lui

A. Nishinaga, T. Itahara, and T. Matsuura, Chem. Ber., 1976, 109, 1530. I. V. Khudyakov, I. Ya. Aliev, and V. A. Kuz’min, Zzvest. Akad. Nauk S.S.S.R.,Ser. khim., 1975, 2598 (Chem. A h . , 1976, 84, 58 225). S. Gasa, N. Hamanaka, S. Matsunaga, T. Okuno, N. Takeda, and T. Matsumoto, Terrahedron Letters, 1976, 553. S. M. Kupchan, C.-K. Kim, and K. Miyano, J.C.S. Chem. Comm., 1976,91.

8;-

Enone Cycloadditions and Rearrangements

297

OH

0 (253)

(254)

@$J

Me0 \

U

U

(258)

(257)

0

(260) a; R1 = R2 = H b; R1 = OH, R2 = C0,Me

(259)

(261)

Photochemical cyclization of the ketal (257) in the presence of o-dichlorobenzoic acid gives initially the cis-cyclized product (258). This reaction was part of a total synthesis of the antibiotic bikaverin (259).14* This type of cyclization of ketals in the presence of acid has been reported Another study 14* 14’

D. H. R. Barton, L. Cottier, K. Freund, F. Luini, P. D. Magnus, and I. Salazar, J.C.S. Perkin I, 1976, 499. D. H. R. Barton, D. L. J. Clive, P. D. Magnus, and G. Smith, J. Chern. SOC.(C), 1971,2193.

298 Photochemistry has examined the base-catalysed cyclizations of such Thus cyclization of (260) into (261) has been accomplished by photolysis in the presence of nonnucleophilic bases [KOBd, NaN(SiMe,),, DABCO, and DBN]. The mechanism suggested for this base-catalysed reaction differs from that originally proposed for the acid-catalysed reaction :14' see Scheme 10. Initial photochemical excitation

(261a) Scheme 10

is followed by electron transfer to give a radical cation-radical anion (262): this is transformed into the biradical (263) which subsequently cyclizes to the product (260). A low yield of photokaranjin (264) is obtained on photolysis of karanjin

o w m

o Iw 1 o

0 OMe

0 (264)

OMe

Et (270) a; R = CO,H b;R=H lo*

Et

D . H. R. Barton, J. H. Bateson, S. C . Datta, and P. D. Magnus, J.C.S. Perkin I, 1976, 503.

Enone Cycloadditions arid Rearrangements

299

Irradiation of the coumarin cabreuvin (266) in methanol (265) in afforded the fission product (267) which is presumably formed by solvent trapping of the keto-ketene (268).150 The other fission fragment is acetylene (269) which forms a 1:l photoadduct with the starting material. Decarboxylation to (270b) occurs when the enone (270a) is irradiated in oxygen-free alkaline solution: a diketone (271) is also formed.151 Miscellaneous Dienones.-Irradiation of the Z-isomer (272) results in isomerization to the thermally unstable E-isomer (273). The influence of substituents upon also occurs in the the reaction has been e ~ a 1 u a t e d . l cis-truns-Isomerization ~~ photolysis of 2,6-dichlorocinnamic acid or its esters (274).153 The formation of R4

R4

R3 R2

the cis-cinnamate is followed by attack of the carbonyl group on the benzene ring to yield a ketene intermediate (275), evidence for the existence of which was obtained by low temperature U.V. studies. This ketene intermediate undergoes thermal cyclization to the observed product (276). A study of the photochemical valence-bond isomerization and thermal reversal of many 2,5-disubstituted troponoids has been ~ e p 0 r t e d . lThe ~ ~ bicyclic ketone (277) is formed in 80% yield by irradiation of a-tropolone methyl ether.lS6 2,2,7,7-Tetramethylcyclohepta-3,5-dien-l -one (278) undergoes exclusive formation of the oxa-di-7r-methane product (279) when irradiated under triplet sensiDirect irradiation at 300 nm yields a decarbonylation tization product, (280). There is no doubt that (279) is formed from the triplet m* state whereas the triene (280) is produced from the Sl n7r* state. Irradiation at 254 nm of the dienone (278) is proposed to yield the S 2 m * state. This state undergoes P. Lakshmi, G. Srimannarayana, and N. V. S. Rao, Indian J. Chem., 1975,13, 1094 (Chem, Abs., 1976, 84, 59 260). l60 0. R. Gottlieb and E. G. Magalhaes, Rev. Latinoam Quim., 1975, 6, 206 (Chem. As., 1976, 84, 105338). 161 N. Detzer and B. Huber, Tetrahedron, 1975, 31, 1937. lSa C. T. Pedersen, C. Lohse, N. Lozac’h, and J.-P. Sauve, J.C.S. Perkin I, 1976, 166. 153 R. Arad-Yellin, B. S . Green, and K. A. Muszkat, J.C.S. Chem. Comm., 1976, 14. Is* T. Kobayashi, T. Hirai, J. Tsunetsugu, H. Hayashi, and T. Nozoe, Tetrahedron, 1975, 31,

us

1483.

lSL 166

A. Greene and P. Crabbe, Tetrahedron Letters, 1975, 2215. J. Eriksen, K. Krogh-Jespersen, M. A. Ratner, and D. I. Schuster, J. Amer. Chem. SOC., 1975,97, 5596.

Photochemistry

300

0

internal conversion to produce the S1m* state which yields the triene (280) or product (279) by intersystem crossing. However the formation of (281) and (282) is in competition with these paths. The observed results correlate well with INDO calculations on the excited states of (278). 5 1,2-,1,3-, and 1,4-Diketones A study of the quenching of fluorescence of a-diketones has been p~b1ished.l~' Steric factors have been reported to be important in the photochemical addition of biacetyl to norbornenes (283).168There is a gradation in the ratio of exo- : endooxetans (284) :(285) as the bulk of substituent R increases. Thus when R = H, Me, and But, the ratios (284) : (285) are 24 : 1, 2.6 : 1, and 1 : 30, respectively.

(283) a; R = H b;R=Me

R = But d;R=OH C;

lS7

lS*

(284) a; RL = H, R2 = Me, R3 = COMe (38%) R1 = H, R2 = COMe, R3 = Me (58%) b; R1 = Me, R2 = Me, RJ = COMe (54%) R1 = Me, R2 = COMe, R3 = Me (21%) d; R1 = OH, R2 = Me, R3 = COMe (74%) R1 = OH, R2 = COMe, R3 = Me (15%) obtained as hemi-acetal

B. M. Monroe, C. Lee, and N. J. Turro, Mol. Photochern., 1974,6,271. R. R. Sauers, P. C. Valenti, and E. Tavss, Tetrahedron Letters, 1975, 3129.

301

Enone Cycloadditions and Rearrangements

' CH,R

& o

(286) R

=

&OH

H or Me

$OH

R (287)

R (288)

Ogata and Takagi ls0 recently published results which suggested that a photoenol was involved in the photochemical conversion of diones (286) into hydroxyindanones (287). Hamer I6O has suggested that a triplet biradical(288) is involved and by the use of SO, as a radical trap (SO, is a poor dienophile) has isolated the adducts of this biradical (289) or (290) from the photolysis of the diones in the presence of SO,. Hydrogen-abstraction from 2,3-dimethylbut-2-ene by the nitrile group of photo-excited benzoylnitrile accounts for the products formed.lel A further publication has dealt with the photochemical conversion of the cyclobutenedione (291) into the bis-lactone (292).ls2 The reaction involves the ring-opening of the cyclobutenedione into a bis-ketene, each function of which is intramolecularly trapped by a hydroxyl group. The formation of a ketene (293) OH I

Ph ZC I OH

Y.Ogata and K. Takagi, J.

Org. Chem., 1974, 39, 1385; Bull. Chem. SOC.Japan, 1974, 47 2255. ld0 N. K. Hamer, J.C.S. Chem. Comm., 1975, 557. lol T. S. Cantrell, J.C.S. Chem. Comm., 1975, 637. Ida F. Toda and E. Todo, Bull. Chem. SOC. Japan, 1975,48, 583. l6@

11

go

302

Photochemistry 90

Me\

(296) a; 2 b; 2

= =

0 CH2

(297) a; 2 = 0

b; 2

(295)

(298) a; X = N2,Y = 2 = 0 b ; X = 0 , Y = N2,Z = CH2

(300) a; R‘ b; R‘ C;

R1

= = =

=

CH2

(299)

R2 = H, R3 = R4 = (CH,),

R2 = R3 .= H, R4 = OEt = R? = R4 = Me

H, R2

C02Et

Ar2COCHC0,Et I COAr’

(

(302) Arl = Ar2 = p-XC,H,, thienyl, naphthyl, or fury1

(301)

(305) a; R1 = H,R2 = R3 = Pri b; R1 = Me, R2 = R q = Pri

R1 = H, R2 = Et, R3 = Ph d; R1 = Ph, R2 = R3 = Et e; R’ = R2 = R3 = Ph f; R1 z H, R2 = R? = Et C;

(306) a; SO% b; c; d; e;

80%

80% 5%

44% f; 60%

Enone Cycloadditions and Rearrangements

303

also occurs in the photolysis of 4,6-dimethylbenzofuran-2,3-dione.The ketene in this instance is trapped intermolecularly as the ester (294).163 The alcohol (295) for this trapping is formed by a hydrogen abstraction-radical combination mechanism when the furandione is irradiated in cyclohexene-benzene. A ketene intermediate (296), which is in equilibrium with the open form (297), is formed by the low-temperature irradiation of the diazo-compounds (298).ls4 Continued irradiation of the ketene (296a) results in decarbonylation and the formation of the carbene (299) which affords benzyne and benzocyclopropenone. In competition with decarbonylation and formation of ketene (293), 4,6-dimethylbenzofuran-2,3-dione undergoes addition to olefins to give the oxetans (30O).ls3 The influence of U.V. irradiation upon the keto-enol equilibrium in the /3-keto esters (301) has been studied.166 Courtot et al.ls6have reported their conclusions concerning the enol-enol photoisomerizations of di- and tri-carbonyl compounds (302). The enol of (303) is also photoreactive and is converted into the selenophen (304) by a cyclization-dehydration path.ls7 The examination of mass spectral fragmentation of substituted 2,4-azetidones has uncovered a striking similarity between the electron bombardment reactions and the photochemistry exhibited by the azetidones.ls8 The ring-expansion of the azetidones (305) to the isoxazolidones (306) is a major pathway in each case.lsS Other pathways such as decarbonylation and cycloreversion occasionally compete. Irradiation of a methanol solution of the triafulvene (307) affords the dimer (308) in 70% yield. The structure of the product has been verified by X-ray ana1~sis.l~~ Phthalimide and its derivatives have been popular substrates for photochemical reactions in recent years, including the past year. Roth and Hundeshagen 171have shown that phthalimide undergoes photochemical reduction addition of solvent to one of the carbonyl functions when irradiated in dioxan-acetone. Kanaoka et aZ.172have demonstrated that the N-methyl derivative (309) undergoes photochemical addition of substituted arenes yielding (310), and of amines (PhNMe,

us W. Friedrichsen, Annalen, 1975, 1545.

0 . L. Chapman, C.-C. Chang, J. Kolc, N. R. Rosenquist, and H. Tomioka, J. Amer. Chem. SOC.,1975, 97, 6586. le6 P. Markov and E. Radeva, J. Photochem., 1975, 4, 179 (Chem. Abs., 1975, 83, 130907). P. Courtot, J. Le Saint, and R. Pichon, Bull. SOC.chim. France, 1975, 2538. le7 A. G. Schulz, J. Org. Chem., 1975, 40,3466. F. Compernolle and F. C. De Schryver, J. Amer. Chem. SOC.,1975,95, 3909. J. A. Schutyzer and F. C. De Schryver, Tetrahedron, 1976,32, 251. 170 T. Eicher, R. Graf, and G. Adiwidjaja, Tetrahedron Letters, 1975, 4243. 171 H. J. Roth and G. Hundeshagen, Arch. Pharm., 1976,309,58 (Chem. Abs., 1976,84, 121 590). 17a Y. Kanaoka, K. Sakai, R. Murata, and Y. Hatanaka, Heterocycles, 1975,3,719 (Chem. Abs., 1976, 84, 4770).

304

Photochemistry

0

0

(312) a ; b; c; d;

(311)

R1-R2 = (CH,), R1-R2 = (CH,),O(CH,), R1 = R2 = Me R1 = R2 = Me,CHCH,

and C8HllNMe2)to yield (311). The intramolecular reactions of the phthalimide derivatives (312) have also been studied.17s,174 The intramolecular reactions of the phthalimides (313) have shown that they undergo the hydrogen-abstraction and cyclization reactions [yielding (314)]

eNa+ /

0

(313)

Me

HO

(314)

which have been encountered in the previous The introduction of electron-donating groups into the phthalimide ring inhibited the hydrogenabstraction reaction. Another study has extended the reaction from phthalimides into the imides (315) of succinic and glutaric The reaction encountered is a typical Norrish Type I1 process and involves photochemical excitation of the carbonyl function, abstraction of y-hydrogen on the N-alkyl function and closure of the resultant biradical. The hydroxyazetidine so formed is unstable and ring-opens to afford the product (316). A sample of the reaction type is shown in Scheme 11. cistrans-Isomerization of 1,Zdiacetylethylene is the primary photochemical event when the ethylene is irradiated ( A =- 330nm) in carbon tetrach10ride.l~~ The photostationary state contained 95% of the cis-isomer. In the presence of 2,3-dimethylbut-2-ene, a slow photoaddition reaction took place to give the 175

17*

H. J. Roth and D. Schwarz, Arch. Pharm., 1975, 308, 631 (Chem. Abs., 1976, 84, 30 934). H. J. Roth, D. Schwarz, and G. Hundeshagen, Arch. Pharm., 1976, 309, 48 (Chem. Abs., 1976, 84, 121 589).

17i5

17'

Y. Kanaoka, C. Nagasawa, and H. Nakai, Heterocycles, 1975,3, 553 (Chem. Abs., 1975,83, 178 701). Y. Kanaoka and Y. Hatanaka, J. Org. Chem., 1976,41,400. Z. Yoshida, M. Kimura, and S. Yoneda, Chem. Letters, 1975, 519 (Chem. Abs., 1975, 83, 130 852).

Enone Cycloadditions and Rearrangements

RlR2 (

0

(315) a; b;

ii = II = C ; 11 = d ; 11 =

e;

ii =

1,

305

R1 = R2 = R3 = R4 = 13 R3 = R4 = H, R2 = Me R4 = H, W-R? = (CH,),

I , R1 = 1, R‘ = 2, R1 = 2, R1 =

R2

= R3 = R4 =

H

R4 = H, R2- R3 = (CH,),

(316) a; b; c; d;

45% 56% 42% 37% e ; 28%

Scheme 11

adduct [(317), 50x1 and the open-chain product [(318), lo%]. The photoaddition could be sensitized by ketonic sensitizers implying that a triplet state (ET = 230.5 kJ mol-l, 55 kcal mol-l) was involved. The photosensitized isomerization of the ester (319) has also been reported1’* to give a photostationary state of esters qualitatively in line with the original report of this A full account of the photochemistry of the phenanthrene-dimethyl fumarate system has been published in which convincing evidence is put forward for the

AC

178

17@

L. W. Jelinski and E. F. Kiefer, J. Amer. Chem. SOC.,1976, 98, 282. R. M. Kellogg and W.L. Prins, J. Org. Chem., 1974, 39, 2366.

306

Photochemistry

R2

R3 0 (323) R3 = R4 = H, Me, o r halogen (324) R 1 = R2 = t i or Me X = OorS Y = 0, N H , or NMc

.4--CO,Me

c1

(325)

C0,Me C0,Me

Br

(327)

involvement of both singlet and triplet exciplexes in the formation of prolS1 The triplet exciplex yields the trans- and cis- [2 + 21 addition product of dimethyl fumarate and phenanthrene whereas the singlet exciplex affords the trans-[2 21 adduct and the oxetan (320). The photochemical dimerization of maleic anhydride in the solid phase has been known for some time to produce the trans-dimer. Dimerization in dioxan solution affords only a low yield of the trans-dimer (321) and the bulk of the product is oligomeric.1s2-1s4More recent work has shown that dimerization also occurs in CC14 and that irradiation at 296, 313, and 334 nm is effective.lss The yield is almost quantitative. It is likely that a triplet state is involved in the process, but evidence from fluorescence studies suggests that the excitation is not of pure maleic anhydride but is that of a CClhanhydride complex (as with the photoaddition to benzene). Maleic anhydride and maleimide are extremely popular substrates for photochemical additions. Maleic anhydride has been added to Ag,lO-octalinyielding (322, 24%).lS6 Maleimide and maleic anhydride derivatives (323) have both been added to the furan and thiophen derivatives (324) yielding [2 21 adducts (325), and [2 41 adducts (326).lS7 The benzophenonesensitized addition of dichloromaleic anhydride to trans-l,2-dichloroethylene yielded, after hydrolysis and esterification, the cyclobutane (327). This provides an interesting example of stereospecificity in a triplet cycloaddition process. A cyclobutane (328) was also obtained from the sensitized addition of dibromomaleic anhydride to vinylene carbonate.les

+

+

lE0 181

+

S. Farid, J. C. Doty, and J. L. R. Williams, J.C.S. Chem. Comm., 1972, 711. S. Farid, S. E. Hartman, J. C. Doty, and J. L. R. Williams, J. Amer. Chem. SOC.,1975, 97, 3697.

m4

IS7

G. W. Griffin, J. E. Basinski, and A. F. Vellturo, Tetrahedron Letters, 1960, 13. G . W. Griffin, A. F. Vellturo, and K. Furukawa, J . Amer. Chem. SOC.,1961, 83,2725. 1. Nagahiro, K. Nishihara, and N. Sakota, J. Polymer. Sci., 1974, 12, 785. P. Boule and J. Lemaire, Tetrahedron Letters, 1976, 865. A. Kunai, K. Kimura, and Y . Odaira, BUN. Chem. SOC.Japan, 1975, 48, 1677. C. Rivas, C. Perez, and T. Nakano, Rev. Latinoam Quim., 1975, 6, 166 (Chem. Abs., 1976, 84, 120785).

*E8

G. Berens, F. Kaplan, R. Rimerman, B. W. Roberts, and A. Wissner, J. Amer. Chem. SOC., 1975,97, 7076.

Enone Cycloadditions and Rearrangements

307

Irradiation of the anhydride (329) in an Argon matrix at 7 K affords laBa product which shows the same i.r. absorption frequencies as those reported lD0, lB1 for the cyclobutadiene obtained from the fragmentation of (330), except that the absorption at 1240cm-l is a doublet. The authorslSBsuggest that free cyclobutadiene is not produced in the irradiation of the lactone (330) but forms a complex such as (331): the 653 cm-1 band is thought to be associated with CO, and not with cyclobutadiene. Thus the tendency of cyclobutadiene to form complexes has to be taken into account in the analysis of the i.r. spectra obtained from the low-temperature studies. The vapour-phase photolysis of the anhydride

F F

F F F

(332)

(333)

$ \

Ph

Ph

Ph

loo

lgl

G. Maier, H.-G. Hartan, and T. Sayrac, Angew. Chem. Internut. Edn., 1976, 15, 226. C. Y. Lin and A. Krantz, Chem. Comm., 1972,1111 ;A. Krantz, C. Y. Lin,and M. D. Newton, J. Amer. Chem. SOC.,1973, 95, 2744. 0. L. Chapman, C. L. McIntosh, and J. Pacansky, J. Amer. Chem. Soc., 1973, 95, 614; 0. L. Chapman, D. De La Cruz, R. Roth, and J. Pacansky, J . Amer. Chem. SOC.,1973,95, 2744.

308 Photochemistry (332) results in the formation of the short-lived tetrafluorocyclobutadiene which readily dimerizes to yield (333).IQ2 The phosphorescence emission of o-dibenzoylbenzene suggests that its lowest excited state is an nn* triplet.lQ3 This excited state is proposed as the reactive intermediate in the formation of the adducts (334, two stereoisomers) and (335) from the irradiation of the diketone in the presence of norbornene. The authors IQ3 suggest that the intermediate (336) is common to the formation of the dioxans (334) and the oxetan. Analogous products are formed with cyclo-octene and 2,3dimethylbut-2-ene. The ground-state conformations of the epimeric adducts (337) and (338) exert control on the photochemistry which they Thus (337) follows the hydrogen-abstraction pathway described for other analogous systems IQ6 and yields (339) and (340). The other epimer (338) by-passes H-abstraction and undergoes two modes of [2 + 21 addition affording the oxetan (341) and (342). The

(342)

(341)

(340)

OR

OR

0 Me Me

Me HO Me

Me

(343) a; R = Ac b;R=H c; R = S0,Me d ; R = SO2C;H; e; R = EtCO ina 193 194

196

HO Me

MeMe

(344)

M. J. Gerace, D. M. Lemal, and H. Ertl, J . Amer. Chem. Sac., 1975, 97, 5584. Y. Shigemitsu, S. Yamamoto, T. Miyamoto, and Y. Odaira, Tetrahedron Letters, 1975, 2819. J. R. Scheffer and B. M. Jennings, J.C.S. Chem. Comm., 1975, 609. J. R. Scheffer, K. S. Bhandari, R. E. Gayler, and R. A. Wostradowski, J. Amer. Chem. Soc., 1975,97, 2178.

309 oxetan is photochemically labile and reverts to starting material (338) which cyclizes to the diketone (342). Thus prolonged photolysis of (338) affords a high 21 Cycloaddition affording a cage-structure is also reported yield of (342). [2 for the phenol acetate (343a) which undergoes a slow photochemical conversion (daylight) into the cage compound (344a) formed by the addition of an enedione to an enone The parent phenol (343b) is also photochemically reactive, but the cyclization to the cage compound (344b) is very slow. The influence of substituents on the rate of cyclization was studied and it was found that the methanesulphonate and the toluene-4-sulphonate derivatives (343c, d respectively) were slow to react, but the propionate (343e) reacted so very rapidly that it was difficult to obtain a pure sample of the uncyclized material. A carbonyl ylid (345) is involved in the photochemically induced addition of the quinone epoxide (346a) to norbornene yielding (347).lg7 An analogous product is formed when the ethylenic compound is N-phenylmaleimide. The reaction to produce the ylid seems to be extremely sensitive to the substitution pattern on the epoxide ring since no ylid-related products are isolated when quinone epoxides (346b, c) are employed with norbornene: oxetans (348) are produced instead.lg7 Enone Cycloadditions and Rearrangements

+

eo+ 0

\

R2

\

0

0

(346) a; R1 = R2 = Me

(345)

b; R1 = Me,R2 = H C; R1 = RZ = H

a T 6 0

(347)

0

(348) a ; R

= Me b;R=H

P

h

R2

0

w

R

R1 lB6

lQ7

3

(349) a; R1 = R2 = H,R3 = Me b; R1 = R2 = Ph, R3 = Me c ; R1 = R3 = H, R3 = Ph d ; R3 = Ph, R1 = Me, R2 = H e; R1 = R3 = ph, R2 = H

F. M. Dean, G. H. Mitchell, B. Parvizi, and C. Thebtaranonth, J.C.S. Perkin I, 1976, 595. K. Maruyama, S. Arakawa, and T. Oysuki, Tetrahedron Letters, 1975, 2433.

310

Photochemistry

(350) a; X = C = 0 b; X = CH,

(351) a; R1 = Ph, R2 = H, X = CO b; R1 = Ph, R2 = H, X = CH, C; R1 = H, R2 = Ph, X = CH2

Thioindigo and 6,6'-diethoxythioindigo undergo trans-cis-isomerization via a long-lived transuid triplet intermediate upon direct irradiation.108 The Norrish Type I1 behaviour of a series of aryl-1,5-diketones [e.g. (349)] has been studied.lo9 The diketone (350a) undergoes photochemical decarbonylation to afford the trans-cyclobutene (351a),200but the monoketone (350b) affords both the cis(351c) and the trans-cyclobutenes (351b).201 6 Quinones The photochemistry of quinones has been reviewed.202 The photoreaction of p-benzoquinone and 1,4-naphthoquinone in water has been The former photoreaction has been shown not to involve free radicals and an electrophilic process is An ab initiu study of the ground and excited states of p-benzoquinone has been published.20S Duroquinone (352a) undergoes photochemical conversion to the aldehyde (353) when irradiated under Nz in CH3CN-Hz0.206 The formation of this aldehyde (353) is the major reaction path of the quinone (352b) which is formed in trace amounts in the irradiation of duroquinone. Creed206reasons that the quinonemethide (354), formed by hydrogen abstraction, is an important intermediate in the photochemical reactions of the quinone. The same methide intermediate can be generated by photolysis of the lactone (355) [this yields

M e O 0M e Me

I

I CH,R

0 (352) a; R b; R lQ8 lee

aoo lol lea

aos 204 2os

aos

M e o C H O

=

=

H OH

Me

/Me

nu OH

(353)

M e 0V H 2' Me

/Me OH (354)

A. D. Kirsch and G. M. Wyman, J. Phys. Chem., 1975,79,543. P.-F. Casals, J. Ferard, R. Report, and M. Keravec, Tetrahedron Letters, 1975, 3909. W. Ried and G. Clauss, Annalen, 1975, 964. W. Ried and G. Clauss, Annalen, 1975, 953. M.J. Bruce, Chem. Quinonoid Compounds, Part 1, 465 (Chem. Abs., 1975, 83, 50 620). S. Hashimoto and H. Takashima, Nippon Kagaku Kaishi, 1975, 6, 1019 (Chem. Abs., 1975, 83, 113 339). M. Shirai, T. Awatsuji, and M. Tanaka, Bull. Chem. SOC.Japan, 1975, 48, 1329. M. H. Wood, Theoy. Chim. Acta, 1975, 36, 345. D. Creed, J.C.S. Chem. Comm., 1976, 121.

311

Enone Cycloadditions and Rearrangements

0

Me M

e/ M e8 OH

M Me

(355)

/eM e HMe M/ M e e

OH

OH (3 54)

(352b) and the dimer (356)l. However, although the methide route can rationalize the formation of the products it is not the main path for the dissipation of triplet energy in the quinone. The author 206 suggests that electron-transfer quenching is important in this instance. A study of the mechanism of the photochemical oxetan formation from olefins and p-benzoquinone has substantiated 207 the earlier claim that a triplet nn* state was The results were obtained from both sensitization and quenching studies, and there is evidence that a singlet state might also be involved in the process. The involvement of biradical intermediates was also shown by the use of 2-methylbut-l-ene as the olefin when two oxetans (357a) and (357b) were produced. The observation of a CIDNP signal could also be taken as evidence for the presence of biradicals, although these were not identified. The mechanism is further complicated by the involvement of an exciplex. Oxetans (358) are also formed by the photoaddition of cycloheptatriene to p-benzoquinone and naphthaquinone.20g Two other 1:l adducts (359; 13%) and (360; 10%) are also obtained from the naphthoquinone reaction. The products obtained in this study are different from those reported earlier.21o

0 (357) a; R1 = R2 = H,R3 = Me,Ra = Et b; R1 = Me, R3 = Et, R3 = R4 = H

(358) a; R1 = R3 = H b; R2-R2 = (CH=CH),, R' C; R'-R1 = (CH=CH)%,R2

(359) *07 208

aoo alo

N. J. Bunce and M. Hadley, Canad. J . Chem., 1975, 53, 3240. D. Bryce-Smith, A. Gilbert, and M. G. Johnson, J. Chem. SOC.(C), 1967, 383. A. Mori and H. Takeshita, Chem. Letters, 1975, 599 (Chem. Abs., 1975, 83, 177 799). R. P. Ghandi, S. N. Dhawan, and S. M. Mukhergi, ZndiunJ. Chem., 1971,9, 283,

= =

H H

312

Photochemistry

(361)

(362)

A patent covering the synthesis of isochromans (361) from the photochemical reaction of alkoxyquinones (362) with olefins has been published.211 The photoaddition of 1,l-diphenylethylene to 2-alkoxy-3-bromo-l,4-naphthoquinone affords the benzanthraquinone (363; 60%).212 Low yields of addition products have been reported following the photolysis of naphtho-l,4-quinone in the presence of bicyclo[4,2,0]oct-7-ene, norbornene, and bicyclo(2,2,2)oct2-ene.213 Three types of addition product were encountered norbornene and bicyclo(2,2,2)oct-2-ene gave oxetans by addition of the olefin to one of the car21 Addition to the double bond of the bony1 groups of the quinone. [2 quinone took place with the same two olefins. The most interesting products were the 2:l adducts [(364), (365)].213

+

(344) 1%

(365) a; n = 1; 10% b; n = 2 ; 4%

The photocycloaddition of 9,lO-phenanthraquinone to several cyclic olefins [cyclohexene, cycloheptene, cis-cyclo-octene, and bicyclo(2,2,l)hept-2-ene] has been reported.214Addition of the same quinone to phenylallene has been studied in detail.21s Rate constants for the addition were measured at various temperatures. The [4 + 21 addition products (366) were obtained together with the oxetan (367; 2%) and the hydrogen-abstraction product (368; 40%). [2 + 21 Cycloaddition of alkyl thioacetylenes to one of the carbonyl groups of phenanthraquinones affords unstable oxetes which ring-open to afford (369).216 The irradiation of anthraquinone in ButOH-C6H6 in the presence of ammonia results in the formation of the adduct (370).217The reaction arises from the 211

21r 216

21e 217

K. Maruyama, T. Otsuki, M. Wakabayashi, and H. Hayashi, Japan Kokai, 75, 53371 C 0 7 D (Chem. Abs., 1976, 84,43 841). K. Maruyama and T. Otsuki, Chem. Letters, 1975, 87 (Chem. A h . , 1976, 84, 17 013). K. Maruyama, Y. Naruta, and T. Otsuki, Bull. Chem. SOC. Japan, 1975, 48, 1553. K. Maruyama, T. Iwai, and Y. Naruta, Chem. Letters, 1975, 1219 (Chem. Abs., 1976, 84, 58 236). R. J. C. Koster and H. J. T. Bos, Rec. trav. chim., 1975, 94, 79. A. Mosterd, R. E. L. J. Lecluijze, and H. J. T. Bos, Rev. trau. chim., 1975, 94, 72. G. G. Wubbels, W. J. Monaco, D. E. Johnson, and R. S. Meredith, J. Amer. Chem. SOC., 1976,98, 1036.

Enone Cycloadditions and Rearrangements

313

(367)

CH2C(OH)Me,

(371) (369) a ; R* = But, R2 = Me b; R1 = R2 = Me c ; R1 = Ph, R3 = Me d ; R1 = Me, R2 = Et

triplet state and the authors217suggest that an exciplex of ammonia with the quinone is the principal intermediate in formation of the product (370). The exciplex is believed to react with ButOH to afford the anthraquinone radical anion, ammonium ion, and the free radical (371).

?

J

Photochemistry of Olefins, Acetylenes and Related Compounds BY W. M. HORSPOOL

1 Reactions of Alkenes Addition Reactions.-Kropp and his co-workers l, have extended the scope of their study of the photoaddition of methanol to tetra-alkyl-substituted olefins (1). The products obtained from these reactions (e.g. Scheme 1) are rationalized in

R

X’

R

(1) a; R = Et b; R = (CH,), C;

(la)----+

R

=

(CH,),

-y-( + -y++ OMe

OMe

Me0

0.8

1 .o

0.7

+

2 0.3

Scheme 1 terms of a Rydberg transition state. The use of unsymmetrically substituted olefins showed that there was little specificity in nucleophilic attack by methanol on the excited-state olefin. A full account of the photoaddition of acetic acid to homoadamant-4-ene (xylene-sensitized) has been r e p ~ r t e d . ~ , The radicalcations of l-phenylcyclopentene, 1-phenylcyclohexene, and 2-phenylnorbornene are produced by the photolysis of the olefins in the presence of l-cyanonaphthalene in aqueous acet~nitrile.~ The radical-cations [e.g. (2) from 1-phenylcyclopentene] react with water (H,O-MeCN) to give cis and trans-2-hydroxy-1-phenylcyclopentane (the anti- Markovnikoff products) in a total yield of 42% (ratio 5 : 9).

a

H. G. Fravel, jun. and P. J. Kropp, J. Org. Chem., 1975, 40,2434. P. J. Kropp, U.S.NTIS, Ad-A. Report, 1975, AD-A012955 (Chem. Abs., 1976,84,30 094). R. Yamaguchi, S. Arimatsu, and M. Kawanisi, Chem. Letters, 1973, 121. R. Yamaguchi and M. Kawanisi, Bull. Chem. SOC.Japan, 1975, 48, 1296. Y. Shigemitsu and D. R. Arnold, J.C.S. Chem. Comm., 1975,407.

314

Photochemistry of Olefins, Acetylenes and Related Compounds

315

Analogous products were obtained from the other two olefins in H,O-MeCN or MeOH-MeCN. The triplet-sensitized reactions of the homoallylic alcohols (3) have been studied.6 Excitation affords a triplet state which can readily be protonated by phenol to give carbonium ion intermediates; these undergo intramolecular addition to afford an oxetan (4), fragmentation to yield an enal which photochemically affords another oxetan (6), or deprotonation to afford an allylic alcohol ( 5 ) . Examples of these reactions are shown in Scheme 2. The allylic

Scheme 2

alcohols ( 5 ) are themselves photolabile and undergo protonation and fragmentation upon excitation in the presence of phenol. The change in reactivity encountered in the allylic and homoallylic series is due to conformational differences. The enamide (7) undergoes ionic addition reactions of water and free-radical processes to benzene when irradiated in benzene (moist) ~ o l u t i o n . ~ ~ Hydrogen Abstraction Reactions.-The intramolecular hydrogen abstraction reactions encountered in the N-(diarylmethy1ene)acetamide (8a) are reminiscent of Norrish Type I1 hydrogen abstraction in o-methylbenzophenones or in the reactions of l-o-tolyl-l-phenylethylene.loHydrogen abstraction by the imine D. Guenard and R. Beugelmans, Tetrahedron, 1976,32,781. J. Boix, J. Gomez, and J.-J. Bonet, Helo. Chim. Acta, 1975, 58, 2545. F. Abello, J. Boix, J. Gomez, J. Morell, and J.-J. Bonet, Helo. Chim. Acta, 1975, 58, 2549. M. Saeki, N. Toshima, and H. Hirai, Bull. Chem. SOC.Japan, 1975, 48,476. lo F. Scully and H. Morrison, J.C.S. Chem. Comm., 1973, 529. a

316

Photochemistry

RPhdNH

0&OAC H

RH,C NAc (8) a ; R = H b;R=D

(7)

+ c

(9)

cl&i3 c1

R3

c1 R'

c1

CI

R?

R 2

R'

group in (8a) leads to an o-quinomethide (9) which is deuteriated in MeOD by Subsequent tautomerization affords the deuterium exchange of the N-H. incorporation product (8b). Hex-l-ene also undergoes 1,5-hydrogen transfer (again reminiscent of a Norrish Type I1 reaction) to yield a 1,4-biradical when it is irradiated (mercury-sensitized) in the gas phase. The resulting 1,4-biradical undergoes fission to propene or recombination to a cyclobutane.ll Various

H & H

4

c1

C0,Me

C1

4

C0,Me /

c1

C0,Me

H

(14)

cQ €Ic1

C0,Me

H C0,Me (15) l1

C0,Me C0,Me

CI

C0,Me (16)

Y. Inoue, S. Takamuku, and H. Sakurai, J.C.S. Chem. Comm., 1975,896.

317

Photochemistry of Olefins, Acetylenes and Related Compounds

hydrogen-transfer processes have been recognized in the gas-phase mercurysensitized irradiation of cis-cyclo-octene.12 Intramolecular hydrogen abstraction reactions are also encountered in the polychloro-hydrocarbons (10) which undergo hydrogen abstraction reactions from the triplet state (acetone sensitization) to afford the cage compounds (1l).l3 A detailed study of the acetonesensitized photochemistry of (10f) has been made.14 At - 30 "C and below the only product is the isomer (12) which arises by two reversible hydrogen transfers. Above - 30 "C, however, the hydrogen-transfer process, which affords biradicals of the type (13) for example, yields three products (14)-(16). Cristol et alls have reported that triplet sensitization (acetone, rn-xylene, or p-methoxyacetophenone)

(17) a; b; C;

R1 = C1, R2 = H R1 = R2 = C1 R1 = H, R2 = C1

of the norbornene derivatives (17) in pentane leads exclusively to (18) by reduction of the double bond. In no case is the halogeno-group lost. Halogeno-01efins.-The photochemistry of the p-iodoacrylamide (19) is dominated have described the photoreactions by C-I fission (Scheme 3).la Two reports of halogenoethylenes. Sket et aZ.17have reported that 1,l-diphenyl-2-chloro(bromo 179

0

Scheme 3 la l3 l4 l8

Y. Inoue, K. Moritsugu, S. Takamuku, and H. Sakurai, J.C.S. Perkin 11, 1976, 569. E. S. Lahaniatis, H. Parlar, S. Gab, and F. Korte, Synthesis, 1976,47. H. Parlar, S. Gab, E. S. Lahaniatis, and F. Korte, Chem. Ber., 1975,108,3632. S. J. Cristol, R. P. Micheli, G. A. Lee, and J. E. Rodgers, J. Org. Chem., 1975, 40, 2179. R. M. Wilson and T. J. Commons, J. Org. Chem., 1975,40,2891. B. Sket, M. Zupan, and A. Pollak, Tetrahedron Letters, 1976, 783. T. Suzuki, T. Sonoda, S. Kobayashi, and H. Taniguchi, J.C.S. Chem. Comm., 1976, 180.

318

Photochemistry or iodo)ethylene undergoes conversion, when irradiated in ether, into diphenylacetylene, 1,l-diphenylethylene, and 1,1,4,4-tetraphenylbutadiene. Diphenylacetylene was also formed from a-chlorostilbene, together with phenanthrene and cis- and trans-stilbene. The authors l7 suggest (without concrete evidence) that the 1,l-diphenylethylenes rearrange to the halogeno-stilbenes by phenyl migration prior to formation of the acetylene. In contrast, Suzuki et aL1* report that 1,l -diphenyl-2-bromoethylene does not afford diphenylacetylene but instead gives the cyclization product 9-phenylphenanthrene by incorporation of solvent benzene, together with 1,l-diphenylethylene. With the anisyl derivatives (20), 1,Zanisyl group migration occurs to give mainly the acetylenes (21), but a competing reaction takes place in the case of (20c) whereby the o-methoxy function

(20) a; R = p-Me0 b; R = rn-Me0 c; R = o-Me0 d; R = o-SMe

(21) a ; 53% b; 86% c; 50%

(22) a; X = 0 b;X=S

undergoes cyclization to afford the furans (22a) and (23). The same type of reaction was found with the thio-derivative (20d) which gave the thiophen (22b). The authors18 suggest that irradiation affords a radical by the fission of the Br-C bond, which in the unsubstituted case reacts with solvent. However, in the substituted examples, electron transfer competes with the reaction with solvent to yield a vinyl cation (24), and it is this intermediate which is considered responsible for group migration and cyclization. Group Migration Reactions.-The gas-phase direct photolysis of cycloheptene has been shown to yield hepta-l,6-diene and vinylcyclopentane at low pressures, but two new products become important as the pressure is increased. These have been identified as methylenecyclohexane and bicyclo[4,1,O]heptane and are proposed to arise via the carbene intermediate (25a) formed by a 1,2-migration (ring-contraction) in the excited state.lB A carbene (25b) is also proposed as the intermediate in the formation of the products (26) and (27) from irradiation of 3-phenylcycloheptene in benzene. The possible involvement of a triplet state was considered since irradiation in heptane or acetonitrile failed to yield products.20 Another example of a photochemically-induced carbene rearrangement in an l9

ao

Y. Inoue, S. Takamuku, and H. Sakurai, J.C.S. Chem. Comm., 1975,577. S . J. Cristol and C. S. Ilenda, J . Amer. Chem. SOC.,1975,97, 5862.

Photochemistry of Olefins, Acetylenes and Related Compounds

319

(27) a ; R1 = Ph, R2 = H b; R1 = H, R2 = Ph

(25) a; R = H b; R = Ph

R1

I

PhzC=CCR2,CR3,R4 (28) a ; R1 = R2 = H, R3 = Me, R4 = Ph b; R1 = D, R2 = H, R3 = Me, R4 Ph c; R1 = R3 = H, R2 = Me, R4 = CHzOH L-

Ph2CR1(=.CR2,CR3,R4 (30)

Ph,CRCH =CHCMe,Ph (29) a ; R = H b;R=D

0

(31) CHPh2

aryl alkene has been provided by the rearrangement of (28a) into tran~-(29a).~l The involvement of carbene (30) in the singlet excited state-induced reaction was demonstrated using the conversion of deuterium-labelled compound (28b) into (29b) and by the formation of the tetrahydrofuran (31) from irradiation of olefin (28c). Isomerization of the 1 , l -diarylpropenes (32a-e) into the cyclopropanes (33) arises from the singlet excited state.22 The ease of formation of the cyclopropanes p-XC H ~cHcHR, P-XGH4

(32) a; R = Me, X = H b; R = H, X = CF, C; R = H , X = CN d;R=X=H e ; R = H, X = Me f ; R = H, X = OMe

falls off from (32a) to (32e), and (32f) is unreactive. Hixson 22 states that electronwithdrawing substituents on the aromatic ring greatly enhance the rate of 1,Zhydrogen migration, whereas electron-donating groups retard the reaction. The reaction is also enhanced by geminal dimethyl substitution at the migration origin. The migratory aptitudes of methyl, phenyl, and hydrogen in the photochemical conversion of the arylbutenes (34) and (35) into the corresponding cyclopropanes (36) and (37) via a singlet state have been The rates of the reactions determined from the kinetic studies are shown in Table 1. It can be seen that the migratory aptitudes are in the order Me c H 4 Ph, which is in line with other experimental evidence. A comparison between the reactivity of 21 22

23

S. S . Hixson, J. C . Tausta, and J. Borovsky, J. Amer. Chem. SOC.,1975,97, 3230. S. S . Hixson, J.C.S. Chem. Comm., 1975, 515. S. S. Hixson, J. Amer. Chem. SOC.,1976, 98, 1271.

320

Photochemistry

J F e

PI1

(34) a ; R = Me b;R=H c ; R = Ph

Me

Me

Ph-Ph

eph

Ph

(35 )

(37)

(36)

Table 1 Rate data for the rearrangements of (34) and 1,3-diphenyIpropene Compound

Product

(344 (34b) (34c) (35)

(36a) (36b) (36c) (3 7)

@f

0.001 0.42 0.014 0.005

(relative)

T,/ns

0.95 0.086 1 .o 0.83

9.7 0.65 9.6 6.3

23

k,/s-l 1.0 x 106 6.5 x lo* 1.4 x los 7.9 x lo6

the alkene (34c) and 1,3-diphenylprop-l-ene (35) has again shown that the influence of the geminal methyl substitution is large. A previous report 24 on direct irradiation of the olefin (38a) had shown that carbene-derived products are formed via a 1,2-rnethylmigration. The present R2

H

R

(38) a ; R1 = R2 = Me b; R1 = R2 = Et C; R'-R2 = (CH,), d ; R1-R2 = (CH,),

(39) a ; R1 = R3 = R4 = Me, R2 = H (18%) b; R3 = R4 = R1 = Et, R2 = Me (7%); R3 = R4 = Et, R1 = Me, R2 = H(19%) c; R1-R2 = (CH,),, R3 = R4 = Me (26%); R3-R4 = (CH,),, R1 = Me,R2 = H (22%) d ; R'-R2 = (CH,),, R3 = R4 = Me (83%); R3-R4 = (CH,),, R1 = Me, R2 = H (10%)

report details a further mode of reaction, viz. 1,3-hydrogen migration to give (39a).25 Other olefins (38b-d) also undergo this positional migration of the double bond in varying yields. The 1,3-migration process has been shown to be intramolecular (Scheme 4). The nature of the excited state involved in the

PD% D

D

83% D4

+

D

78% D4 Scheme 4

85% D,

process is unsure, but the authors 25 suggest that a singlet state (Rydberg) is involved. Unlike the other Rydberg processes which Kropp and his co-workers have reported,2s the 1,3-hydrogen migration takes place even with di- and trisubstituted olefins. Other 1,3-migration reactions have been reported in the 2a 26

2a

P. J. Kropp and T. R. Fields, J. Amer. Chem. SOC.,1974,96,7559. P. J. Kropp, H. G. Fravel, jun., and T. R. Fields, J. Amer. Chem. SOC.,1976,98, 840. See refs. 1 and 2.

Photochemistry of Olefins, Acetylenes and Related Compounds F3C

CF3

X F3C CF3

32 1

F&FF2CF3

F3C

(40)

(44) tz

=

I , 2, 3, 4, or 5

conversion of (40) into (41) by a fluorine migrationz7 and for the formation of (42), as a mixture of Z- and E-isomers, from (43).z8 cis-trans-1somerization.-A full account of the work dealing with excited-state 30 Crosby and deactivation of the phenylcycloalkenes (44) has been published.z*~ Salisbury3' have shown that rotation about the double bond in excited-state styrenes is an important non-radiative deactivation process. However, other structure-dependent processes are also important. A study of the acetophenone-sensitized transcis-isomerization of three sets of olefinic pairs, 2,2-dimethylhex-3-ene, 4,4-dimethylpent-2-ene, and 3,4-dimethylpent-2-ene, has been reported.32 The results obtained have shown that steric hindrance can divert olefin-acetophenone encounters from energy transfer into energy wasting. The authors 32 suggest that triplet exciplexes are important in these quenching processes. The cis-trans-isomerization of 1-phenylbut-2-ene was earlier postulated to involve a triplet state, but without definite proof.33 Morrison et aLa4have now used xenon in solution to enhance intersystem crossing rates, and have concluded that the isomerization does indeed arise from the triplet state. A similar effect of xenon is seen in the photoisomerization of 2-methylenebenzonorbornene, and again a triplet state is implicated even though that molecule has a free Application of the xenon technique is particularly useful when the triplet state which might be involved cannot be quenched by conventional means, and can be diagnostic of a singlet mechanism when it leads to decreased quantum efficiency, as in the intramolecular cyclization of 6-phenylhex-2-yne. Ferrocene has been used as a triplet sensitizer for the photochemical cis-trans-isomerization of 1,Zdichloroethylenes and b ~ t - 2 - e n e s . ~ ~ A theoretical study of the cis-trans-isomerization of stilbene has suggested ~ ~ question of that a singlet excited state is operative for direct i r r a d i a t i ~ n . The 28 29

so 31

sa s3

s4

se s7

A. N. Bell, R. Fields, R. N. Haszeldine, and I. Kumadaki, J.C.S. Chem. Comm., 1975, 866. P. Brownbridge and S. Warren, J.C.S. Chem. Comm., 1975, 820. H. E. Zimmerman, K. S. Kamm, and D. P. Werthemann, J. Amer. Chem. SOC.,1975,97,3718. H. E. Zimmerman, K. S. Kamm, and D. P. Werthemann, J. Amer. Chem. SOC.,1974, 96, 7821. P. M. Crosby and K. Salisbury, J.C.S. Chem. Comm., 1975, 477. A. Gupta and G. S. Hammond, J. Amer. Chem. SOC.,1976,98, 1218. H. Morrison and R. Peiffer, J. Amer. Chem. SOC.,1968,90,3428; H. Morrison, J. Pajak, and R. Peiffer, Ibid., 1971, 93, 3978. H. Morrison, T. Nylund, and F. Palensky, J.C.S. Chem. Comm., 1976, 4. S . S. Hixson, P. S. Mariano, and H. E. Zimmerman, Chem. Rev., 1973,73, 531. J. Wojtczak and A. Jaworska-Augustyniak, Chem. Stosow., 1975, 19, 359 (Chem. Abs., 1976, 84, 73 396). M. C. Bruni, F. Momicchioli, and I. Baraldi, Chem. Phys. Letters, 1975, 36,484.

322

Photochemistry

the involvement of thermal activation has also been Another interpretation of the cis-trans-isomerization has suggested the existence of a low-lying second excited singlet state whose energy is 5000 cm-1 lower than that of the S1 A review lecture had dealt with the involvement of the triplet state in stilbene isomeri~ation.~~ Quantum yields for cis-trans-isomerization of a series of diarylethylenes have been rnea~ured.~,A phenyl vinyl ketone-2-vinylnaphthalene copolymer has been used for the sensitized isomerization of s t i l b e n e ~ .The ~~ synthesis and photochemistry of stilbenes has been described as a useful undergraduate laboratory exercise.43 Irradiation of 4-nitro-4’-methoxystilbene in petroleum ether leads to a photostationary state where 92% of the cis-form is In the more polar solvents methanol and DMF, the proportions of the cis-isomer are 28 and 12% respectively. This solvent dependence is not exhibited by 4-cyano-4’-methoxystilbene. The authors 44 interpret the behaviour to a difference in excited state in the two cases. Thus the cyanomethoxystilbene isomerizes from the singlet state and the nitromethoxystilbene isomerizes from the triplet state. The enhancement by molecular oxygen of trans +- cis-isomerization in five dinaphthylethylenes and some stilbenes has been examined.45 The influence of temperature upon the reaction was also assessed (see also ref. 38). The authors 45 suggest that the extent of the enhancement depends on a number of factors including the absolute value of cf> in the absence of oxygen, the lifetime of the excited state, and the structure of the molecule. The results are in accord with a mechanism whereby oxygen enhances S, + intersystem crossing. A triplet s) is also involved in the trans-cis-isomerization (Otis = state (lifetime 2 x 0.16) of trans-l-phenyl-2-(2-naphthyl)ethylene.46Energy transfer from biacetyl to styrylpyridines has been The synthesis and photochemistry of pyridylnaphthylethylenes have been 2 Reactions involving Cyclopropane Rings Methyl chrysanthemate (45) has been synthesized by irradiation of the diene (46a), which undergoes a di-r-methane rearrangement .49 The chrysanthemate is obtained as a mixture of cis- and trans-isomers in a ratio of 1 : 2 (it is not known at what stage this isomerization occurs). The principal product of the irradiation of (46a) is the diene (46b) formed by trans-cis-isomerization via a triplet state a8

F. Momicchioli, G. R. Corradini, M. C. Bruni, and I. Baraldi, J.C.S. Furuduy IZ, 1975, 71, 215.

8B 40 41

4a

Is 44 45 40 47

48

4#

G. Orlandi and W. Siebrand, Chem. Phys. Letters, 1975, 30, 352. J. Saltiel, D. W. L. Chang, E. D. Megarity, A. D. Rousseau, P. T. Shannon, B. Thomas, and A. K. Uriarte, Pure Appl. Chem., 1975, 41, 559. N. P. Kovalenko, B. Yu. Shekk, L. Ya. Malkes, and M. V. Alfimov, Izvest. Akud. Nauk, S.S.S.R., 1975, 298 (Chem. Abs., 1975, 83, 18 896). S. Irie, M. Irie, Y. Yamamoto, and K. Hayashi, Macromolecules, 1975, 8,424. J. R. Davy, P. J. Jessup, and J. A. Reiss, J. Educ. Chem., 1975, 52, 747. D. Schulte-Frohlinde and D. V. Bent, Mol. Photochem., 1974, 6, 315. G. Fischer and E. Fischer, Mol. Photochem., 1974, 6, 463. M. Sunitani, S. Nagakura, and K. Yoshihara, Chem. Phys. Letters, 1974,29,410. G. Favaro, G. Bartocci, and P. Bortolus, Z.phys. Chem., 1975,96, 161 (Chem. Abs., 1975,83, 113 344). G. Galiazzo, P. Bortolus, and F. Masetti, J.C.S. Perkin ZI, 1975, 1712. M. J. Bullivant and G. Pattenden, J.C.S. Perkin I, 1976, 256.

Photochemistry of Olefins, Acetylenes and Related Compounds

323

R'

\/

C0,Me (45)

'0 (47)

W

R

2

(46) a; R1 = H, R2 = C02Me b; R' = CO,Me, R2 = H

(48) a ; R1 = R2 = H, R3 = Me b; R1 = OMe, R2 = Me, R3 = H c; R1 = OAc, R2 = Me, R3 = H d ; R1 = R3 = H, R2 = Me e; R1 = R2 = R3 = H

(free-rotor effect?3s)and it is therefore concluded that a singlet state is involved in the di-r-methane reaction. The cyclopentenone (47a) also undergoes efficient di-r-methane rearrangement to afford a single product (48a) (60%).50ss1 The chemical efficiency of this reaction is to be contrasted with the inefficient rearrangement of the enones (47b-e) which also undergo the di-7r-methane reaction to yield (48b-e). This conversion is thought to arise from the triplet state (direct and sensitized irradiation was effective; quenching by 2,Sdimethylhexa-2,4-diene stopped the reaction). Earlier work by Zimmerman et a1.35had suggested that compounds having a free rotor would undergo reaction from the singlet manifold. However, in this instance the triplet state is reactive. The introduction of substituents into the side-chain as in (49) does affect the reaction and

R1wR 0

R'

R2

=

= R2 =

R3

OAc or Me, R2 = Me, R5 = Me or Et, R3 = R4 = H

R5 = Me, R1 = R3 = R4 = H Me, R3-R4 = (CH,),, R1 = R5

=

H

(49)

only E-2-isomerization takes place with no evidence for cyclization. The inefficienciesencountered in the rearrangement of the cyclopentenonesare thought to be due to a competing 1,2-hydrogen transfer, but this of course is impossible in the case of (47a) where the isomerization to (48a) is efficient. Such disubstitution has always been thought to be practically essential for the success of the di-n-methane process.35 The triplet state of (50), populated by sensitization (ET> 57 kcal mol-l, 239 kJ mol-l), only undergoes cis-trans-isomerization, but direct irradiation, via the singlet state, yields the vinylcyclopropane (51a) from the 6o b1

M. J. Bullivant and G. Pattenden, J.C.S. Perkin I, 1976, 249. M. J. Bullivant and G . Pattenden, J.C.S. Chern. Cornrn., 1972, 864.

324 Photochemistry cis-isomer (50a) and (51b) from the t r a ~ ( 5 0 b ) A . ~detailed ~ account of the photochemical reactivity of the allene-olefin system (52), originally published in note form,53 has appeared.54 The 2-acetonaphthone- or m-methoxyacetophenonesensitized irradiation of the vinyl diene (53) affords a single photoproduct (54a)

R1 = Ph, R2 = H b; R1 = H, R2 = Ph

(50) a ;

fi

GR1

Q

PI1

-.R2 H

K'

R1 = H, R2 = Ph R1 = Ph, R2 = H

I1

R1 = CH=CH2, R2 = Ph b; R1 = Ph, R2 = CH=CH,

(54) a;

(53)

(52)

(83%, 0 = 0.006) as a result of vinyl migration. The authorsss reason that in the triplet excited state the excitation energy is localized in the diene portion (ET ca. 251 kJ mol-l, 60 kcal mol-l) of the molecule (no free-rotor effect). Direct = 0.08) through phenyl irradiation, however, gives mainly (54b) (39%, migration, together with two minor products. In this instance the energy is thought to be localized in the phenyl group (Es= 452.5 kJ mol-l, 108 kcal mol-l). The dienes ( 5 5 ) and (56) both undergo the di-7-methane reaction to afford the photoproducts (57) and (58) respectively.66 This result clearly shows that there is no demand on the reaction for either a cis (59) or a trans (60) arrangement in the H

sa 63

66

P. S. Mariano and D. G. Watson, Tetrahedron Letters, 1975, 3439. D. C. Lankin, D. M. Chihal, N. S. Bhacca, and G. W. Griffin, Tetrahedron Letters, 1973,4009, D. C. Lankin, D. M. Chihal, N. S. Bhacca, and G. W. Griffin, J . Amer. Chem. SOC.,1975, 97. 7133. J. S. Swenton, R. M. Blankenship, and R. Sanitra, J . Amer. Chem. SOC.,1975,97,4941. H. E. Zimmerman and L. M. Tolbert, J. Amer. Chem. Sac., 1975,97,5497.

325

Photochemistry of Olefins, Acetylenes and Related Compounds

biradical leading to product. A review of Zimmerman's own work in the area of organic photochemistry has been published.67 The problem of the free-rotor effect in photochemical reactions (alluded to in refs. 49-52 and 55) has been further examined in the photochemical phenyl migration of the bicyclic dienes (61).58 In both cases the triplet and the singlet excited states are reactive. However, the efficiency of product formation from the triplet state depends on the flexibility (free-rotor deactivation) of the diene moiety (see also refs. 29 and 30). Thus the larger ring system (61a) rearranges to products (62a and b) and (63a and b) from the triplet state with an efficiency of

(61) a; n = 2 (62) a; n b; n b;n=l

2, R1 = 2,R1 = C; n = 1, R1 = d; n = 1, R1 = =

=

Ph, R2 = H (63) a; iz = b; IZ = H , R 2 = Ph C; n = Ph, R2 = H d ; KZ = H, R2 = Ph

2, R1 = 2, R1 = 1, R' = 1, R1 =

Ph, R2 = H H, R2 = Ph Ph, R2 = H H, R2 = Ph

0 = 0.003 whereas (61b) rearranges to (62c and d) and (63c and d) with an efficiency of 0 = 0.36. These results are to be compared with the non-reactivity of the triplet state of (64). Surprisingly the triplet state of (53) is reactive.s5 The singlet-state reactivities of the dienes (61a and b) and (64) were measured as

R2&COzMe

C0,Me 0

(67) a; b; c; d; e; f; g;

R1 = Me, R2 = R3 = H, R4-R5 = OCH, R1 = R2 = Me, R3 = H, R4-R5 = OCH, R1 = R2 = H, R3 = Me, R4 = OH, R5 = Me R1 = R2 = H, R3 = Me, R4-R5 = OCH, R1 = R3 = Me, R2 = H, R4-R5 = OCH, R1 = R3 = Me, R2 = H, R4 = OH, R5 = Me R1 = But, R2 = H, R3 = Me, R4-R5 = OCH,, n

R A R 3

Me

O7

H. E. Zimmerman, Science, 1976,191, 523. H. E. Zimmerman, F. X. Albrecht, and M. J. Haire, J. Amer. Chem. Soc., 1975, 97, 3726.

326

Photochemistry

@ = 0.11, 0.12, and 0.11 respectively. In both the singlet- and the triplet-state reactions of dienes (61a and b) the main product was the trans-endo-isomer [(63a) from (61a); O8 = 0.09, @b = 0.002; (63c) from (61b); @g = 0.1, at, =0.23. The dihydroindene (65) also undergoes phenyl migration in the excited state to afford the di-.rr-methaneproduct (66).5n The bicyclo-octadienones (67) undergo regiospecific di-.rr-methanereactions to yield (68) when subjected to acetophenone-sensitized irradiation.g0 The products (68a and b) were identified by comparison with that obtained from the sensitized conversion of (69) into (70). [The direct irradiation (singlet state) of (67a-c, e, g) gave the corresponding phthalate and the novel keten (71).] An earlier

report 61 of the acetone-sensitized photochemistry of homobarrelene (72a) stated that the principal product was the barbaralene (73a). However, a reinvestigation of the reaction using benzene-acetone as solvent and sensitizer resulted in the formation of a mixture of three products of which barbaralane (73a) (0.9%) was a minor component.62 The main product was the homosemibullvalene (74a) (30%), and a minor product (2.8%) remains unidentified. The cyano-derivative (72b) also yields a homosemibullvalene (74b) (26.8%) as the sole product of the

(72) a; R = H b;R=CN

(75)

(73) a; R b; R

= =

H CN

(74) a; R = H b; R = CN

(76)

irradiation. No evidence for the formation of (73b), which was synthesized independently by irradiation of (79, was found. The authors 62 reason that the formation of the homosemibullvalene is regiospecific and involves intermediate biradicals (76) similar to those required for the rearrangement of (67). Deuterium-labelling studies have shown that exclusive vinyl-vinyl bridging is involved in the photochemical conversion of the barrelene (77) into the cyclo0o

62

W. Eberbach, Chem. Ber., 1975, 108, 1052. H.-D. Becker and B. Ruge, Angew. Chem. Internat. Edn., 1975, 14, 761. J. Daub and P. von R. Schleyer, Angew. Chem. Internat. Edn., 1968,7, 468. T. Kumagai and T. Mukai, Chem. Letters, 1975,1187 (Chem. Abs., 1976,84,4548).

327

Photochemistry of Olefins, Acetylenes and Related Compounds

octatetraene (78).s3 This result is in direct contrast to the suggestions put forward by Grovenstein 64 and Epiotis 65 for systems substituted with electron-withdrawing groups. Grovenstein et aZ.64reported originally that the singlet state of the barrelene (77) yielded the cyclo-octatetraene (78). Bender and Brooks 63 concur with this observation but suggest that charge transfer is involved. A study of the production of biradicals (79), thought to be intermediate along the barrelene photochemical rearrangement pathway, by the thermally and photochemically

induced decomposition of (80) has been published.6s Direct irradiation of 2,3-dicyanobarrelene (81) yields 1,2-dicyanocyclo-octatetraenein almost quantitative yield.67 The cyclo-octatetraene could also be obtained in 20% yield under acetone-sensitized photolysis of the barrelene, but two dicyanosemibullvalenes (82a) (4%) and (82b) (56%) were also produced. The formation of (82b) is thought to arise via a di-r-methane route but the formation of (82a), although it

&

CN

(82) a; R1 = CN,R2 = H

CN

could arise by a similar path, might occur via a carbene intermediate (83) (cf. the rearrangement of tryptycene).68 Another carbene pathway is suggested for the photorearrangement of the dihydroxytriptycene (84) into (85).6s The triptycenes Oa

64

C. 0. Bender and D. W. Brooks, Canad.J. Chem., 1975,53, 1684. E. Grovenstein, jun., T. C. Campbell, and T. Shibata, J. Org. Chem., 1969,34,2418. N. D. Epiotis, J. Amer. Chem. SOC.,1972, 94, 1941. H. E. Zimmerman, R. J. Boettcher, N. E. Buehler, and G. E. Keck, J. Amer. Chern. SOC.,1975, 97, 5635.

68

OB

K. Saito and T. Mukai, Bull. Chem. SOC.Japan, 1975,48,2334. H. Iwamura and K. Yoshimura, J. Amer. Chem. SOC.,1974, 96, 2652; H. Iwamura, Chem. Letters, 1974, 5 . H. Iwamura and H. Tukada, J.C.S. Chem. Comm., 1975,969.

328

Photochemistry

(86) and (87) are both photochemically reactive. Compound (86) undergoes sensitized conversion into (88) 'O while acetone sensitization of (87a) affords a ring-opened product (89a).71 A ring-opened product (89b) is also obtained when the irradiation is carried out in CH,CI,. No evidence was obtained for formation of the semibullvalene (90a), although the products (89) are thought to arise by anionic ring-opening of the cyclopropyl moiety of (90a) with anti-attack favoured in CH,C12 (or benzene) and syn-attack favoured in acetone. The acetate (87b) yields the semibullvalene

(87) a;

R

=

b; R =

(88)

H

AC

(89) a; R1 = CO,Me, R2 = H (90) a; R1 = OH,R2 = H b; R1 = H,R2 = OAC b; R1 = H, R2 = C0,Me

The diols (91) both undergo di-n-methane-type photochemical rearrangements to afford the products (92).7a The reaction was shown not to be influenced by oxygen, and there was also a lack of enhancement by sensitizers: no definite 'O

71

73

N. K. Saxena, Mrs. Maya, and P. S. Venkataramani, Indian J. Chem., 1975,13, 1075 (Chem. Abs., 1976, 84, 17 016). K. E. Richards, R. W. Tillman, and G. J. Wright, Austral. J. Chem., 1975,28, 1289. S. J. Fuerniss, C. R. Olander, D. M. S. Wheeler, A. T. McPhail, and K. D. Onan, J.C.S. Perkin I, 1976, 550.

Photochemistry of Olefins, Acetylenes and Related Compounds

329

decision could be made on the nature of the excited state involved. Edman 73 had previously shown that related compounds rearranged only in the presence of sensitizers, but the latest work is not in agreement with that proposal, and the authors72suggest that a singlet state is involved. The use of deuteriated compounds showed that the reaction did not involve a 1,3-sigmatropic shift and that

mln HO

HO (91) a; n = 1

(92) a; n = 1, 44%

b;

b;n=2

(93) a; R

= Me0 b;R=CN C; R = C02Et

11 =

2,48%

R1 = MeO, R2 = H b; R1 = H, R2 = CN C; R1 = H, R2 = C02Et

(94) a;

the process took place by fission of the C-l-C-8a bond and formation of a C-1-C-3 bond. Other workers7* have studied the influence of polar substituents on the reaction of the closely related benzonorbornadienes (93). Contrary to the results of Wheeler et al.,72triplet-sensitized photolysis converted each diene into a single photoproduct (94). The results7* indicate that there is a dramatic change in bonding preference on changing the substituent R in (93) from Me0 to CN. This effect was examined further in the conversion, again by

R

=

b; R

=

(95) a;

(97)

74

Me0

CN

(96) a; R1 = MeO, R2 = H b; R' = H, R2 = CN

(98) a; R1 = MeO, R2 = H b; R2 = CN, R1 = H

J. R. Edman, J. Amer. Chem. SOC.,1969, 91, 7103. L. A. Paquette, D. M. Cottrell, R. A. Snow, K. B. Gifkins, and J. Clardy, J. Amer. Chem. SOC.,1975, 97, 3275.

330

Photochemistry

triplet excitation, of the quadricyclic compounds (95). The methoxy-derivative (95a) gave two products (96a) and (97), whereas the cyano-derivative (95b) gave only (96b). The authors 74 make the observation that, in the latter two examples, the biradical (98b) is preferred as a result of exclusive benzo-vinyl bonding para to the cyano-substituent, whereas (98a) is preferred in the methoxy case. The benzophenone-sensitized rearrangement of the triene (99) affords the barbaralene derivative (100) 75 (see also ref. 62 for a similar rearrangement). The silatriene (101) undergoes a similar conversion (either direct or sensitized) into the barbaralene (102) (20%), but also yields the valence-bond isomer (103) (80%).76

/fjco2* H 2oc[:-f

-

(99)

Me,

SiMe,

,Me

(103)

Breslow et aL7' originally suggested that an intermediate prismane derivative was involved in the conversion of the bicyclopropenyl (104a) into the corresponding benzene. Weiss and Kolbl 78 have reinvestigated the process utilizing the bicyclopropenyl(104b) and have observed that this compound is photochemically (254nm, C,H6) converted into a 2 : 1 mixture of 2,3,4,5-tetraphenyltolueneand

R' R2 Ph

(104) a; R1 = R2 = H,R3 = R4 = Ph b; R1 = Me, R2 = H, R3 = R4 = H c; R1 = R2 = Me, R3 = R4 = Ph d; R1 = R2 = Ph, R3 = Me, R4 = H e; R' = R2 = Ph, R3 = R4 = Me

Ph

Ph

76 76

E. Vedejs and R. A. Shepherd, J. Org. Chem., 1976,41,742. T . J. Barton and M. Juvet, Tetrahedron Letters, 1975, 2561. R. Breslow, P. Gal, H. W. Chang, and L. J. Altman, J. Amer. Chem. Suc., 1965, 87, 5139. R. Weiss and H. Kolbl, J. Amer. Chem. Soc., 1975, 97, 3222.

Photochemistry of Olefins, Acetylenes and Related Compounds

331

2,3,4,6-tetraphenyltoluene. If a prismane intermediate (105) had been involved, the independent generation of this by photochemical ring-closure of (106) should have afforded the same two benzene products, but in fact 2,3,4,5-tetraphenyltoluene and 2,3,5,64etraphenyltoluenewere produced. A further study 79 identified another reaction path for the bicyclopropenyls (104b and c). This was observed following irradiation at 320 nm when (104c) was converted into (104e) and 1,2-dimethyl-3,4,5,6-tetraphenylbenzene, and (104b) into (104d) plus the two benzene derivatives originally reported. Weiss and Kolbl 79 rationalize the transformation in terms of a photo-Cope process involving an intermediate (107) which either aromatizes or else reverts to bicyclopropenyl (cf. Cundall e f al., ref. 79a). Dimerization and isomerization are involved in the conversion of the triafulvene (108a) into the quino-methide (109a) and in the formation of the products (110) and (111) from (108b) and (108c) respectively.sO NC CNH

(108) a; b; C;

R' R1 = R2 = CN R' = COPh, R2 = CN R'-R2

=

Ph

R1

o-COC~H~CO (109)

\

HNC CN

The problem of isomerization of the cyclopropane (112a) into (112b) and vice versa has been investigated.81*8 2 The isomerization can be achieved by direct irradiation (@)a+b = 0.10; @b+a = 0.13) or by acetone-sensitized irradiation (a&+)-, = 0.13; @b+a = 0.40). By the use of optically active (112a) and (112b) it is possible to differentiate between the two isomerization paths, i.e. breaking of bond a or bond b. Breaking bond b converts (112a) into its enantiomer (113), which is a different product from (112b) which is obtained by breaking bond a in (112a).82 The analysis of the reaction indicates that under direct irradiative conditions 81% of the isomerization arises by bond a fission whereas under sensitization 98% arises in this manner. Thus the triplet states show a greater preference for breaking the outside bond, in accord with expected orbital overlap control. It is an important fact that in the direct irradiation experiments ca. 17% of the isomerization arises from the singlet state. Direct irradiation of (112) also R. Weiss and H. Kolbl, J. Amer. Chem. SOC.,1975, 97,3224. R. B. Cundall, D. A. Robinson, and A. J. R. Voss, J. Photochem., 1974, 2, 221, 231, 239. T. Eicher and R. Graf, Tetrahedron Letters, 1975,4021. S . S . Hixson and J. Borovsky, J.C.S. Chem. Comm., 1975,607. S . S . Hixson and J. Borovsky, J. Amer. Chem. SOC.,1975,97,2930.

7w

332

Photochemistry

(112) a; R1 = H, R2 = CH,OH b; R' = CH,OH, R2 = H

(114) a; R1 = b; R1 = c ; R1 = d ; R1 =

H, R2 = OH OH, R' = H H, R2 = OMe OMe, R2 = H

mcH=

affords the ring-opened products (114).s1 [Care was taken to ensure that this ringopening process (of homoallyl type) was not catalysed by acid.] Hixson and Borovsky 81 conclude that the mechanism of formation of ring-opened products involves a carbonium ion (115). Evidence for this comes from an experiment in which (112a) and (112b) were irradiated in methanol and the ethers (114c) and (114d) were produced together with the alcohols (113a) and (113b). The ethers could of course arise by secondary photolysis of the alcohols. An ionic mechanism is also involved in the photoconversion of the acetate (116) labelled with l80on the ether oxygen into (117), where the l80label has undergone s ~ r a m b l i n g . ~ ~

Recovered starting material, which had undergone trans-cis-isomerization, had all the label in the ether oxygen. This suggests that the cis-trans-isomerization process arises by the fission of a cyclopropyl bond, i.e. a process distinct from the isomerization to (117) which is formally a 1,3-migration. The 1,3-hydrogen migration in 2,2-dimethyl-l-phenylcyclopropane, yielding 2-methyl-4-phenylbut-l-ene,has been studied in detaiLS4The results from deuteriation clearly indicate that there is a preference for migration from the trans-methyl group. From analysis of isotope effects on the reaction, the authors suggest that either a concerted [as2 aa2]process is involved or that hydrogen migration takes place immediately after C-1-C-2 cleavage prior to rotation of the benzylic carbon at C-l.s4 Mazzocchi and Lustig have also reported the synthesis of (27)-( )-2,2-dimethylphenylcyclopropane-1-01 and its photochemical conversion into (S)-( )-2-methyl-4-phenylbut-l-en-4-01. A full report of the photochemistry of the allylcyclopropane (1 18)

+

+

83 04

+

S. S. Hixson and R. E. Factor, Tetrahedron Letters, 1975, 3111. P. H. Mazzocchi and R. S. Lustig, J . Amer. Chem. SOC.,1975, 97,3707. P. H. Mazzocchi and R. S. Lustig, J . Amer. Chem. SOC.,1975, 97, 3714.

Photochemistry of Olefins, Acetylenes and Related Compounds

333

87 The influence of substituents upon the photochemical has been ring-opening of the bicyclobutane derivative (119a) to give (120a) has been reported.88,89 The authors 89 believe that the ring-opening follows an ionic pathway (121) and this appears to be borne out by the influence of substituents. Thus when the substituent on the aryl ring is hydrogen, p-methyl, or m-methoxy, the principal products are (120a-c), but with electron-withdrawing substituents the formation of (122) becomes more important. Thus with (119d) and (119e) 88q

Ar

(119) a; Ar = Ph, R1 = H (120) a; Ar b; Ar = p-MeC,H,, R1 = H b; Ar C ; Ar = nZ-MeOCcH,, R' = H c; Ar d; Ar = p-CIC,H,, R1 = H d; Ar e; Ar = I?I-CCl,C6H,, R1 = H e; Ar f; Ar = Ph, R1 = Me

(122) d ; Ar e; Ar

= =

Ph

Ph; 54% p-MeC,H,; 67% = p-MeOC,H,; 70% = p-ClC,H4; 40% = m-CCI,C,H,; 41% =

(121)

=

H p-CIC,H4; 19% ni-CCI,C,H,, 39%

two products (120d and e) and (122d and e) are formed in each case. Substitution in the ring of (119f) also affords two products (123) and (124). The rearrangement of the zwitterionic intermediate (121) is a rapid process since irradiation of the parent compound (119a) in methanol failed to yield alcohol addition products. Photoisomerization of (125) into (126) has been shown to proceed by inversion at C-7.90 This was demonstrated by establishing that the starting material had an R configuration at C-7 whereas the product had the opposite configuration. The rearrangement (Scheme 5 ) probably involves an intermediate such as (127). [In relation to this work the ester (128) ring-closes photochemically to (129) in a disrotatory fashion.] Swenton et aLgl have also studied this class of reaction (Scheme 6 ) in some detail and have shown the importance of 1,5-migrations in the conversion of the norcaradiene (130) into the cycloheptatriene (131), via the isomeric norcaradiene (132) [cf. (127)]. Pomerantz and Gruber 92 previously studied the photoreactivity of cycloheptatrienes of type (131). They observed that a singlet state was involved in the conversion of (133a) into the norcaradiene (134a). This compound is formed by a ring-closure of the intermediate (135) 87

*O

H. E. Zimmerman and C. J. Samuel, J . Amer. Chem. SOC.,1975, 97,4025. H. E. Zimmerman and C. J. Samuel, J . Amer. Chem. SOC.,1975,97,448. K. Fujita, T. Nakamura, K. Matsui, and T. Shono, Tetrahedron Letters, 1975, 4385. K. Fujita, T. Nakamura, K. Matsui, and T. Shono, Tetrahedron Letters, 1975,2441. M. Kato, M. Funakura, M. Tsuji, and T. Miwa, J.C.S. Chem. Comm., 1976, 63. J. S. Swenton, K. A. Burdett, D. M. Madigan, T. Johnson, and P. D. Rosso, J. Amer. Chem. SOC.,1975,97, 3428.

82

M. Pomerantzand G. W. Gruber, J. Amer. Chem. SOC.,1967,89,6799; 1971,93,6615.

12

334

Photochemistry

c

CH2CO2Me

CH,CO,Me

-

hv, 267 n m (I, = 0.35

IIV, 300 nni Q, = 0.09

(131)

Scheme 6

a

C02Me

R3

(133) a ; R1 = R2 = R3 = H b; R1 = R2 = H, R3 = C0,Me C ; R' = Cl, R2 = R3 = H d; R1 = R3 = H, R2 = CN e; R1 = R2 = H, R3 = Me f; R1 = R2 = H, R3 = 2-propenyl g; R1 = R2 = H, R3 = CN

Photochemistry of Olefins, Acetylenes and Related Compounds

335

(0= 0.1) formed by 1,7-hydrogen migration. Swenton et ~ 1have . ~ reinvesti~ gated the reaction and agree that a singlet state is involved, but they report the much higher quantum yield of 0.79. This recent work has sought to evaluate the influence of a C-5 substituent on the reactions of the benzotropilidenes (133b-g). No 1,3-migrations were detected in these reactions, and the main products were the benzonorcaradienes (134) and cyclobutenes (1 36). The quantum yields for the reactions are recorded in Table 2: the processes are reasonably efficient. It

Table 2 Quantum yields for photoreactions of (133) Benzotropilidene (133a) (133b) (133b) (133e) (133e) (133f) (133g) (133g)

93

@ [cyclobutene (136)]

@ [norcaradiene (134)]

-

0.79 0.088 0.17 0.42 0.81 0.54 0.21 0.44

0.61 0.64 0.11 0.006 0.37 0.28

Solvent C6H12 C6H12

MeCN

GHl2

MeCN

c6 H12

GHl2

MeCN

should also be noted that the modest solvent dependency suggests a degree of polar character in the transition state for hydrogen migration. A review dealing with the utilization of deuterium labelling in the study of photochemical rearrangements has been published.94 3 Isomerization of Dienes Benzophenone-sensitized irradiation of neat cis- and trans-piperylene gives only isomerization and d i m e r i z a t i ~ n .The ~ ~ overall process is less efficient than the same reaction in dilute solution, and the authors reason that this is a result of quenching of the diene excited state by ground-state diene. Earlier work had shown that direct irradiation (253.7 nm) of cis- and trans-penta-l,3-dienes in solution led to 3-methylcyclobutane and 1,3-dirnethylcyclopropene,together with the geometric isomer of the diene.96 A recent studye7 has now revealed a wavelength effect in this system which was evident by the lack of cyclobutene and cyclopropene products when the penta-l,3-dienes were irradiated at 228.8 nm. Triplet-triplet energy transfer from solid benzophenone (deposited on a light pipe) results in the cis-trans-isomerization of penta-l,3-diene vapour. The photostationary state obtained by this technique contains 56.5% of the trans~liene.~~ The two vinyl allenes (137) and (138) both undergo photochemical transformation into the same photostationary mixture (1 : 1) by rotation about the allenyl Simpler allenes (1 39) undergo photochemical addition of acetic acid when they are irradiated by direct or sensitized procedures.100 The O3 J. S. Swenton, K. A. Burdett, D. M. Madigan, and P. D. ROSSO, J. Org. Chem., 1975,40,1280. J. S . Swenton, Isotop. Mol. Rearrangements, 1975,241 (Chem. Abs., 1975, 83,205 337). M. Bigwood and S. Boue, Bull. SOC.chim. beiges, 1973,82,685 (Chem. Abs., 1976,84,88 985). S. Boue and R. Srinivasan, J. Amer. Chem. SOC.,1970, 92, 3226. O7 P. Vanderlinden and S. Boue, J.C.S. Chem. Comm., 1975, 932. 98 J. S. Deguzman and G . R. McMillan, J. Phys. Chem., 1975,79, 1377. 99 J. A. van Koeveringe and J. Lugtenburg, Rec. Trav. chim., 1976, 5, 80. l o o K. Fujita, K. Matsui, and T. Shono, J. Amer. Chem. SOC.,1975, 97, 6256. 86

O6

336

Photochemistry

R

(137)

(138)

direction of the addition of acetic acid to afford the enol acetates (140) is directly opposite to the mode of addition encountered in the ground-state process, and the authorsloosuggest that a polarized excited state is responsible. Clearly the presence of the phenyl group plays a large part in determining the mode of addition since the allenes (139d and e) add acetic acid upon photochemical excitation to yield products (141) and (140d) respectively, which are the same as are formed by thermal reaction.

R x R 3 R 4

(139) a;

b; c; d; e;

R2 R4 R1 = Ph, R2 = Me, R3 = R4 = H R1 = Ph, R2 = R3 = Me, R4 = H R1 = rn-MeOC,H,, R2 = Me, R3 = R1-R3 = (CH,),, R2 = R4 = H R1 = W C ~ H ,R2 ~ , = R3 = R4 = H

R2 R4 =

H

OAc

(140) a; 65% b; 45% c; 40%

A c O q H (141) 3%

d; 2%

A study of the photochemistry of the dienes (142) in methanol has been reported.lol Several products (dimers, cyclobutenes, and alcohol addition products) were obtained from all the dienes investigated. The diene (142c) affords 1,2-diphenylcyclobutene, 1,2-diphenyl-l-methoxycyclobutane, and the cyclopropyl ether (143). These latter two products have previously been shown to arise from an ionic addition of methanol to the cyclobutene.f02 The singlet and triplet energies of a series of steroidal dienes [e.g. (144)] have been determined.lo3 The photochemical reactivity of these compounds [e.g. (144), Scheme 71 has been studied.lo4 lol lo4

loS lo’

P. J. Baldry, J.C.S. Perkin I, 1975, 1913. M. Sakuragi and M. Hasegawa, Chem. Letters, 1974, 29. J. Pusset and R. Beugelmans, Tetrahedron, 1976, 32, 791. J. Pusset and R. Beugelmans, Tetrahedron, 1976, 32, 797.

337

Photochemistry of Olefins, Acetylenes and Related Compounds

\C,H,-MeOH-HO-

(144)

+

+

OMe

30%

a; R1 = OMe, R2 = H; 10% b; R1 = H, R2 = OMe; 10%

20%

Scheme 7

Irradiation of the alcohol (145) results in the loss of the C-10 f~nctiona1ity.l~~ An X-ray diffraction study of the molecule shows that the ring junction of the product is as shown in (146). The likely mechanism for the loss of the substituent was checked by deuterium labelling and is shown in Scheme 8. Several products

(145)

Scheme 8

are formed when the diene (147) is irradiated in xylene-MeOH, mainly the cyclized isomer (148).lo8 The exact reaction path is not known, although the absence of deuterium incorporation suggests that intramolecular hydrogen transfer is involved.

(147)

(148)

Intramolecular lY3-hydrogentransfer is involved in the photoisomerization of cyclo-octa-l,5-diene to cyclo-octa-l,4-diene in the presence of RhCl (as a complex). The mechanistic path was verified by deuterium-labelling studies.lo7 Similar isomerization was found with the acyclic 3,3-dimethylhexa-l,5-diene,which was converted into a cis-trans mixture (ratio of 1 : 4) of 3,3-dimethylhexa-l,4-dienes. lo6 log

H. Paaren, R. M. Moriarty, and J. Flippen, J.C.S. Chem. Comm., 1976, 114. R. E. K. Winter and R. F. Lindauer, Tetrahedron, 1976,32,955. R. G . Salomon and N. El Sanadi, J. Amer. Chem. Soc., 1975,97, 6214.

Photochemistry

338

Direct irradiation of cis-3,3-dimethylhexa-174-diene failed to yield an appreciable quantity of the trans-isomer, thus indicating that both the cis- and the trans-1,4diene arise from the 175-diene. Benzophenone-sensitized isomerization of ~ i s - 3 ~ 3 dimethylhexa-l,4-diene gave the trans-isomer (photostationary state ratio of 1 : 1). A 1J-hydrogen transfer is involved in conversion of the ionol (149) into the isomer (15O).lo8 This y-ionol (150) is itself photolabile and is converted into the cyclobutene (151).lo8 Cyclobutene formation (152) is also encountered in the irradiation of the diene (153).lo9 A full account of the products formed in the

(1 54)

benzophenone-sensitized irradiation of ~,2-dimethylenecyclobutenehas been published.ll0Sll1 The dimer (154) obtained from this reaction is thermally labile and is converted into (155). Irradiation of this compound afforded a 1 : 1 mixture of (154) and (156). Calculations dealing with electronic overlap populations in the excited and ground states for photocyclizations in dienes, trienes, naphthylethylenes, etc. and for the dimerization of anthracene, naphthalenes, and stilbenes have been published.l12 An all-electron calculation 113 on the butadiene-cyclobutene ringclosure and a generalization of the Woodward-Hoffmann rules 114in terms of a valence-bond approach have been put forward. A. Van Wageningen, A. A. M. Roof, and H. Cerfontain, Synrh. Comm., 1975, 5, 217 (Chem. Abs., 1975, 83, 114 675). log G. Ohloff, C.Vial, H. R. Wolf, and 0. Jeger, Helo. Chim. Acta, 1976, 59, 75. W. T. Borden, I. L. Reich, L. A. Sharpe, R. B. Weinberg, and H. J. Reich, J. Org. Chenr.,

Io8

1975,40, 2438.

n1 W. T. Borden, I. L. Reich, L. A. Sharpe, and H. J. Reich, J. Amer. Chem. Soc., 1970,92,3808. 112

113 114

K. A. Muszkat, G. Seger, and S. Sharafi-Ozeri,J.C.S. Furaday 11, 1975, 71, 1529. D. Grimbert, G . Segal, and A. Devaquet, J. Amer. Chem. SOC.,1975, 97, 6629. D.M.Silver and M. Karplus, J . Amer. Chem. SOC.,1975,97, 2645.

Photochemistry of Olefins, Acetylenes and Related Compounds

Ph (157) a; R = H

339

Ph (158) R = H

b;R=Me

(159) R = Me

The photochemical ring-opening of the dienes (157) gives rise to trienes (158) and (159)respectively in each case. Courtot and Salaun 115suggest that only one isomer is formed because only one conrotatory mode is possible for the ringopening as a result of the methyl group adopting a pseudo-axial position. and 1 -cyanoIrradiation of l-cyano-2-methoxy-5,5-dimethylcyclohexa-l,3-diene 2-(N-azetidino)-5,5-dimethylcyclohexa-l,3-dieneyields the ring-opened trienes 2-cyano-3-methoxy-6-methyl-and 2-cyano-3-(N-azetidino)-6-methyl-hepta-1,3,5triene respectively.l16 Vycor-filtered irradiation of the cyclohexadienes (1 60) in ether solution at temperatures lower than 0 "C led to the formation of the all-cis trienes (161).l17 These unstable compounds were trapped as the dihydrothiepin 1,l -dioxides (1 62) by reaction with SOz. Direct irradiation of ergosterol (1 63)

6

R2\

. R3

R4

R2 R3

R4 R3

leads to a mixture of starting material, previtamin D, (164), and tachysterol in a ratio of 1 : 2 : 1.l18 Thermal treatment of this gives only an 11% yield of vitamin D, (165). The use of fluorenone as a sensitizer for further photolysis of the mixture obtained by direct photolysis enhances the yield of previtamin D, and thence the vitamin itself (28% isolated yield). lib

11*

P. Courtot and J. Y . Salaun, J.C.S. Chem. Comm., 1976, 124. P. Margaretha, Helv. Chim. Acta, 1975, 58, 929. W. L. Mock and J. H. McCausland, J. Org. Chem., 1976,41,242. S . C. Eyley and D. H. Williams, J.C.S. Chem. Comm., 1975, 858. J. W. Lown, M. H. Akhtar, and W M. Dadson, J. Org. Chem., 1975,40, 3363.

Photochemistry

340

Irradiation of the diazabicyclo-octadiene (166) at - 78 "C in ether affords the pyrrole (167) in high yield.lla The stereochemistry of the process was checked by the use of optically active starting material. The reaction is thought to involve ring-opening to the triene (168) followed by photochemical 1,3-sigmatropic

Me

migration of the alkyl group with retention (80%) of configuration of the migrating species. The new triene (169) so produced undergoes cyclization and elimination to afford the final product (167). The ionylidene epoxide (170a) is converted into the cyclopropyl ketone (171) (49% yield) on irradiation in pentane The pathway to the product could involve the formation of an intermediate carbene (172) by fission of the epoxide ring. Heavier methyl substitution on the ionylidene epoxide diverts the reaction, and in the case of (170b) only cis-trans-isomerization to (173) has been detecfed.l2O

(170) a; R = H b;R=Me

(171)

(172)

(173)

4 Reactions of Trienes and Higher Polyenes A full account of the studies by Courtot and his co-workers 121-123 on the photochemistry of hexatrienes (174) has been published. In particular, they have examined the dependence of ground-state conformation on the photochemistry lao lZ1

lZz

A. P. Alder, H. R. Wolf, and 0. Jeger, Helu. Chim.Actu, 1976, 59, 907. P. Courtot, R. Rumin, and J. Mahuteau-Corvest, Tetrahedron Letters, 1973, 899. P. Courtot and R. Rumin, J.C.S. Chem. Comm., 1974, 168. P. Courtot and R. Rumin, Tetrahedron, 1976, 32, 441.

341

Photochemistry of Olefns, Acetylenes and Related Compounds

6

J&

R' R

(174)

=

Ph

H or Me

(175) a; b;

R1

R1 = H, R2 = Me R1 = Me, R2 = H

()m R'

R2

(176) a; R1 = Me, R2 = H b; R1 = H, RZ = Me

that the compounds undergo. The triene (175a), obtained from ring-opening of l-phenyl-6-methylcyclohexa-l,3-diene, is converted upon direct irradiation into the isomer (175b) which in turn gives (176a).ll5 Iodine-catalysed photoisomerization of the triene (175a) gives (176a) and (176b) (70 : 30), whereas sensitized irradiation gives a photostationary state the composition of which is dependent upon the triplet energy of the ketonic sensitizer (Table 3). These results confirm

Table 3 Photostationary state composition from sensitized irradiation of (175a) 115 Sensitizer (ET/kJ mol-l)

Benzophenone (289) Michler's ketone (256) Fluorenone (222)

Photostationary state composition (%) (175a) (175b) (176a) (176b) 50 5 20 7 50 5 20 25 80 4 6 10

that a singlet-state reaction causes preferential isomerization of the terminal bond of the triene, whereas triplet sensitization leads to isomerization about the central bond. Contrary to that postulate is the observation that direct irradiation of the hexatriene (177) in pentane affords the 2-isomer (178) by central-bond isomerization together with the cyclobutene (179).124

Further studies 125 on the photochemistry of dihydronaphthalenes (180) have shown that products (181) and (182) from [1,5]- and [l,7]-hydrogen migrations respectively are produced together with products from the photochemical Diels-Alder reaction (183) (Scheme 9). These reactions take place at low temperatures (ca. - 130 "C), and at - 181 "CU.V. absorption evidence (Amx = 402 nm) for an o-quinodimethane intermediate was obtained. The authors 125 point out that the [1,7]-hydrogen migrations occur with remarkable facility in these phenylated systems in comparison with non-phenylated systems. Similar ring-opening to that observed in (180) occurs in the dihydroquinolines (184) which are converted by irradiation (Pyrex filter) into the benzoazetine (185) and also (186) [from lZ4

lz6

J. M. G. Bonfrer, H. J. C. Jacobs, and E. Havinga, Tetrahedron Letters, 1975, 3741. U. Widmer, H. Heimgartner, and H. Schmid, Helv. Chim.Acta, 1975,58,2210.

342

Photochemistry

@Me a; R1 = Ph, R2 = Me b; R1 = H, R = Ph

R2

J + R2

(181)

others

R2 (182) Scheme 9

(183)

(184a)].lZ6 The benzoazetine (185) also undergoes photochemical conversion into the tricyclic compound (186). These reactions can best be summarized by the route shown in Scheme 10 involving the formation of a quinodimethane intermediate (187).

Ac (184) a; R = Me

b;R=H

(187) I

1

hv R = Me

Ac (186)

Scheme 10

A perturbation treatment has provided a method for predicting the direction of cyclization in the photochemical valence-bond isomerization of cycloheptatrienes.12' An alternative method for the rationalization of the cyclization modes has been published previously.128 120

M. Ikeda, S. Matsugashita, F. Tabusa, H. Ishibashi, and Y . Tamura, J.C.S. Chem. Comm., 1975, 575.

12' 128

T. Tezuka and 0. Kikuchi, Tetrahedron Letters, 1976, 1125. A. R. Brember, A. A. Gorman, and J. B. Sheridan, Tetrahedron Letters, 1971, 653.

343

Photochemistry of Olejins, Acetylenes and Related Compounds

bR

R H

RQR H

The valence-bond isomerization of azepines has received considerable attention recently. The triplet state of the azepine (188) is photochemically unreactive and the conversion into (189) arises from the singlet excited state.12g Valence-bond isomerization to (190) occurs on irradiation of the diazepine (191),130 and a closely similar process is involved in the conversion of (192) into (193).131 The

R3

R3

N H R' (194) R' H H Me Me Me

R2

H OMe OMe H H H Ph I3 H PhCHz H

H R1

R3

(195)

H H H H Ph H Ph Ph

130

G. Jones, jun. and L. J. Turbini, J. Photochern., 1976, 5 , 61 (Chem. Abs., 1976, 84, 89 282). C. D. Anderson, J. T. Sharp, E. Stefaniuk, and R. S. Strathdee, Tetrahedron Letters, 1976,

lS1

305. J. P. Luttringer, N. Perol, and J. Streith, Tetrahedron, 1975, 31, 2435.

120

344 Photochemistry isomerization of the benzodiazepines (194) to the isoindole derivatives (195) has also been d e m ~ n s t r a t e d . ~ ~ ~ Valence-bond isomerization is also a common photoreaction in hydrocarbons, as is demonstrated by the photochemical ring-closure of the triene (196a) to the diene (197a) by irradiation at 280 nm.133 The photochemical retro-process

C0,Me

(197) a; R1 = R2 = H, X = CH, b; R' = C02Me, R2 = H, X = CH, c; R1 = CO,Me, R2 = H, X = spiro-C,H,

R (198) b; R

=

R

=

c;

(199)

CO,MC, X C02Me, X

=

=

CH, spiro-C,H,

(200) a ; R1 = H, K" = CO,Me b; R1 = CO,Me, R2 = H

(197) -+ (196) also takes place efficiently. The trienes (196b and c) undergo [2 21 cycloaddition to yield (198b and c) when irradiated with light of wavelength > 280 nm. The o-quinodimethane (199), obtained by the photochemical decarbonylation of (200), is also photoreactive and is converted into three products (201a), (201b), and (202) when irradiated at 366 nm.134 The ratio of products obtained from this reaction is invariant with temperature and solvent. The products (201) arise by a valence-bond isomerization whereas (202) is formed by a 1,4-addition of the ester C=O to the termini of the o-quinomethane system. A Aash-photolytic study of various 1,l-diarylindenes has shown the existence of transients which are proposed to be isoindenes (203) formed by a 1,2-aryl migration.136

+

182 188

134 135

A. A. Reid, H. R. Sood, and J. T. Sharp, J.C.S. Perkin I, 1976,362. W. Eberbach, Chem. Ber., 1975,108, 1052. D. S. Weiss, J. Amer. Chem. SOC.,1975, 97, 2550. J. J. McCullough and A. J. Yarwood, J.C.S. Chem. Comm., 1975,485.

34 5

Photochemistry of Olefins, Acetylenes and Related Compounds OMe

/

(202) 2.6%

(201) a ; b;

R1 = H, R* = C0,Me; 1% R1 = CO,Me, R2 = H; 10.2%

(203) a ; R b; R c; R

= =

=

H, Ar = Ph Ar = Ph H, Ar = p-CNC,H4

The singlet excited state of the vinyl stilbene (204) gives a high yield of the [2 + 21 crossed addition product (205) (70%).136 A small amount (2%) of the corresponding endo-isomer is also formed. This mode of reaction is to be contrasted with that of 1,2-divinylbenzenewhich yields (206) as the main p r o d ~ c t13* .~~~~

The authors13* suggest that this divergence in reactivity could be due to conformational effects which would make the vinyl group almost perpendicular to the stilbene moiety. An extension of this type of process has provided a synthesis of (207) (Scheme l l ) , a dibenzo-derivative of a new C,,H,, system. Binkley and Schumann lS9had observed previously that the propane derivative (208a) yielded biphenyl, amongst other products, arising by a T +. T interaction pathway. However, the system (208b) originally studied by Fischer 140 apparently only yielded fission products. A reinvestigation of this system by Binkley et has demonstrated that biphenyl is indeed formed. Biphenyl is also formed when (208c) is irradiated in ether. Other fission products (2-ethoxy-l-phenylpropane, benzaldehyde, and 2-ethoxy-l-phenylpropan-1-01)are also produced, and it is thought that the intermediate (209) collapses to biphenyl and diphenyloxiran, which is unstable under the photolysis conditions. Interestingly, the 136 13'

M. Sindler-Kulyk and W. H. Laarhoven, J. Amer. Chem. SOC., 1976, 98, 1052. M. Pomerantz, J, Amer. Chem. SOC.,1967, 89, 694; M. Pomerantz and G. W. Gruber, Ibid., 1971,93, 6615.

138

140

J. Meinwald and P. H. Mazzocchi, J. Amer. Chem. SOC.,1967, 89, 696. R. W. Binkley and W. C. Schumann, J. Amer. Chem. SOC.,1972,94, 1769,8743. M. Fischer, Chem. Ber., 1968, 101, 731. R. W. Binkley, S.-C. Chen, and D. G . Hehemann, J. Org. Chem., 1975, 40, 2406.

Photochemistry

346

Scheme 11 n + n interaction process can be quenched by piperylene. The sulphide (208d)

does not yield biphenyl when irradiated under the same conditions. The aza[l3]annulene (210) is prepared by irradiation of the adducts (211) and (212a):14, its configuration is uncertain. Irradiation of the adduct (213a) also afforded the annulene (210), having the configuration shown in (214). Oxa[13]annulenes (215), whose gross formula only is known, can also be prepared by photolysis of the epoxides (212b) and (213b).14s A review lecture dealing with the synthesis of heteronins, including photochemical routes, has been p~b1ished.l~~

Ph2CHXCHPh, (208) a; X = CH, b;X=NH

Ph

c;x=o

Ph

d;X=S

H H A

(212) a; X = NC02Et b;X=O

(210) X = NC0,Et (215) X = 0

(213) a; X

N

C0,Et

G. Frank and G. Schroder, Chem. Ber., 1975,108, 3736. W. Henne, G. Plinke, and G. Schroder, Chem. Ber., 1975,108, 3753. lr14 A. G. Anastassiou, Pure Appl. Chem., 1975,44, 691.

142

143

=

NC0,Et

b;X=O

Photochemistry of Olefins, Acetylenes and Related Compounds 5 [2

347

+ 21 Intramolecular Reactions

The quadricyclane (216) was prepared by the photosensitized cyclization of the norbornadiene derivative (217).145 Reactions of this type have been a sourcc of considerable activity over the years both from a mechanistic and from a synthetic viewpoint. Thus the sensitized photoreaction of (218) affords (219),146and (220)

dYPh siinilarly yields (221).14' The yield of product in this last reaction was quantitative and there was no evidence for the involvement of a di-rr-methane process. The study of the closely related benzo-derivative has been published previ0us1y.l~~ Cyclization also occurs in heavily substituted derivatives, as in the conversion of (222) into (223).149An n.m.r. analysis of the oxaquadricyclane (223) was carried out. The trifluoromethyl-substituted derivatives (224) also undergo cyclization to yield (225) with no adverse effects as a result of increasing the number of

(224) a; b;

140 lo' 148

la

11 = ?I =

1 2

W. G . Dauben and J. W. Vinson, J. Org. Chem., 1975, 40, 3756. I. Tabushi, K. Yamamura, and J. Ueda, J . Amer. Chem. Soc., 1975,97,4039. H. Prinzbach and H. Babsch, Angew. Chem. Znternat. Edn., 1975,14, 753. G. R. Ziegler, J . Amer. Chem. SOC.,1969, 91, 446. R. Hogeveen and B. J. Nusse, Tetrahedron Letters, 1976, 699.

348 Photochemistry carbon atoms in the bridge.lS0 The photochemistry of the quadricyclane (226) has also been studied.l5' Hart and Kuzuya report that the acetone-sensitized [2 + 21 addition reactions of the trienes (227) give the tetracyclic compounds (228):152p153 cf. ref. 154. They have also described the photoisomerization of (227b) into (228b) as a proof

-@ R1 ~1 R2

(227) a; b;

R'

R1 = Ph, Ra = H R1 = R2 = Me

Fb (230)

&* R2

(229)

(228)

Meo2c&02~e

Me02C Meo&02Me

MeO,C C0,Me (231) a; R = H b; R-R = CH2-CH2

C02Me

(232)

of structure for a product obtained from the rearrangement of bicyclo[3,2,1]octadienyl cation^.'^^^ 156 Another product from the cationic rearrangement was the triene (229), which was photochemically converted into (230) by a [2 + 21 intramolecular cycloaddition. Cyclizations of this type have also been reported by Eberbach133in the conversion of the trienes (231) into (232). A cage compound (233) is also produced when (234) is subjected to acetone-sensitized irradiation.16' [2 + 21 Addition has been reported for the diene (235), which yields (236) (85%) upon direct irradiati~n.'~~

lSo lS1 lS1 16*

lSs lS6 lS7

lS6

P. G. Gassman and T. H. Johnson, J. Amer. Chem. SOC.,1976,98,861. H. Babsch, H. Fritz, and H. Prinzbach, Tetrahedron Letters, 1975, 4677. H. Hart and M. Kuzuya, J. Amer. Chem. SOC.,1975, 97, 2450. H. Hart and M. Kuzuya, J. Amer. Chem. Soc., 1975, 97, 2459. H. Hart and M. Kuzuya, J. Amer. Chem. SOC.,1974,96, 3709. M. Kuzuya and H. Hart, Tetrahedron Letters, 1973, 3891. H. Hart and M. Kuzuya, J. Amer. Chem. SOC.,1976, 98, 1551. L. A. Paquette, R. K. Russell, and R. L. Burson, J. Amer. Chem. SOC.,1975, 97, 6124. T. Katsushima, R. Yamaguchi, and M. Kawanisi, J.C.S. Chem. Comm., 1975, 692.

349

Photochemistry of Olefins, Acetylenes and Related Compounds

Another mode of [2 + 21 cyclization involves the addition of a n-bond to a a-bond. Thus the norbornene derivative (237) undergoes photocyclization to yield (238).lsS The formation of products of type (239) from the reaction of bicyclo[2,2,2]octa-2,5-diene and methyl phenylpropiolate or 4-phenylbut-3-yn2-one can also be rationalized in this fashion. Thermal addition of the acetylene to the diene yields an adduct (240) which undergoes photocyclization to yield (239).lS0

R&

R

R (237) a; R = C0,Me b; R = CHzOH

P11 R (239) R = C0,Me or COMe

(238)

&? R

Ph

+ +

The intramolecular [2 21 product (241) was produced on irradiation of a benzene solution of the cis,cis-compound (242a).la1 Phenanthrene, a decomposition product of the [2 21 product (241), was also formed. The trans&compound (242b) yielded phenanthrene together with the cis,cis-compound (242a), whereas the trans,trans-compound (242c) gave the [2 21 product (243) under the same conditions.lsl Irradiation (Pyrex filter) of the oxide (244) in benzene solution affords the [2 21 cycloadduct (245).lg2 Oxidation of this compound affords the previously unknown cis-syn-cis-dimer of dibenzo[a,d]cyclohepten-5-one.

+

+

T. Toda, K. Nakano, A. Yamae, and T. Muka , Tetrahedron, 1975,31, 1597. K. Fujita, K. Matsui, and T. Shono, Nippon Kagaku Kaishi, 1975,6, 1024 (Chem. Abs., 1975, 83, 146 784). G. Wittig and G. Skipka, Annalen, 1975, 1157. laa J. Rokach, Y. Girard, and J. G. Atkinson, J.C.S. Chem. Comm.,1975,602. lS9

lB0

Photochemistry

350

(242) a; cis,cis b; cis,traiis c; traiis,tranS

(243)

0

6 Dimerization and Intermolecular Cycloaddition Reactions Both the direct and sensitized additions of diphenylvinylene carbonate to dienes (4-methylpenta-l,3-diene,Z- and E-penta-l,3-diene) afford mixtures of [2 + 21 adducts (246).163 However, there is a difference between the regioselectivity shown by the singlet and the triplet excited states. Thus the singlet excited state

R4P h (246) a; R1 = R2 = R4 = H; R3 = CH=CMe, or Z- or E-CH=CHMe b; R' = R2 = R3 = H; R4 = CH=CMe2 or Z - or E-CH=CHMe c ; R1 = H, R2 = CH=CH2, R3 = R4 = H or Me d ; R1 = CH=CH,, R2 = H, R3 = R4 = H or Me

of the vinylene carbonate shows a moderate regioselectivity with 68-76% addition to the less substituted double bond, whereas the triplet exhibits 95% addition to the same double bond. However, the authors163 conclude that initial bond formation between the singlet or triplet diphenylvinylene carbonate 163

F. D.Lewis and R. H. Hirsch, Tetrahedron Letters, 1975, 2651.

Photochemistry of Ole$ns, Acetylenes and Related Compounds

351

and the diene is comparably regioselective and that the apparent high regioselectivity of the triplet reaction is due to selective collapse of the more substituted biradical intermediate [e.g. (247)l. Such a mechanism would account for the observed isomerization of the starting diene. Lewis and his co-workers 164 have attempted to rationalise the abnormal regioselectivity in the photochemical addition of singlet trans-stilbene to conjugated dienes. The stereochemical outcome of the reaction could be the result of exciplex formation, although the evidence for such a complex is tenuous in this case. The acetonaphthonesensitized reaction of trans,trans-hexa-2,4-dienewith 1,l -dichloro-2,2-difluoroethylene leads to the formation of hexadiene dimers and mixed adducts of [2 21 and [2 41 addition:lSSsee Scheme 12.

+

+

+

hv

2-acetonaphthone

Fz

‘Me

Fz

‘Me

a; R = a-trans-CH=CHMe a; R = a-cis-CH=CHMe b; R = fLfraizs-CH=CHMe b; R = P-cis-CH=CHMe

Scheme 12

The study of dimerization of styryl derivatives (styrene, p-methylstyrene, and a-methylstyrene) in the presence of tetracyanobenzene has continued.ls6 Irradiation of the three styrenes in acetonitrile or benzene (but not hexane) results in the formation of products (248) and (249) by [2 + 21 addition and (250) by a [2 + 41 addition mode.lsa The direct irradiation of 3,3-biphenylene-l-bromo-l-phenylallene yields the methylenecyclobutane (251) (30%) and the hexatriene (252) A r n A r H ii (248) Ar = Ph, or y-MeC,H,

H a A r Ar H (249) Ar

=

Ph, or p-MeC,H,

(250) R1 = R2 = H, Ar = Ph R1 = Me, R2 = H, Ar. = p-MeC,H, R1 = H, R2 = Me, Ar = Ph 184

186 166

F. D. Lewis, C. E. Hoyle, and D. E. Johnson, J. Amer. Chem. SOC.,1975,97,3267. P. D. Bartlett and J. J.-B. Mallet, J. Amer. Chem. SOC.,1976, 98, 143. T. Asanuma, M. Yaniamoto, and Y . Nishijima, J.C.S. Chem. Comm., 1975, 608.

3 52

Photochemistry R

Ph

c=c=c/

\

Ra

\/Hr C=CBr-C

R-R

\ /

R

=

g /

\

Ph

(252)

(10%). Fluorenone-sensitized irradiation failed to yield cyclobutane and gave the hexatriene in an enhanced yield (46%) and also its debromination product (5%). 167

The co-dimerization of benzofuran with 2-phenyl- and 2-(3-pyridyl)-benzofuran affords the two dimers (253) and (254).168 The reaction is thought to involve the addition of excited singlet benzofuran to the ground-state 2-arylbenzofuran. When benzofuran and methyl benzofuran-2-carboxylate are

R = Ph b; R = 3-pyridyl

(253) a;

(254)

(255) a; b;

R1 = H, R2 = C0,Me R1 = R2 = C0,Me

0

irradiated only one co-dimer (255a) is obtained. Another dimer, the homodimer (255b), is also formed, and in this instance it is thought that singlet excitedstate furancarboxylate is involved with ground-state benzofuran. This proposal has further substantiation from the isolation of the two keto-compounds (256) and (257) which are most likely formed by addition of the carboxylate group to benzofuran yielding an unstable oxetan or an unstable adduct such as (258), either of which could collapse thermally to the observed products. A study of the photochemical dimerization of 5,6-dichloroacenaphthylene(259a) has shown that the two dimers (260a) and (260b) arise from the triplet manifold.lsO Only a triplet state is involved as seen by the lack of influence on product distribution in the presence of a triplet quencher (ferrocene). The influence of the intramolecular heavy-atom effect is obviously important in this respect. External le7

168 1119

K. Ueda and F. Toda, Chem. Letters, 1975, 257. K. Takamatsu, H A . Ryang, and H. Sakurai, J. Org. Chem., 1976, 41, 541. J. C. Koziar and D. 0. Cowan, J . Amer. Chem. Suc., 1976,98, 1001.

353

Photochemistry of OleJins, Acetylenes and Related Compounds

(260) a; /3-H b; or-H

(259) a; R = C1 b;R=H

(258)

heavy-atom effects are also measurable when the irradiations are carried out in the presence of various concentrations of ethyl iodide ( 0 - 1 .O moll-1). Above this concentration, a reduction in dimerization efficiency is found. The dimerization of this acenaphthylene (259a) is very similar to that of acenaphthylene (259b).170

7 Reactions of Acetylenic Compounds The hydrogen-abstracting properties of ynones continue to be exploited. Thus the irradiation of 1 -phenylprop-2-yn-l-one in alcohols (methanol, ethanol, propanol, or isobutanol) affords 1 : 1 adducts (261) which cyclize to the furan PhCOCH=CHCHROH (261) R

=

Ph

Ak R 0

H, Me, Et, or Pri (262)

derivatives (262).171 Calculations dealing with the shapes of the excited states of acetylene have been p~b1ished.l~~ The photoaddition of methyl phenylpropiolate and 4-phenylbut-3-yne-2-one to cyclohexa-l,4-diene yields the cyclobutene (263) and the bicyclopropyl

0; (263) R

=

C0,Me or COMe

(264) R

gb

@

Ph

Ph R C0,Me or COMe

=

(265)

Ph

0% 'Ph

Ph

Ph (268)

(266) 171 172

D. 0. Cowan and J. C. Koziar, J. Amer. Chem. SOC.,1974,96, 1229. T. Nishio and Y . Omote, Chem. Letters, 1976, 103 (Chem. Abs., 1976, 84, 89 920). D. Demoulin, Chem. Phys., 1975,11,329 (Chem. Abs., 1976,84,89 341).

Photochemistry

354

derivative (264).160 The triarylphosphine (265) is converted into (266) upon irradiation in benzene The route to product involves fission of a C-P bond to yield the radicals (267) and (268); recombination of these gives the product. 8 Miscellaneous Reactions The interest shown during the past few years in the photochemical ring-opening of azirines [e.g. (269)] to produce ylides [e.g. (270)] has been maintained. A detailed reinvestigation of the photochemistry of the azirine (269) has shown that the ylide (270) is formed quantitatively at - 196 "Cin a DMBP glass.17*Addition of the ylide to (269) to yield (271) takes place at - 160 "C. A related report

R1R2

Y

N- Ph

(272) a; R1 = R2 = Me b; R1 = R2 = H c; R1 = Me, R2 = H d; R1 = R2 = Ph

describes the addition of the ylides derived from the azirines (272) and (269) to esters substituted with electron-withdrawing g ~ 0 u p s . lThe ~ ~ addition of the ylides from the azirines (272a-c) to diethyl benzoylphosphonate affords the phosphonate (273) by a regiospecific but not stereospecific addition.176 The additions to diethyl ethoxycarbonylphosphonate and diethyl vinylphosphonate have also been re~0rted.l'~ The azirine (274a) undergoes ready conversion into the oxazoline (275) when irradiated in benzene.17' As with other azirine systems, it is likely that this reaction proceeds by ring-opening to the ylide (276a) which is trapped intramolecularly by the hydroxyl function. The azirine (274b) also ring-opens to an ylide which in the presence of methyl trifluoroacetate yields the adduct (277). However, in the absence of a dipolarophile a 1,4-group migration takes place to give the diene (278b). This 1,4-migration occurs when a good leaving group is attached to the azirine, and the rearrangement yielding (278c-e) is encountered 178 174 l7~i

176

177

W. Winter, Tetrahedron Letters, 1975, 3913. A. Orahovats, H. Heimgartner, H. Schmid, and W. Heinzelmann, Helv. Chim. Actu, 1975, 58, 2662. P. Gilgen, H.-J. Hansen, H. Heimgartner, W. Sieber, P. Uebelhart, H. Schmid, P. Schonholzer, and W. E. Oberhansli, Helu. Chim.Acta, 1975, 58, 1739. N. Gakis, H. Heimgartner, and H. Schmid, Helv. Chim.Acta, 1975, 58, 748. A. Padwa, J. K. Rasmussen, and A. Tremper, J.C.S. Chem. Comm., 1976, 10.

Photochemistry of OleJns, Acetylenes and Related Compounds

(274) a; X b; X c; x d; X e; X f; X g; X

= = = =

= = =

(275)

OH OAC

355

(276) a; X = OH g; X = OMe

(277)

c1

Br

OCOPh OSiMe, OMe P h y N y H

MeO*Ph

"Y" CH,OMe

X (278) b, c, d, and e.

(279)

(280) a; IZ = 3 b; it = 2 c;n=l

(281) n = 3, 2, or 1

(282) n

=

3, 2, or 1

d;n=O

with the azirines (274c-e). When trimethylsilyloxy is the substituent, as in (274f), extensive decomposition takes place and no diene is formed. With a methoxy-group, as in (274g), addition of the ylide (2768) to the azirine yields (279).177 The spiroazirines (280a-c) all undergo photochemical ring-opening to the ylides (281) which are trapped by methanol as the imines (282) in high yields.17* These imines can be readily hydrolysed to cycloalkanones and benzaldehyde. The spiroazirine (280d) behaves in a different fashion and affords many products (Scheme 13) which appear to result from three modes of fragmentation, viz. (i) formation of carbene (285) followed by trapping as (283) and photochemical ring-opening to an ylide (286) or isocyanide (284), (ii) fission to benzonitrile, and

(280d)

-% PhCN

+

' " y N v

PhCH(0Me)NC

(284) 8%

PhCH(OMe), i-

5%

4%

+

+

+

6%

PhCH(0Me)CN 8%

+

Me0 )=NCH,OMe Ph

Scheme 13 17*

yN

Ph OMe (283) 20%

A. Padwa and J. K. Rasmussen, J. Amer. Chem. SOC.,1975,97, 5912.

14%

Photochemistry

356

(iii) formation of an ylide (287) which can be trapped when the irradiation is carried out in the presence of methyl trifluor~acetate.~~~ Review articles have dealt with the cycloaddition reactions of 1 -azirines 170 and also with the intramolecular reactions of nitrile ylides generated photochemically from azirines.laO Padwa and Carlsen have examined the photochemistry of

R+

R2

(288) a; R1 = R2 = H b; R1 = H, R2 = Me c; R1 = Me, R2 = H

(289)

the substituted azirines (288) and have observed that the products formed (289) are the result of 1,l-addition. The authors reason that as a result of molecular constraints the normal ‘two-plane’ orientation of the nitrile ylide is impossible.

(290)

(291)

(292) R

~“CH,CH=CH,

N 4 (295) 170 180 lS1

H or Me

PhFN

N

Ph

=

CH,CH= CH,

(296)

D. J. Anderson and A. Hassner, Synthesis, 1975, 483. A. Padwa, Angew. Chem. Internat. Edn., 1976, 15, 123. A. Padwa and P. H. J. Carlsen, J. Amer. Chem. SOC.,1975,97, 3862.

\“CH,CH=CH,

(297)

Photochemistry of Olejins, Acetylenes and Related Compounds

357

However, rehybridization, as suggested by Salem,lSzwould permit addition of the bent ylide to the isolated double bond. Interestingly this carbene-like addition takes place with inversion of the geometry of the r-system. A carbene-like intermediate (290) is also proposed to account for the formation of the product (291) [obtained from the irradiation of the azirine (292)] uia the ylide (293) in the absence of d i p o l a r ~ p h i l e s .When ~ ~ ~ such a reagent is added to the reaction mixture, the ylide (293) is trapped as e.g. (294). Intramolecular cycloaddition, yielding the imidazole (295), has been reported for the nitrile ylide (296) formed on irradiation of the azirine (297).lS4 The azirine (298a) also underwent photochemical ring-opening, but in this instance cyclization did not occur and the Ph

yJ C02Et

R2 (298) a; R1 = Me, R2 = C0,Et b; R1 = Ph, R2 = H

(299)

R (300) a; R1 = Me, R2 = C0,Et b; R' = Ph, R2 = H

(301) R

=

C02Me or CN

azatriene (299) was produced by a 1,5-hydrogen shift. The formation of the ylide (300a) in this instance was demonstrated by the formation of the adducts (301) when methyl acrylate or acrylonitrile were used as dipolarophiles. Trapping of the ylide (302) was demonstrated for photolysis of the azirine (303) when (304a) and (304b) were isolated. The styrylazirine (298b) is also photolabile, but

(303)

(304) a; R1 = CO,Me, R2 = H b; R1 = H, RS = C0,Me

(305)

in this instance the ylide is intramolecularly trapped as the benzazepine (3O5).ls4 A full account of the scope of the intramolecular cycloaddition reactions of vinylazirines [e.g. (306)]has been p ~ b 1 i s h e d . l ~ ~ At low temperature (77 K), the oxirans (307) undergo ring-opening on irradiation in 2-methyltetrahydrofuran.ls6 The intense colour developed under these L. Salem, J. Amer. Chem. SOC., 1974, 96, 3486. A. Padwa, A. Ku, A. Mazzu, and S. I. Wetmore, jun., J. Amer. Chem. SOC.,1976, 98, 1048. la' A. Padwa, J. Smolanoff, and A. Tremper, J. Org. Chem., 1976,41, 543. lab A. Padwa, J. Smolanoff, and A. Tremper, J. Amer. Chem. SOC.,1975,97,4682. la6 G. W. Griffin, D. M. Gibson, and K. Ishikawa, J.C.S. Chem. Comm., 1975, 595. lB2

Photochemistry

358

R

(306) R

=

R1 COzMe, CN, COPh, or CHO (307) a; R1 = Rz = H b ; R1 = NOz, R2 = H C; R1 = C1, R2 = H d; R1 = Br, R2 = H e; R1 = Me, R2 = H f ; R1 = H,R2 = C1 g; Rf = H, R2 = I

conditions is possibly attributable to the formation of a carbonyl ylide (308). At room temperature the irradiation of a degassed solution of (307a) gives phenylmethoxycarbonylcarbene, which can be trapped by addition to 2,3-dimethylbut2-ene. The influence of protic solvents (MeOH, EtOH, Pr'OH, or MeC0,H) on the photochemistry of the a7/3-epoxy-esters(309) has shown that a heterolytic

(309) R1-R2

=

(CH,),

R1-R2 = (CH,), R1 = Et, Rz = Me bond-fission process is important in formation of the product (31O).ls7 No evidence for free-radical fission was reported, although this is usually the principal process when such compounds are photolysed in non-protic media. Brightwell and GriffinlS8reported earlier that direct irradiation of the arene oxide (311a) induces an oxygen 'walk' followed by ring-opening to the oxepin (312) (Scheme 14). Two further examples of this rearrangement reported by Griffin and his co-workersla9 involve the rearrangement of (311b and c) to the oxepins (312b and c). The rearrangement is thought to arise from the singlet state, and in corro-

hv I

direct

(311) a; R = H b;R=CN C; R = COzMe

Scheme 14 la'

M. Tokuda, V. V. Chung, A. Suzuki, and M. Itoh, J. Org. Chem., 1975,40, 1858. N. E. Brightwell and G. W. Griffin, J.C.S. Chem. Comm., 1973, 37.

Photochemistry of Olefins, Acetylenes and Related Compounds

359

OH

AOM ACOMe (313)

u C 0 , M e

0

'DozMe (314)

($f0" .OH .

"C0,Me

.

'C0,Me

(3 15)

0

'C0,Me

OH (3 16)

(3 17)

boration of this the rearrangement of the acetyl derivative (313) yields only the phenanthrol (314). Griffin et aZ.lBDreason that the incorporation of the acetyl group ensures the formation of the triplet state of the molecule upon irradiation and that under these conditions the oxygen walk reaction does not take place. Irradiation of the endoperoxide (315) in methanol or cyclohexane gave a low yield of the bis-epoxide (316) (6%) and the diol (317) (15%).lgo Irradiation of the compound (318) at - 190 "C and at room temperature by selective excitation of the biphenyl group (300 nm) leads to the formation of the

R = Ph b; R = 2-naphthyl

(318) a;

ring-opened compound (319).lD1 This pathway for bond fission is preferred since it offers maximum relief of steric strain by the removal of unfavourable interactions.1s2 Under the same conditions, the isomeric compound (320) yields the phenanthrene and trans-stilbene, again following the path which best eliminates steric interactions within the molecule. No evidence for a wavelength effect was found with the compounds (318) when they were irradiated either into the phenyl groups E253.7 nm in (318a)l or into the naphthyl groups [253.7 or 313 nm in (318b)], but a wavelength effect was observed for (320). When the phenyl groups in this compound are irradiated (253.7 nm) the cis-phenyl groups experience considerable interaction in two directions : two reaction paths result, G . W. Griffin, S. K. Satra, N. E. Brightwell, K. Ishikawa, and N. S. Bhacca, Tetrahedron Letters, 1976, 1239. lgo S. A. Cerefice and E. K. Fields, J . Org. Chem., 1976,41, 355. lS1 G. Kaupp and W. H. Laarhoven, Tetrahedron Letters, 1976, 941. Iga G . Kaupp, Angew. Chem. Znternat. Edn., 1974, 13, 817.

360 Photochemistry one yielding phenanthrene and stilbene in 82% yield, and the other giving the ring-opened material (319a) in 18% yield.lQ1 Chapman et aZ.lQ3have demonstrated that matrix-isolated benzocyclobutene (321) does not undergo photochemical degradation into benzyne and acetylene when it is irradiated at 8 K. The cis-di-iodide (322a), the precursor of the benzocyclobutene, is photochemically labile. Irradiation of the trans-di-iodide (322b) R1

Ph

, K * = i

(323) a ; R, R

=

CHOEt

b;R=H

gives only the cis-isomer (322a) as the primary photochemical product. The cis-isomer, however, produces the trans-isomer (as the major product) together with the benzocyclobutene. Irradiation of the pyrylium salt (323a) in 0.5-5 mol-1 sulphuric acid affords (323b).lQ4 A reinvestigation of the photochemistry of dioxan has confirmed the formation lQ6Two of a pair of diastereomeric dioxan dimers of gross structure (324).lQ6~ other products, the diastereoisomeric alcohols (325), were also obtained in this

(325)

(326) a; R1 = R2 = alkyl b; R1 = Me, R2 = H

study.lQs Benzophenone (or any other good hydrogen-abstracting ketone) has long been known as a useful reagent for the photochemical generation of free radicals. This technique provides a method for the decomposition of cyclic acetals,lQ7presumably via free radicals such as (326a) ID*and (326b).lQQProducts AcOH,C

AcoH2cQoAc AcO" OAc

0 <.CONH2

r;l

AcO"

(329) R

R

lg3 lg4 lg5 leg lD7 lD8 lDe

R

OAc

= =

CY-OAC S-OAC

AcOH,C

0 -,CMe,OH

AcO'.V

OAc

OAc (330)

0. L. Chapman, C. C. Chang, and R. N. Rosenquist, J . Amer. Chem. Soc., 1976,98,261. V. P. Karmazin, E. P. Olekhnovich, M. I. Knyazhanskii, and G. N. Dorofeenko, Zhur. org. Khim., 1975, 11, 1137 (Chem. Abs., 1975, 83, 193 005). P. H. Mazzocchi and M. W. Bowen, J. Org. Chem., 1975,40,2689. K. Pfordte, Annalen, 1959, 625,30. D. L. Rakhrnankulov, E. P. Serebryakov, V. N. Uzikova, and S. S. Zlotskii, Trudy Ufim. Neft., 1974,16,266 (Chem. Abs., 1975, 83, 163 318). C. Bernasconi and G . Descotes, Compt. rend., 1975, 280, C, 469. R. D. McKelvey, Carbohydrate Res., 1975, 42, 187 (Chem. Abs., 1976, 84, 120 769).

Photochemistry of Olefins, Acetylenes and Related Compounds

361

also arise via a free-radical path involving (327) when the ethyl acetal of acrolein is irradiated in the presence of benzophenone.200 Free radicals produced in this manner can be trapped by suitable substrates, as by the addition of tetrahydrofuranyl radicals to phenylacetylene 201 or of formamidyl radicals to the pyranose (328), yielding (329) and (330).202 aoo

R. Sastre, M. V. Dabrio, and Y.J. L. Mateo, Anales de Quim., 1974, 70, 905 (Chem. Abs.,

aol

H. Hasegawa, T. Satake, and T. Mikami, Nippon Kagaku Kaishi, 1974, 12, 2356 (Chem. A h . , 1975, 82, 118 133). A. Rosenthal and M. Ratcliffe, Cunad.J. Chem., 1976,54,91.

1975, 83, 177 805). aoa

4 Photochemistry of Aromatic Compounds BY A. GILBERT

1 Introduction The format of this chapter follows that used in Volume 7. The subject matter reviewed in this section is restricted to the more chemical aspects of the photochemistry of aromatic compounds, and in particular only reactions in which the arene ring is chemically involved will be considered. The more physical and spectroscopic aspects are reviewed in Part I. 2 Isomerization Reactions Some years ago it was pointed out from orbital and state symmetry considerations that the then known photochemistry of benzene could be rationalized by ascribing to the S1 state of benzene (lBau) a tendency to transform into a 1,3-biradical (1) (possibly with some zwitterionic character): the Tl state (3B,,) could have the characteristics of a 1,4- (2) or 1,2-biradical (3).l It has since been

I

shown that not only the 1,3-processes, but also some of those which involve the 1,2- and 1,4-positions arise from the S, state;2 but 1,Qbonding to form Dewarbenzene involves the S2 state (1B,u).2aOther factors such as charge-transfer in the ground and/or excited states are important in certain cases3 Irradiation of benzene in the presence of Me3SnH has not provided any clear evidence for free radical intermediate^.^ There was no observable reaction with triplet benzene: excited singlet benzene did apparently react, but 'only scarcely' and the products were not described. It would seem that the Me3SnH acted as a moderately effective intersystem crossing catalyst. Light-induced interconversions of the xylenes have been studied in past year^,^ and the investigations have now been extended to the perfluoroD. Bryce-Smith and H. C. Longuet-Higgins, Chem. Comm., 1966, 593. D. Bryce-Smith, Pure Appl. Chem., 1973, 34, 193, and references therein. m D. Bryce-Smith, A. Gilbert, and D. A, Robinson, Angew. Chem. Internat. Edn., 1971,10, 745. D. Bryce-Smith, Chem. Comm., 1969, 806. * A. Delaby, D. Rondelez, and S. BouC, J . Phorochem., 1975,4,399. ti See R. B. Cundall, D. A. Robinson, and A. J. R. Voss,J . Photochem., 1974, 2,221, 231,239, and references therein. a

362

Photochemistry of Aromatic Compounds

363

derivatives.6 In the gas phase, the ortho isomer is the most reactive, and the para isomer the least. para-Bonded and prismane valence bond isomers are considered to be probably involved: of the possible six isomers of the former type, perfluoro-1,2-, -1,3- and -2,5-dimethylbicyclo[2,2,0]hexa-2,5-dienes(4a, b, and c) have been characterized, and the 2,3- and 2,6-isomers (4d and e) tentatively identified. The reported observations are uniquely accommodated by the processes outlined in Scheme 1 where the interconversions involve six para-

M F * F

F

TL CF3

M

=acF3 F

F

F

M. G . Barlow, R.N.Haszeldine, and M. J. Kershaw, J.C.S. Perkin Z, 1975,2005.

364 Photochemistry bonded isomers and three prismanes. No evidence was found for the formation of benzvalenes and triplet species: the former are believed to be involved in many other light-induced positional isomerization reactions of non-fluorinated benzenoid compounds. The same workers have also examined the dienophilic reactivity of some perfluorobicyclo[2,2,O]hexa-2,5-dienes and benzvalenes towards furan, cyclopentadiene, and buta-l,3-diene.' Ratajczak has described photochemical para-bonding in some substituted perfluorobenzenes (Scheme 2), and has studied the corresponding thermal X

Fii F5 X = F, CF3, H, Me, Br, or OMe Scheme 2

rearomatizations in the gas phase: this back reaction is unimolecular, and the first and rate-determining step is reported to be formation of a biradical, presumably by homolysis of the 1,4-bond.* Last year Barltrop and Day suggested that the analysis of photo-transposition reactions of six-membered aromatic compounds can be greatly simplified if attention is fixed on the pattern of transposition, and they pointed out that in no case has the movement of all six ring atoms been followed throughout a r e a c t i ~ n . ~ Their novel mode of analysis has now been applied to the light-induced transposition reactions of various 2- and 4-hydroxypyrylium cations, first reported by Pavlik and Clennan,lo and, for the first time, the fate of all the ring carbon atoms in such processes has been defined.ll Various mechanisms are consistent with the observed permutation patterns described as P4 [e.g. ( 5 ) and ( 6 ) ] , but the

BH (5)

authors consider that outlined in Scheme 3 involving oxabicyclohexenyl cations (7) to be the most attractive: ab initio SCF calculations support the view that 2,6-bonding is a most likely consequence of electronic excitation of 4-hydroxypyrylium cations. The treatment of these processes by permutation patterns

* lo l1

M. G. Barlow, G. M. Harrison, R. N. Haszeldine, R. Hubbard, M. J. Kershaw, and D. R. Woodward, J.C.S. Perkin Z, 1975, 2010. E. Ratajczak, Pr. Nauk Znst. Chem. Technol. Nafty. Wegla. Politech. Wroclaw, 1974, 17, 3 (Chem. Abs., 1975, 83, 130 901). J. Barltrop and A. C. Day, J.C.S. Chem. Comm., 1975,177. P. W. Pavlik and E. L. Clennan, J. Amer. Chem. Soc., 1973,95, 1697. J. Barltrop, R. Carder, A. C. Day, J. R. Harding, and C. Samuel, J.C.S. Chem. Comm., 1975, 729.

Photocheniistry of Aronlatic Compounds

365

Scheme 3

and the assertion in ref. 9 that there is virtually no case in which any connection has been established between the formation of Dewar and prismane isomers and the occurrence of phototransposition have prompted Chambers and co-workers l2 to draw attention to their earlier work on the isolation of such valence isomers in the pyridazine-pyrazine conversion l3 and to the similar studies by Haszeldine and co-workers on the intermediacy of valence isomers in the light-induced rearrangement of perfluor~alkylbenzenes.~~ They have suggested that the substituent labelling of the 4,5- and 3,5-positions in the pyridazine l3 constitutes an example in which the permutation pattern is clearly defined,12 but it should be noted that the two nitrogens involved in the pyridazine-pyrazine conversion are not distinguishable in the absence of isotopic labelling. The photoisomerization of pentakis(pentafluoroethy1)pyridine to the ‘Dewar’ isomer and azaprismane was reported in 1 9 7 3 9 irradiation of the pyridine derivatives (8) has now provided evidence for the type of rearrangement previously noted with pyridazines.12 In CF,ClCF,Cl as solvent, (8) undergoes quantitative conversion into a 1 : 1 mixture of the two azaprismanes (10) and (ll), together with a small amount of (9). It is consistent with the results reported in ref. 15 that (10) and (11) only slowly rearomatize at 175 “C into (12), (13), and (14). The pathway for formation of (10) has not been definitely established, but (15) and (16) are suggested as likely intermediates. Such isomers as (11) and (17) have been previously postulated but not isolated in the phototranspositions of some pyridines.16 The same group have also published full details of their earlier report 13a on experiments designed to isolate the intermediates in the pyridazinepyrazine conversion.17 Various perfluoroalkylpyridazine derivatives (1 8) have been irradiated at 254 and 300 nm in a flow system. Since the same results were observed at both wavelengths it is assumed that absorption of energy by either the rr* or nr* states can lead to rearrangement (this does not necessarily imply that the bonding changes proceed directly from either state). Perfluoro-l,2diazobicyclo[2,2,0]hexa-2,5-dienes (19) have been isolated in some cases, and l2 l3

l4 l6 l6

l7

R. D. Chambers, R. Middleton, and R. P. Corbally, J.C.S. Chem. Comm., 1975, 731. ( a ) R. D. Chambers, W. K. R. Musgrave, and K. C. Srivastava, J.C.S. Chem. Comm., 1971, 264; (b) R. D. Chambers, J. A. H. McBride, J. R. Maslakiewicz, and K. C. Srivastava, J.C.S. Perkin I , 1975, 396. M. G. Barlow, R. N. Haszeldine, and M. J. Kershaw, J.C.S. Perkin I , 1974, 1736. M. G. Barlow, R. N. Haszeldine, and J. D. Dingwall, J.C.S. Perkin I, 1973, 1542. T. J. van Bergen and R. M. Kellogg, J. Amer. Chem. SOC.,1972,94, 8451. R. D. Chambers, J. R. Maslakiewicz, and K. C. Srivastava, J.C.S. Perkin I, 1975, 1130.

13

366

Photochemistry

R' h / N R

/R3 2

z&R3

(16)

R2 R1= CF, CF(CF,), R3 = CF2CF,

R2

(17)

these are converted thermally or photochemically, or in a process catalysed by firebrick, into the corresponding 2,5-diaza-compounds (20). The transformation of these p-bonded isomers into the pyrazines (21) establishes a new process of lY3-shiftsin aromatic systems. Similar results were noted with the perfluoropyridazine (22), and the authors also comment on the remarkable thermal stability of perfluoro-lY4-diethyl-2,5-diazabicyclo[2,2,0]hexa-2,5-diene (23).

F

(20) (23)

(19)

(18)

(21)

R1 = R2= C2F6

R1,R2 = perfluoroalkylgroups

(22)

Permutation pattern analysis has been used to interpret the phototranspositions observed with 2- and 3-~yanopyrroles.~* By this approach it has been shown that irradiation of (24), (25), and (26) at 254 nm causes the nitrogen atom and the J. Barltrop, A. C . Day, P. D. Moxon, and R. R. Ward, J.C.S. Chem. Comm., 1975, 786.

367

Photochemistry of Aromatic Compounds

Me

H

Me

CN

H

LN

H

major

(26)

minor

C-5 carbon to interchange. Of the four patterns (Figure l), Pais suggested to be common to the transformations, and a mechanism (Scheme 4) involving initial 2,5-bonding followed by skeletal rearrangement is proposed: see also Part 111, Chapter 6 .

P1 Figure 1

5

H'

-

L l Y

major

minor Scheme 4

3 Addition Reactions The irradiation of benzene in aerated aqueous solution to yield an oxygenated product was reported in 1968.1e Over the years there has been controversy concerning the structure of this, and indeed whether oxygen is necessary for its formation: all the workers do, however, appear to agree that benzvalene is an intermediate in the process.2o These contradictions over the requirement for oxygen have been re-investigated by Stein and co-workers, who claim to have shown that when the reaction is carried out in the absence of oxygen in l8 2o

G. Farenhorst, Tetrahedron Letters, 1968, 4835. See Vol. 5, p. 474 for earlier reports.

368

Photochemistry

degassed solution, the photoproduct rapidly reaches a low concentration beyond which its formation is very slow.21 The results are thus consistent with a previous proposal that oxygen is essential for the formation of appreciable yields of the product. The formation of the photoproduct in the presence of oxygen at 254 and 214 nm apparently follows regular kinetics, whereas in the absence of oxygen (and the authors point out that they are never certain that this is completely achieved) it was difficult to establish the formation of product let alone determine its yield. The structure of the product is still in some doubt, for while Stein and his co-workers now agree that it is a cyclopentadienyl aldehyde, they appear to favour the R group in (27) as OH whereas the original proposal had R = H.22 They point out that although resolution of this difference is necessary for the elucidation of the detailed mechanism, such a discrepancy does not affect their conclusions concerning the importance of oxygen in the reaction. The process appears to be independent of wavelength over the range 214265 nm. This result, considered in conjunction with the known wavelength dependence of benzene fluorescence, supports the view that the essential photochemistry originates in non-relaxed states prior to the formation of the

thermalized fluorescent level : this is wholly consistent with formation of benzvalene as the primary step, and the production of this valence isomer from such states has been substantiated by studies in the presence of xenon.23 The use of xenon as a probe for the multiplicity of the photoreactions of aromatic compounds, where more common methods may be ambiguous or inconclusive, has been further advocated by Morrison and his co-workers, and used by them to show that the intramolecular 1 ,2-cycloaddition of 6-phenylhex2-yne to yield (28) is a singlet process.24 It has been reported that in 1,2-cycloadditions of ethylenes to benzene to yield products of type (29), donor and acceptor ethylenes give exclusively endo and exo products, re~pectively.~~ It was further shown from n.m.r. evidence that these stereospecificities were matched by corresponding stereospecific ground-state

21 22

23 z4 25

Y. Ilan, M. Luria, and G. Stein, J. Phys. Chem., 1976, 80, 584. L. Kaplan, L. A. Wendling, and K. E. Wilzbach, J . Amer. Chem. SOC.,1971,93, 3819. Y. Ilan and G. Stein, Chem. Phys. Letters, 1975, 31, 441. H. Morrison, T. Nylund, and F. Palensky, J.C.S. Chem. Comm., 1976,4. D. Bryce-Smith, A. Gilbert, B. H. Orger, and H. M. Tyrrell, J.C.S. Chem. Comm., 1974, 334.

Photochemistry of Aromatic Compounds

369

interactions between the ethylenes and benzene.2s For example, acrylonitrile has an exo ground-state orientation with 27 and the photoadduct 28 of the reactants has now been shown to have the exo structure (30) by comparison of its maleic anhydride adduct with that synthesized by conventional thermal procedure^.^^ Using n.m.r. spectroscopic methods, the fumarodinitrilebenzene ground-state complex is deduced to have the orientation (31),27 and irradiation of this system yields 1 : 1 (major) and 2 : 1 (minor) adducts of unspecified It is well known that although tetracyanoethylene forms a charge-transfer complex with benzene, irradiation of the system does not yield adducts. However, irradiation within the charge-transfer absorption band of the complex of this dienophile with toluene has been reported to yield 3-phenylpropane-l,1,2,2-tetracarbonitrile. The reaction is accelerated in methanol solution, probably due to the involvement of a proton-transfer step.31 The 2 : 1 photoaddition of maleic anhydride and maelimides to benzene has been known for many years but examples involving substituted derivatives of the addends continue to be reported. Thus the photoreaction of N-ethylmaleimide and isopropylbenzene has been examined in detail, and the equilibrium constant of the ground-state complex between the addends has been determined The products from 254 nm radiation are C-cumylby n.m.r. spectros~opy.~~ N-ethylsuccinimide, the 1,2-cycloadduct, and the 2 : 1 adduct bearing an isopropyl group on the ethylenic bond, consistent with previous studies on such Although the 1,3-cycloaddition of ethylenes to benzene has been known for ten years, until this year it has been restricted to hydrocarbons and vinyl ethers as the e t h y l e n e ~ .Two ~ ~ reports have now appeared which describe the 1,3-cycloaddition of functionalized ethylenes to b e n ~ e n e34. ~ ~The ~ former of these describes the facile 1,3-cycloaddition of vinyl acetate to benzene which yields the adduct (32) in a purity >95% from preparative experiments: its use as a convenient route to semibullvalene, however, appears at present not to be viable.33 It has earlier been reported that the relative quanta1 efficiency for formation of 1,2- and 1,3-cycloadducts [i.e. (29) and (33), respectively] of ethylenes and benzene may be predicted from the ionization potential difference between the addends.25 In particular, where an ethylene has marked acceptor or donor properties with ionization potentials >9.6 or <8.65 eV, it was found that the quantum yield for 1,Zaddition was larger than that for the corresponding 1,3-process,thereby indicating the necessary involvement of an element of chargetransfer to or from the ethylene in the former process: the work described in ref. 33 supports this prediction. On the other hand, Heine and Hartmann have now claimed that the major product from the photoaddition of benzene and D. Bryce-Smith, A. Gilbert, and H. M. Tyrrell, J.C.S. Chem. Comm., 1974, 699. P. Schuler and H. Heusinger, J. Mol. Structure, 1975,28,25. 28 B. E. Job and J. D. Littlehailes, J. Chem. SOC. ( C ) , 1968, 886. 29 R. J. Atkins, G. I. Fray, and A. Gilbert, Tetrahedron Letters, 1975, 3087. 8 o H. Tamm and H. Heusinger, J. Mol. Structure, 1975, 26, 303. M. Ohashi, S. Suwa, and K. Tsujimoto, J.C.S. Chem. Comm., 1976,404. 31e See for example Vol. 7, p. 357. 32 See Vols. 1-7 for pertinent past references. 53 A. Gilbert and M. W. bin Samsudin, Angew. Chem. Internat. Edn., 1975, 552. 54 H. G. Heine and W. Hartmann, Angew. Chem. Internat. Edn., 1975, 708. 28

27

370

Photochemistry

vinylene carbonate (ionization potential 10.1 eV) is the 1,3-adduct (34), with minor amounts of the isomer (35) and the 1,4-cycloadduct (36).34 It would appear at first sight that the vinylene carbonate-benzene reaction is an exception to the rule outlined in ref. 25 in that, 1,Zadducts would have been expected as major products. It is, however, important to realize that the predictions referred to quantum yields and not the chemical yield described in ref. 34, and that the concentrations (9 : 1 benzene : ethylene) employed in ref. 34 would not be conducive to the production of 1,2-adducts (which is known to be generally favoured by high ethylene c ~ n c e n t r a t i o n s )35. ~ The ~ ~ reason for this is that some 1,Zadducts are very photolabile, and high benzene concentrations where the yield of TI benzene is high 36 favour triplet photosensitized decomposition. Thus it remains to be seen whether the empirical rule in ref. 25 has really been violated by the result now reported for vinylene carbonate. Srinivasan has continued to use the 1,3-cycloaddition reaction as a route to novel polycyclic ring systems, and has described the chemical transformations of the 1,3-photoadducts of cyclobutenes with benzene and anisole to give (37) by a four-stage Some years ago two groups independently reported the 1,2- and 1,4-cycloadditions of triplet dichlorovinylene carbonate to benzene.38 Scharf and his group have now studied further aspects of this process, and have described in detail the addition of this ethylene to both benzene and n a ~ h t h a l e n e . ~ ~ Sensitizers having ET > 68 kcalmol-1 are effective: the triplet of dichlorovinylene carbonate yields the 1,2- and 1,4-~ycloadductsin consecutive reactions with both benzene and naphthalene whereas S1naphthalene yields the products in parallel processes via a common intermediate. Both 1,Zadducts (38) and (39), and 1,4-adducts (40) and (41) are formed from naphthalene. Three isomeric 1 : 1 photoadducts of vinylfluoride and benzene have been detected, but their structures are as yet unknown.40

3B

K. E. Wilzbach and L. Kaplan, J. Amer. Chem. SOC.,1971,93,2073. R. B. Cundall and D. A. Robinson, J.C.S. Faraday ZZ, 1972,68, 1691. G. Subrahmanyam and R. Srinivasan, Tetrahedron, 1975,31, 1797. G. Hesse and P. Lechtken, Angew. Chem., 1971, 83, 143; Annalen, 1971,754, 1; H. D. Scharf and R. Klar, Tetrahedron Letters, 1971, 517; Chem. Ber., 1972, 105, 575. H. D. Scharf, H. Leismann, W. Erb, H. W. Gaidetzka, and J. Aretz, Pure Appl. Chem., 1975,

40

41 581. S. Tsunashima, H. E. Gunning, and 0. P. Strausz, J. Amer. Chem. SOC.,1976, 98, 1690.

86 97 88

371

Photochemistry of Aromatic Compounds

-.q (39)

(41)

(40)

The early observations by Koltzenberg and Kraft on photoadditions of 1,3dienes to benzene 41 have been followed up by several groups of Yang and co-workers have contributed much to the understanding of these reactions and now report the addition of 1,2-dihydrophthalic anhydride to benzene, naphthalene, and anthracene to yield adducts (42), (43), and (44) respecti~ely.~~ The formation of (42) is most interesting not only because cyclohexa-1,3-diene undergoes ring-opening in preference to benzene addition, but more importantly because this adduct is a very realistic precursor for the as yet unknown 1,41',4'-dimer (45) of benzene: cf. ref. 3. We look forward to reading soon of the

"

(45)

(46) 'l

4L

K. Kraft and G . Koltzenburg, Tetrahedron Letters, 1967, 4357, 4723. N . C. Yang, C. V. Neywick, and K. Srinivasachar, Tetrahedron Letters, 1975,4313.

372

Photochemistry

synthesis and properties of this ClzHlzisomer. Preliminary results suggest that sensitized irradiation of (42) yields (46). The original two independent reports of the photoaddition of furan to benzene appeared to be in conflict, particularly over the structures of the a d d u c t ~ .The ~~ workers have now collaborated and it transpires that most of the discrepancies arise from the thermal and photolabilities of the major adduct (47) and its Cope-rearranged isomer (48).44 Thus when high-intensity light sources are used and/or temperatures above ambient in the work-up procedure (i.e. preparative g . ~ . adducts ~ ~ ~ ) (47) and (48) are destroyed and the more photochemically (at

254 nm) and thermally stable minor adducts (49), (50), and (51) remain to be isolated. The question of the quantum efficiency of the process still remains in dispute. It is interesting to note here that not all 1,4-1',4'-diene-benzene adducts simply yield Cope isomers thermally: thus the adduct (52) from 1,2-dimethylenecyclohexane and benzene 45 undergoes a series of intramolecular and retroDiels-Alder reactions which result in the formation of buta-l,3-diene and tetralin.4s During a study of the S1and Tl energies of steroidal transoid dienes, Pusset and Beugelmans observed the formation of four unidentified adducts with both benzene and na~hthalene.~' Light-induced acyclic additions of amines to aromatic hydrocarbons have been known for some ten years,32but with some substituted arenes replacement of the substituent group also tends to occur. Thus primary and secondary aliphatic amines photoreact with fluorobenzenes to give adducts and substitution products, and evidence for an addition-elimination mechanism in the substitution reaction has been p r e ~ e n t e d .Irradiation ~~ (254 nm) of, for example, fluorobenzene with diethylamine gives NN-diethylaniline and the adducts (53)-(55). Adducts reflecting attack of the amine nitrogen at the 1-position and those which contain (a) J. C. Berridge, D. Bryce-Smith, and A. Gilbert, J.C.S. Chem. Comm., 1974, 964; (b) T. S. Cantrell, Tetrahedron Letters, 1974, 3959. 44 J. C. Berridge, D. Bryce-Smith, A. Gilbert, and T. S. Cantrell, J.C.S.Chem. Comm., 1975,611. 46 J. C. Berridge, D. Bryce-Smith, and A. Gilbert, Tetrahedron Letters, 1975, 2325. A. Gilbert and R. Walsh, J . Amer. Chem. SOC.,1976, 98, 1606. 47 J. Pusset and R. Beugelmans, Tetrahedron, 1976, 32, 791. IaD. Bryce-Smith, A. Gilbert, and S. Krestonosich, J.C.S. Chem. Comm., 1976,405. 43

373

Photoclzemistry of Aromatic Compounds

U N E t 2

or

FQtEt2

FDE

&HNEt2

'

(54)

(53)

H

(55)

a HCF group were not detected, and it is suspected that such adducts are unstable, and eliminate HF to give the aniline. Evidence that this may well be the case was provided from the reactions of difluorobenzenes with diethylamine which gave cine-substitution products together with the expected corresponding monofluoroaniline derivatives and various 1,2- or 1,4-acyclic adducts. Major contributions from an aryne intermediate in the formation of the cine-substitution products have been discounted, and an addition-elimination mechanism has been proposed which involves either zwitterionic Wheland-type intermediates [e.g. (56) and (57) from meta-difluorobenzene] and/or unstable chemical adducts [e.g. (58) and (59)]. These reactions provide the first known examples of photochemical cine-subst it ut ion. 48 Two reports within the year have described light-induced cleavage of a benzenoid ring. The irradiation of aromatic nitro-compounds which have either 3n7r* or 3 7 r ~ * lowest states in the presence of aromatic methoxy-compounds leads to selective addition of the nitro-group at the 1,2-positions of the latter a ~ e n e .The ~ ~ resulting adduct is very labile and undergoes fission to yield the diene (60). The second report claims that there is some evidence that the

6"' OMe

ArNO,*

'

A

Me0,C \

C

R2 O

R

'

aliphatic products from irradiation of phenylalanine at 254 nm result from a cleavage reaction of the aromatic ring.50 Although photoreduction reactions are reviewed in Chapter 5 of Part 111, it is worth noting here that aromatic hydrocarbons and phenols undergo photoreduction by aqueous sodium borohydride,61 and that the former are also photoreduced by 1,4-di~yanobenzene.~~ Indeed there have been several accounts in the literature this year in which cyanobenzenes have been incorporated into 4g

6o 61

I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC. Japan, 1975,48, 2865. C . Hasselmann and G. Laustriat, Phorochem. and Photobiol., 1975, 21, 2, 133. D. Bradbury and J. Barltrop, J.C.S. Chem. Comm., 1975, 842. K. Mizuno, H. Okamoto, C. Pac, and H. Sakurai, J.C.S. Chem. Comm.,1975,839.

374

Photochemistry

reaction mixtures, sometimes with very significant changes in pathways. Thus although styrenes normally yield cyclobutane-type dimers, their irradiation in the presence of 1,2,4,5-tetracyanobenzenealso gives l-phenyI-l,2,3,4-tetrahydron a ~ h t h a l e n e .This ~ ~ type of dimer is suggested to arise via an ionic mechanism through photodissociation of the exciplex of the styrene derivative with the cyanobenzene. Photoaddition reactions of naphthalenes, and in particular naphthonitriles, continue to attract considerable interest. McCullough and co-workers have studied the reactions of 1- and 2-naphthonitriles with tetramethylethylene and reported that in benzene solution exciplexes are intermediates in the cycloaddition reaction, but that electron transfer dominates the chemistry in polar While both exciplex emission and adduct (61) formation still occur in acetonitrile, both have lower efficiencies than in benzene. Similarly in methanol the formation of adduct (62), previously described by C a n t ~ e l lfrom , ~ ~ 2-naphthonitrile is totally quenched and the photoreduction products (63) and (64) are

q:; H H

(61)

(62)

R1 = CN, R2 = H R1 = H, R2 = CN

Me Me

NC

RO

(67)

formed. Since the fluorescence of 2-naphthonitrile is quenched by the ethylene at a diffusion-controIled rate in methanol, it is suggested that both the photoaddition reaction and the quenching process in methanol and benzene have different mechanisms. The photoaddition of alkyl vinyl ethers to 2-naphthonitrile has been studied in great detail, and Pac and co-workers have published full details 66 of their earlier communication on this Using 313 nm radiation, only the single endo-[2 + 21 cycloadduct (65) is formed in 80-90% yield, whereas Pyrex-filtered irradiation (A > 280 nm) gives the cyclobutene (66) as the ultimate major product together with various 1 : 1 adducts (67)-(70) and the cyclobutane dimer of (67). These differences in selectivity of the vinyl ether addition are interpreted in terms of differences in the stability of the conE.* 65

56 67

J. J. McCullough, R. C. Miller, D. Fung, and W. s. Wu, J. Amer. Chem. SOC.,1975,97, 5942. T. S. Cantreli, J . Amer. Chem. SOC.,1972, 94, 5929. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Perkin I, 1975, 2221. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1973,219.

Photochemistry of Aromatic Compounds

NC

Q -

RO (68)

375

Ncq % ROSS ,-'

I

NC

(69)

OR

(70)

figurations of the intermediate exciplexes,68as deduced from the solvent dependence of quantum yields and fluorescence quenching. Adducts (67), (68), and (70) are not products of secondary photoreactions, but (69) is formed from (68) and (66) likewise arises from (67). The intermediate (71) is suggested for (66), (67), and (68). Readers should also be aware that the 2-naphthonitrile-methyl vinyl ether system has also been investigated by Chamberlain and McCullough when, together with the products described above, the 1 : 1 adduct (72) was isolated and suggested to arise from the fulvene derivative (73).59 The photochemical behaviour of 1-naphthonitrile has also been studied in the presence of phenylacetic acid derivatives, and from mechanistic studies the S1naphthaleneacid exciplexes, whose reactions are again found to be solvent-dependent (see Scheme 5), are suggested as intermediates.s0 The report describes the photoArCN*

+ RCH,CO,H

+ [ArCN .., RCH2C02H]* p H 6

MeC{

A~CN'

photoproducts

t--

RCH,CO~H

HArCN'

starting materials

+ RCH;I + CO2

Scheme 5

reduction and reductive alkylation of the naphthonitrile by rn- and p-methoxyphenylacetic acid and by phenoxyacetic acid. Thus p-methoxytoluene, 1,4-dihydro-l-naphthonitriie, (74), ( 7 9 , and (76) are formed from the naphthonitrile using the p-methoxy-acid derivative. Libman has also investigated this reaction with acridine as the electron-acceptor component, and the formation 63 b8 68

6o

T. Asanuma, M. Yamamoto, and Y . Nishijima, J.C.S. Chem. Comm., 1975, 609. C. Pac, T. Sugioka, K. Mizuno, and H. Sakurai, Bull. Chem. SOC.Japan, 1973,46, 238. T. R. Chamberlain and J. J. McCullough, Canad. J. Chem., 1973, 51, 2578. J. Libman, J. Amer. Chem. SOC.,1975,97,4139.

376

Photochemistry NC C H 2 e O M e

(74)

I H (77)

of (77) has been suggested to arise via simultaneous or consecutive electron- and proton-transfer from the carboxyl group of the acid to the acridine.61 Since the first report in 1965, there have appeared many enlightening accounts of the additions of diphenylacetylene derivatives to naphthalenes. A further account has now appeared which describes the formation of the adduct (78) from 1,4-diinethoxynaphthaleneand diphenylacetylene, and its thermal conversion into (79) and (80).62 U.V. irradiation of (79) yields (81), and (80) gives (81) more slowly than the corresponding conversions of (82), (83), or (84).

(78) R1 = R2 = OMe (81) R1 = H, R2 = OMe

(79) R1 = OMe, R2 = H (80) R1 = R2 = OMe (82) R' = H, R2 = OMe (83) R1 = K2 = Me (84) R' = R2 = H

This year has seen the publication of several important accounts of the interaction of 1,3-dienes with naphthalene and anthracene derivatives: the role of exciplexes in the photochemical processes has received particular attention. Libman has studied the details of the photochemical behaviour of octafluoronaphthalene towards conjugated dienes and reports that the reaction is markedly dependent on solvent polarity (as with so many other donor-acceptor Thus in cyclohexane solution the naphthalene and 2,4-dimethylpenta-l,3-diene give an 80% yield of a 1 : 1 mixture of the adducts (85) and (86) whereas with acetonitrile as solvent, although there is slow conversion of the naphthalene, the diene is consumed rapidly to yield the diene dimers (87) and (88) together with (85). As with other systems, although. the quenching efficiencies of the 61 62

63

J. Libman, J.C.S. Chem. Comm., 1976, 198. T. Teitei, D. Wells, and W. H. F. Sasse, Ausrral. J . Chem., 1975, 28, 571. J. Libman, J.C.S. Chem. Comm., 1976, 361.

377

Photochemistry of Aromatic Compounds

Ye

AY”

(87)

Me

(88)

naphthalene fluorescence by the diene are similar in the two solvents, the addition efficiencies vary greatly, and in acetonitrile the quantum yield decreases as the ionization potential of the diene decreases. It is suggested from the qualitative correlation between ionization potential of the diene and the quantum yield for the diene dimer formation in acetonitrile (at the expense of adduct formation) that the dimerization involves the intermediacy of solvated charge-transfer complexes or ion pairs. Formation of such intermediates is attributed to the occurrence of a solvent-induced crossing between the covalent and ionic potential energy surfaces of the naphthalene-diene system in acetonitrile. Sensitization of the diene with octafluoronaphthalene does not result in enhanced intersystem crossing, and it is concluded that Tl diene does not play an important role in the naphthalene-induced d i m e r i ~ a t i o n . ~ A~three-component exciplex 65 is suggested as an intermediate in the d i m e r i ~ a t i o n . ~ ~ With diene-arene systems, it is frequently postulated that the light-induced processes involve initial formation of an ‘encounter complex’ which can lead to the exciplex or chemical products. In many systems, the involvement of an exciplex has been conclusively proved and emission from such species is well documented; but as Yang and collaborators point out, the same cannot be said for encounter complexes. From a study of the fluorescence of the 9,lO-difluoroanthracene-2,5-dimethylhexa-2,4-diene(DMHD) system, however, these workers have now observed dual emission in a number of solvents.66 One of these emissions is only 440 cm-1 displaced from the anthracene, exhibits fine structure, and is not affected by change in solvent polarity. It is thus considered that this structured emission has all the characteristics of those expected for an encounter complex ‘and is so identified’. The other fluorescence is featureless and displaced well to the red. Further studies have been reported by the same group on the detection and characterization of exciplexes from anthracene and its halo- and cyano-derivatives with DMHD and their relationship to the photochemistry of 64 66

66

J. Libman, J.C.S. Chem. Comm., 1976, 363. J. Saltiel, D. E. Townsend, B. D. Watson, and P. Shannon, J. Amer. Chem. SOC.,1975,97,5688. N. C. Yang, D. M. Shold, J. K. McVey, and B. Kim, J. Chem. Phys., 1975,62,4559.

378 Photochemistry these The anomalously high quenching constant ( k , ~= 1500) for the fluorescence of 9,lO-dicyanoanthracene by D M H D is accounted for by a ground-state complex of the reactants, and end-absorption extending beyond 430 nm is observed. Two other groups have also commented upon arene-diene exciplexes. Saltiel and co-workers have reported details 66 of the emission properties of the 9,lO-dichloroanthracene-DMHD system in both methanol and acetonitrile.6s In the former solvent, the exciplex has T = 7.4 ns, emits with Am= ca. 485 nm, and is formed reversibly with an equilibrium constant at room temperature of 20 k 1 1 mol-l. In contrast, no exciplex emission was evident in acetonitrile at intermediate concentrations, but at higher diene concentrations triplex fluorescence (Arnx ca. 543 nm and T = 3.6 ns) was observed. These workers call for a thorough study of the photochemistry of these systems. Lewis and Hoyle have also emphasized the importance of reversible exciplex formation and noted that rate constants for fluorescence quenching generally decrease with increasing ionization potential of the diene.ss The involvement of exciplexes in addition processes has been further studied, and the relationship between the nature of the exciplex and the orientation of photocycloaddition in arene-1,3diene systems has been The reactions investigated in this context involved D M H D and cyclohexa-l,3-diene with 9,10-difluoro-, -dichloro-, -dibromo-, and -dicyano-anthracenes, acridine, and naphthalene under conditions such that 87.5-99% of the arene fluorescence was quenched by the diene: thus most of the S1arene was intercepted by the diene as an exciplex and hence photocycloaddition has to proceed mostly uia the exciplex. It was found that the majority of the arenes reacted with cyclohexa-1,3-diene in a 47ra 47ra mode to yield adducts of type (89), whereas only anthracene and naphthalene reacted with DMHD to give the respective47~g 4raadducts [e.g.(go)] : most of the anthracene

+

+

(91) R = F, CI, Br or CN O7

'O

(92)

N. C. Yang, D. M. Shold, and J. K. McVey, J. Amer. Chem. Soc., 1975,97, 5004. For a preliminary account see J. Saltiel and D. E. Townsend, J . Amer. Chem. SOC.,1973, 95, 6140. F. D. Lewis and C. E. Hoyle, J. Amer. Chem. SOC.,1975,97, 5950. N. C. Yang, K. Srinivasachar, B. Kim, and J. Libman, J. Amer. Chem. SOC.,1975, 97, 5006.

Photochemistry of Aromatic Compounds

379

derivatives gave the 47rs + 27r8adducts (91) with this diene. This greater reactivity of the cyclohexadiene over DMHD with excited arenes in 47r, + 47rBprocesses is explained by the more favourable overlap of the 7r-systems with the former diene. Also, adducts of type (90) and those from other transoid dienes have much greater strain than those of type (89), and this strain must have existed in the transition state thus making this process less favourable. It is interesting that acridine and anthracenes which have a halogen in the meso position react with cyclohexadiene in their terminal ring. On the other hand, (92) is formed from acridine and DMHD. The authors point out that there is an excellent correlation between the polarity of arene-l,3-diene exciplexes and the orientation observed in the cycloaddition processes. The photoaddition of cyclic dienes and cycloheptatriene to anthracene has been the subject of two recent reports. Common biradical precursors have been suggested for both 47r8 + 2r8and 4n8 + 47r8 adduct~,'~ and 4r8 47r8 and 47r8 67r8 adducts, re~pectively.~~ From work described in ref. 70, however, it is clear that the reaction pathways in these systems may be influenced by the nature of the exciplex, and further relevant information has been provided from a study of the 9-cyanoanthracene-cycloheptatrienesystem.73 Three adducts (93), (94), and (95) are formed from the irradiation of this system and the results

+

+

(93)

(94) R' (97) R'

= =

CN, R2 = H H, R2 = CN

+

suggest that the 47rs + 27~sadduct (93) and the 4r8 47r8 adduct (94) arise via different reaction pathways. Consistent with previous singlet excited states are involved in this reaction, and the formation of (93) (which is symmetryforbidden as a concerted process) is suggested to proceed via polar exciplexes and radical intermediates, the most stable of which is (96). Since collapse of (96)to a 4r8 + 4r8 adduct would, however, yield (97) and not the observed isomer (94), a 71

72

79

G. Kaupp, R. Dyllick-Benzinger, and I. Zimmermann, Angew. Chem. Internat. Edn., 1975,14, 491. T. Sasaki, K. Kanematsu, and K. Hayakawa, J. Amer. Chem. SOC.,1973,95, 5632. N. C. Yang and K. Srinivasachar, J.C.S. Chem. Comm.,1976,48.

380

Photochemistry

duality of mechanisms is suggested. The cyano and CH, groups of the addends are considered to orient in the same direction in the exciplex to give the most favourable dipole interaction (98). Such a species as (98) may either collapse to give (94) by a 47r, + 47r8 concerted reaction or follow a stepwise route to the biradical (96) which then gives (93). The light-induced acceleration of the Diels-Alder reaction of maleic anhydride and anthracene was reported 15 years and a similar study has now been made with acrylonitrile as the dienophile using 365 nm light.76 From sensitization experiments TI anthracene is discounted in the process, and a mechanism involving exciplexes is proposed: the reaction rate is increased by an increase in the acidity and polarity of the solvent. The involvement of exciplexes in the photoaddition of amines to arenes has previously been In particular, the reaction between anthracene and aniline has been suggested to involve such intermediate^,^^ and solvent effects have now been extensively s t ~ d i e d . ' ~The main finding is that fluorescence quenching is low in solvents in which the rate of adduct (99) formation is high: Ph

the latter process is again favoured in non-polar solvents. The overall mechanism is suggested to involve electron transfer followed by proton transfer, as in the case of the addition of tertiary amines to benzene.'* In polar solvents, electron transfer may almost be complete in the exciplex, which then rapidly yields a highly solvated ion-pair and this dissociates to solvated radical-anions and -cations. The solvent effects noted in the current work are rationalized by taking into account the effects on the ion-pair as well as on the radical pair formed after proton transfer. Hydroxylic solvents do not fit into the general scheme of solvent polarity effects, and this finding is attributed to the occurrence of proton transfer via two steps in such media (ix.from N to the oxygen of the alcohol and subsequently from the oxonium ion to the anthracene). Such a relay mechanism allows sufficient time for the molecules to separate and thus the addition efficiency is reduced. Over the past four years there has been renewed interest in the 9,lO-photoaddition of dienophiles to phenanthrene, a reaction first reported for maleic anhydride in 1961.79 The earlier controversy over the involvement of exciplexes 74

75 70

77 '1.3

78

J. P. Simons, Trans. Faraday Soc., 1960, 56, 391. N. Selvarajan and V. Ramakrishnan, Z . Phys. Chem. (Frankfurt), 1975, 96, 167. V. R. Rao, S. Vaidyanathan, U. K. Menon, and V. Ramakrishnan, Indian J . Chem., 1973,11, 231; S. Vaidyanathan and V. Ramakrishnan, Z . Phys. Chem. (Frankfurt), 1973, 85, 130. S. Vaidyanathan and V. Ramakrishnan, Indian J. Chem., 1975, 13, 257. D. Bryce-Smith, M. T. Clarke, A. Gilbert, G. Klunklin, and C. Manning, Chem. Comm., 1971, 916. D. Bryce-Smith and B. Vickery, Chem. and Ind., 1961, 429.

381

Photochemistry of Aromatic Compounds

or singlet biradicals in the addition of dimethyl fumarate to phenanthrene was settled last year in favour of the former by Caldwell and co-workers,80and this year further details concerning the intermediacy of both singlet and triplet exciplexes in the process have appeared.81 Reactions were performed in benzene solution using 347 nm radiation, and kinetic and quenching experiments were consistent with a diffusion-controlled reaction of the S1arene with So dimethyl fumarate to yield a weakly emitting singlet exciplex. This intermediate variously yields the oxetan (100) (2.4%), the cyclobutane adduct (101) (0.1%) stereospecifically, and undergoes intersystem crossing to the triplet exciplex (5.373, and decay to starting materials (92%). The triplet exciplex yields both (101)

PR: \

R3

C02Me, / R1 = R2 = H

C02Me (100)

(101) R4 = R3 = (102) R4 = R2 = H, R' = R3 = C02Me (103) R4 = R2 = H, R1-R3 = CO-0-CO

(3.2%) and its isomer (102) (1.7%), dissociates to Soolefin and Tl arene (67.2%), and decays to the Sostarting materials (27.9%). This latter process is presumed to arise via dissociation of the biradical which precedes the formation of (101) and (102). Hence the earlier evidence for the intermediacy of both singlet and triplet exciplexes in this addition process is greatly substantiated. The effect of alkyl substituents on the phenanthrene on the formation of the 1 : 1 arenemaleic anhydride adduct (103) has been investigated.82 The alkyl chain length does not seemingly affect the reactivity, but substituents on the 9,lO-positions increase the rate of reaction whereas those in other positions have the reverse effect. From time to time the photo-Diels-Alder reaction receives comment, but little has been reported on such processes with five-membered heterocycles :in particular, the additions to thiophen appear only to have been investigated with acetylenedicarboxylic The photocycloadditions of maleimide and maleic anhydride derivatives to five-membered heterocycles have now been reported.84 Both 2,3and 2,5-attack of the dienophile on the heterocyclic compound (104) occur and yield stereospecifically the endo and exo adducts (105) and (106), respectively. Photoreduction of the furan ring in benzo[b]furans by aliphatic amines to yield (107) has been reported by Lablache-Combier and co-workers.86 The chemical yields of such products increase with decrease in ionization potential

8a

83

*6

D. Creed and R. A. Caldwell, J . Amer. Chem. Soc., 1974, 96, 7369; R. A. Caldwell and L. Smith, J . Amer. Chem. SOC.,1974, 96, 2994. S. Farid, S. E. Hartman, J. C. Doty, and J. L. R. Williams, J . Amer. Chem. Soc., 1975,97,3697. E. G . Lekveishvili and E. G . Akhalkatsi, Soobschch. Akad. Nauk. Gruz, 1974, 76, 633. R. Helder and W. Wunberg, Tetrahedron Letters, 1972, 605; H. J. Kuhn and K. Gollnick, ibid., p. 1909; Chem. Ber., 1973, 106, 674. C. Rivas, C. Perez, and T. Makano, Rev. Latinoam. Quim., 1975, 6, 166. C. Parkanya, A. Lablache-Combier, I. Marko, and H. Ofenberg, J. Org. Chem., 1976,41, 151.

3 82

Photochemistry

+ (104)

R1,R2 = H or Me X=OorS

0

R3,R4 = H, Me or halogen Y

=

0, NH or NMe

of the amines, hence tertiary amines are the most effective. Exciplex intermediates are again suggested. 4 Substitution Reactions Two excellent reviews concerned with light-induced aromatic substitution reactions have been published this 1 3 ~ The authors are renowned for their very important contributions to this area of research and whether the reader is concerned with synthetic applications of these reactions or wishes to have a summary of present knowledge of photosubstitution reactions he will find either or both reviews invaluable. There are some basic ‘rules’ which describe the Thus as noted orientation of nucleophilic substitution in the excited many years ago, the nitro and methoxy groups are meta and ortho-para directing respectively, contrary to their effects in ground-state nucleophilic substitutions. With polynuclear aromatic compounds, certain positions (e.g. 1- in naphthalenes and azulenes, and 9- in phenanthrene) are more reactive than others, and merging (resonance) stabilization during product formation is an important consideration. Most of the substitution reactions involve the triplet m r * state of the arene, but others involve the singlet m*state, and there are indications that in some cases the aromatic molecule in its excited state undergoes dissociation to produce an ion which subsequently reacts with the nucleophile. There are few reported examples in which benzene undergoes light-induced substitution, hence a report concerned with the nitric oxide-benzene system is 86

E. Havinga and J. Cornelisse, Chem. Rev., 1975, 75, 353. J. Cornelisse, Pure AppE. Chem., 1975,41, 433.

Photochemistry of Aromatic Compounds

383

all the more interesting :** nitrobenzene, o-nitrophenol, p-nitrophenol, 2,4-dinitrophenol, and 2,6-dinitrophenol are all reported to be formed. The photosubstitution of halogen in aryl halides has in past years been studied in detail by Russian workers, who now report displacement reactions by sulphite, nitrite,89and cyanide ions.no Thus sulphite reacts with RC6H4X(R, X = p-Et2N, C1; p-Me2N, C1; p-EtNH, C1; p-NH2, C1; p-NH2, F; p-NH2, Br; p-NH2, I; p-PhNH, C1; 0-NH,, CI;and p-ONa, CI), and with 4-chloro-o-toluidine, 2-amino5-chloropyridine, and 1-amino-4-bromonaphthalene by substitution of the halogen: the reactions are deduced to arise from the triplet states of the halogen compounds. The reaction of NaNO, and Na,SO, with p-ClC6H4NR2(R = Me or Et) has been studied in the presence of other ions, and the quenching of the process was found to vary with the oxidation potential of the added ions, their redox reaction constants with nascent hydrogen, and their energy of chargetransfer by the ion to the solvent. Photosubstitution of halogen by the cyanide ion has been investigated in the naphthalene series where irradiation of aqueous t-butyl alcohol solutions of potassium cyanide and l-amino-4-chloro-, 1-amino4-bromo-, 2-amino-l-chloro-, or 1-amino-2-chloro-naphthalenes gives the corresponding aminocyanonaphthalenes.no The photocyanidation of aromatic hydrocarbons has been reported to be enhanced in the presence of both ‘crown ethers’ n1 and 1,4-dicyanoben~ene.~~ In the former account, naphthalene, phenanthrene, anthracene, and biphenyl all reacted readily with potassium cyanide dissolved in the cyclic polyether 18-crown-6 in anhydrous acetonitrile.nl Under such conditions, of course, the cyano nucleophile is unsolvated whereas when water is present the ion is strongly solvated. An extensive systematic study of the use of crown ethers in light-induced nucleophilic substitution reactions would be most informative. Similarly the photocyanidation of naphthalene and phenanthrene is observed to occur efficiently with sodium cyanide when 1,ddicyanoIn this case the reaction medium is dimethylformamidebenzene is water (3 : 1). The Pyrex-filtered irradiation of phenanthrene in this system gives 9-cyanophenanthrene (679, its photo-dimer, 9-cyano-9,lO-dihydrophenanthrene (4979, and dicyanated products. The authors stress the necessity for the presence of water and the dicyanobenzene otherwise the reaction rate is reduced to about 7% of that in their presence. The reaction is interpreted in turns of CN- attack on the cation-radicals of the arenes generated by electron-transfer from the excited hydrocarbon to the 1,6dicyanobenzene. No ground-state complexation was observed. With naphthalene the results were very similar, and l-cyanodihydronaphthalenes, 1-cyanonaphthalene, and dicyanotetrahydronaphthalene were formed together with other unspecified compounds. Both 0-and p-dicyanobenzenes are, however, photochemically labile in the presence of triethylamine, and the 254 nm irradiation of their acetonitrile solutions gives reasonable yields (56% from the p-isomer) of the corresponding ethylbenzonitriles together with 88

eo

K. Nojima, K. Fukaya, S. Fukui, and S. Kanno, Chemosphere, 1975, 4, 77. A. N . Frolov, E. V. Smirnov, N. I. Rtishchev, 0.V. Kulbitskaya, and A. V. Eltsov, Zhur. org. Khim., 1975, 11, 1464. A. N. Frolov, A. V. Eltsov, 0. V. Kulbitskaya, and V. V. Yunnikov, Zhur. org. Khim., 1975, 11,2623. R. Beugelmans, M. T. LeGoff, J. Pusset, and G. Royssi, J.C.S. Chem. Comm., 1976,377. K . Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975,553.

Dl e2

3 84

Photochemistry

smaller amounts of (108).93 rn-Dicyanobenzene is inert and oxygen also inhibits formation of the other ethylbenzonitriles. With methanol as solvent the starting materials are recovered from the irradiation, as with the tetracyanobenzenetoluene reaction,g4but in contrast with benzene and trieth~lamine,~~ and tetracyanoquinodimethane and toluene.95 The proposed mechanism for this cyano substitution involves initial addition of the amine to the arene ( c - refs. 48 and 78) to form, e.g., (109)zwhich loses HCN to yield the aromatized product (108).

6

€1 CN 6 / +

CN

E

t

3

N

%

a

N C ,CHMe Et,N (109)

-HCN\

CN /

Et

CHMe

Et,N' (108)

The step from (108) to p-ethylbenzonitrile is considered to be photochemical and-indeed irradiation of (108) in triethylamine does yield the observed product. Irradiation (254nm) of 1,2,4,5-tetracyanobenzene in tetrahydrofuran has been reported to yield tetrahydro-2-(2,4,5-tricyanophenyl)furan(1 lo), and as with the above system an addition-elimination mechanism is proposed (Scheme 6).98

(1 12)

Thus the primary step is considered to involve electron-transfer followed by radical coupling to yield the zwitterion (1 11). It is pointed out that the suggested rearrangement of (111) to the adduct (112) is analogous to that previously proposed for the intermediate (113) in the photoaddition of diethyl ether to benzene in the presence of trifluoroacetic acid to give (114).97 O3 O6 O6 O6

97

K. Tsujimoto, K. Miyake, and M. Ohashi, J.C.S. Chem. Comm., 1976, 386. A. Yoshino, K. Yamasaki, T. Yonezawa, and M. Ohashi, J.C.S. Perkin Z, 1975, 735. K. Yamasaki, T. Yonezawa, and M. Ohashi, J.C.S. Perkin Z, 1975, 735. M.Ohashi and K. Tsujimoto, Chem. Letters, 1975, 8, 829. D. Bryce-Smith and G. B. Cox, Chem. Comm., 1971,915.

Photochemistry of Aromatic Compounds

385

Each year a number of reports describe the photochemical dehalogenation of aryl halides, and in particular the dechlorination of chlorinated biphenyls, an environmental interest which has been highlighted by the recent accident at Seveso in Italy involving tetrachlorodibenzo-p-dioxin.The photodechlorination of 1,2,4-trichlorobenzene has been studied in both cyclohexane and propan-2-01 solutions and the primary products shown to be the 1,3- and 1,4-dichlorobenzenes.gs The product ratio is significantly different on direct and acetonesensitized irradiation, and in propan-2-01 solution under aerated conditions is fairly efficient (0= 0.44). Irradiation of cyclohexane solutions of tri- and tetra-chlorobiphenyls at 300 nm gives dechlorinated products, but quantum yields were reported to be only ca. 10-2.99 The reactivity of the biphenyls is dependent upon the position of the chlorine substituent ; o-chlorines cleave first and faster when p-chlorines are present on the same ring. The photoreduction of 3- and 4-chlorobiphenyl has been reported to occur in the presence of sodium borohydride or triethylamine, but with each isomer and reagent the mechanism of the reaction seemingly differs.loOThus, whereas the reaction of the 3-isomer with the borohydride is deduced from deuterium-labelling experiments to occur via a hydride-proton transfer mechanism, the 4-isomer reacts via a radical-chain mechanism of a type previously proposed for similar systems.101 With triethylamine as the reagent, the 4-isomer is efficiently' reduced via an electron-transfer process to yield biphenyl, but in the same system the 3-isomer yields 3-chloro-1,4-dihydrobiphenyl(0= 0.1) together with biphenyl (0= 0.3). Photolysis of both hexachloro- and hexabromo-biphenyls in methanol solution gives the corresponding tetra- and penta-halogenobiphenyls, but in aerated methanol the reaction rates are decreased by ca. 5O%.lo2 In the case of bromobiphenyls, evidence has been presented to show that the C-Br fission is preceded by electron transfer, and that the reaction is assisted by trieth~1amine.l~~ In this latter system, it is suggested that the triplet aryl bromide interacts with the amine to yield the amine radical-cation, the bromide anion, and an aryl radical which abstracts hydrogen from the solvent. The same workers have also investigated the photodegradation of polychloronaphthalenes in methanol s01ution.l~~ Dechlorination and formation of binaphthyl derivatives occur and, of the 20 B. Akermark, P. Baeckstrom, U. E. Weslin, R. Gothe, and C. A. Wachtmeister, Acta Chem. Scand. ( B ) , 1976,30,49. 98 L. 0. RUZO, S. Safe, and M. J. Zabik, J. Agric. Food Chem., 1975, 23, 594. l o o K. Tsujimoto, S. Tasaka, and M. Ohashi, J.C.S. Chem. Comm., 1975,758. Iol J. Barltrop and D. Bradbury, J . Amer. Chem. SOC., 1973, 95, 5085. Ioa L. 0. Ruzo and M. J. Zabik, Bull. Enuiron. Contam. Toxicof., 1975, 13, 181. l o 3 N. J. Bunce, S. Safe, and L. 0. Ruzo, J.C.S. Perkin I, 1975, 1607. lo4 L. 0. Ruzo, N. J. Bunce, S. Safe, and 0. Hutzinger, Buff.Enuiron. Contam. Toxicol., 1975,14, 341. 88

386

(ys& -1

Photochemistry

R1

0

S0,Me

(116) R' = R2 = C1, R3 = H (117) R1 = H, R2 = R3 = C1

(118) R1 = -

S

e

C

l

R2 = CI, R3 = H

R3 (119a) R1 = F, R2 = Me, R3 2 H (119b) R1 = C1, R2 = H, R3 = Me (120a)

R1 = -S o M e , R 2 = Me, R3 = H

(120b) R1 = - S G M e , R 2

R

H, R3 = Me

O

(121)

R

= C1 or F

put

(122)

___, \

But

OH But

OH But

OMe

Jiv

OMe

=

/

+

-

OH (123)

+

OMe

Me0

\ / But (124)

+

\ / OMe

Photochemistry of Aromatic Compoiinds

387

compounds examined, 1,8-dichloronaphthalene had the highest rate of reaction and the 1,2,3,4-tetrachloro-derivativethe lowest. In the light of current intensive efforts to dehalogenate aromatic compounds, it is interesting to note that for some workers at least, the photobromination of halogeno- and dihalogenobenzenes is still an absorbing and worthwhile area of research.lo5 Study of the irradiation of the thioester (115) has led to the observation of a somewhat exotic light-induced substitution of chIorine.lo8 Neither of the expected products (116) and (117) was detected, but (116) was implicated in the process because (118) (33%) was formed, together with 3,3’,4,4’-tetrachlorodiphenyl sulphide (15%) and 2-methylsulphonylbenzaldehyde (27%). Consistent with the intermediacy of (116) in the formation of (118), irradiation of (119a and b) in the presence of p-methylthiophenol gave (120a and b) in respective yields of 62 and 58%: irradiation of (121) also yielded the cyclized halogensubstituted product (122). Although photodealkylation of amines is well known,lo7N-aryl bond cleavage in aryl amines is unusual. This year, however, the photochemical deamination of phenylenediamines in acid solution has been described.lo8 For example, the irradiation of NN-dimethyl-p-phenylenediaminehydrogen sulphate or hydrochloride in methanol yields aniline as the major product via bond homolysis. It appears that the monoprotonated diamine is the reactive species. The formation of biphenyl derivatives from the photolysis of benzenoid compounds has again been noted with several systems. Thus irradiation of 2-t-butyl-4-methoxyphenol in benzene solution has been reported to yield the biphenyl derivatives (123)-(125), together with the intriguing adduct (126),lo9 and from methyl benzenesulphonate in methanol solution, biphenyl, anisole, and benzene are formed via radical reactions.l1° Irradiation of N-arylsulphonylSS-dimethylsulphoximides (127) in either benzene or toluene leads to biphenyl, again by aryl radical coupling.lll Photoisomerization of the 0- and m-methylbiphenyls yields mixtures of the three isomers in which the m-isomer predominates. Under the reaction conditions (254 nm radiation) the p-isomer is photostable: benzvalene intermediates are suggested.lll Lablache-Combier and his group have done much to help achieve an understanding of the light-induced substitution reactions of pyridine, quinoline, and isoquinoline, and their derivative^.^^, 112 They have now published full details concerning the mechanism of photosubstitution of such compounds by methanol in neutral and acidified (HCI) media, and report that pyridine, quinoline, 4-methylquinoline, isoquinoline, and 9-phenylacridine give initially the corresponding In neutral media hydrogen-abstraction semiquinone radicals in both lo6

lo8 lo’ lo*

loS 110 111

113

P. Gouverneur and J. P. Soumillion, Tetrahedron Letters, 1976, 133. G. Buchholz, J. Martens, and K. Praefcke, Tetrahedron Letters, 1975, 3213. See A. Schonberg, G. 0.Schenck, and 0.A. Neumiiller, ‘Preparative Organic Photochemistry’, Springer-Verlag, New York, 1968, p. 255. D.P. Specht, J. L. R. Williams, T. H. Chen, and S. Farid, J.C.S. Chem. Comm., 1975, 705. M. Mihara, T. Kondo, and H. Tanabe, Shokuhin Eiseigaku Zasshi, 1974, 15, 270. Y.Izawa and N. Kuromiya, Bull. Chem. SOC.Japan, 1975,48, 3197. R. A. Abramovitch and T. Takaya, J.C.S. Perkin I, 1975, 1806. A. Lablache-Combier, ‘616ments de Photochimie AvancCe’, ed. P. Courtot, Hermann Press, Paris, 1972, p. 293. A. Castellano, J. P. Catteau, and A. Lablache-Combier, Tetrahedron, 1975, 31, 2255.

388

Photochemistry

occurs from the nrr* excited state in a single-photon process, whereas in acidified methanol the photoreaction apparently involves two photons. In this latter case the reaction arises via electron-transfer from the methanol to a protonated upper excited triplet state of the aza-aromatic (see Scheme 7). The authors conclude Quinolinium ion

uinolinium] ion

hv

H++ &H,OH

.5'1 +

[Quinolinium] ion

m ., bH --.-.. +

.+

\__*-

7'1

uinolinium]

Tn

+ p p H 3 0 H ]

N

I H Scheme 7

that when n electrons are available, the nn* excited aza-aromatic compounds react with the hydrogenated solvents, but on protonation of the N-atom the photoreaction follows another path. Unsensitized photosubstitution of quinoline2- and -4-carbonitriles in alcohol solvents to yield the corresponding l-hydroxyethyl derivatives has been well researched.l14 The process has now been examined in ethanol with benzophenone sensitization, and differences from the unsensitized reaction have become 8 ~ p a r e n t . l ~Quinoline-2-carbonitrile ~ (128) yields no substitution product, but triazapentaphene (131) results. 2-(Hydroxy-

(128) R1 d = CN,RR2 = H 1 (129) R' = CN, R2 = Me (130) R' = CN, R2 = C1 ,Ph (132) R ' = -C,OH, R2 = Me

& \

Ph

N

(131)

NN

xN '

N'

CN

(1 33)

diphenylmethyl)-4-methylquinoline(132) and 4,4'-bi(2,2'-dicyano)quinoline (133) are formed from (129) and (130), respectively. It would appear that the conversion of (128) into (131) proceeds from the %n* state via energy transfer, whereas formation of (132) and (133) involves the primary formation of the ketyl radical of benzophenone and its subsequent reaction with ground-state (129) and (130). A further example of the light-induced Friedel-Crafts reaction has been reported, involving the benzoylation of anthracene.lls Benzoyl-, p-toluoyl-, N. Hata and I. Saito, Bull. Chem. SOC.Japan, 1974, 47,942, and references therein. nS N. Hata and R. Ohtsuka, Chem. Letters, 1975, 1107.

114

116

T. Tamaki, J.C.S. Chem. Comm., 1976, 335.

Photochemistry of Aromatic Compoiinds 389 and p-anisoyl chlorides were all observed to react photochemically with the arene to yield 2- and 9-aroylanthracenes. The anthracene fluorescence is quenched by the aroyl chloride, and the involvement of an exciplex between the S1aromatic compound and the ground-state acid chloride is suggested, but as with so many systems this could not be substantiated by exciplex emission. The formation of acetophenone by irradiation of acetyl chloride (but not acetic anhydride) in benzene has previously been reported.llsa Photochemical substitution in 9,lO-anthraquinone and its derivatives continues to attract the attention of several groups. Filipescu and co-workers have reported that photohydroxylation of the parent quinone occurs readily in concentrated sulphuric acid with near-u.v. or visible light to yield 2-hydro~yanthraquinone.~~~ Unlike the l-isomer, 2-hydroxyanthraquinone is normally tedious to prepare but the present one-step reaction can readily provide it in > 80% chemical yields. There appears to be some controversy over the mechanism of the previously reported photohydroxylation of sodium 9,10-anthraquinone-2-s~lphonate.~~~ A new mechanism which rules out the participation of kinetically free hydroxyl radicals has been proposed,llg but Stonehill and Clark have criticized the mechanistic conclusions as inconsistent with some of their results.120 Further it would appear that the mechanism outlined in ref. 119 would not operate in aerobic systems for which mechanisms proposed by the authors of ref. 120 were designed. The photosynthesis of aminoanthraquinone sulphonates has been reported from the irradiation with visible light of aminoanthraquinone and sodium sulphite in aqueous pyridine.121 A 92.6% yield of l-aminoanthraquinone2-sulphonate is obtained under air or nitrogen, but yields with an oxygen atmosphere are poor as the sulphite is oxidized to sulphate. A mechanism involving the triplet quinone (Scheme 8) is suggested. Two groups have commented on the photosubstitution of halogens in halogenoanthraquinones by 3ArH"

+

SO,T

SO,'-

+

H

__j

+

Ar'

SOa

H ArH +

&'\

so,-

+ 0,

+ ArS0,-

+

Hb2

./H + ArH

ArS0,'-

+

Ar\

Ar

\

H

-/

H

SO3-

Scheme 8 D. Bryce-Smith, G. B. Cox, and A. Gilbert, Chem. Comm., 1971, 914. G. G. Mihai, P. G. Tarassoff, and N. Filipescu, J.C.S. Perkin I, 1975, 1375. 11* A. D.Broadbent and R. P. Newton, Canad. J. Chem., 1972, 50, 381. lleJ. L. Charlton, R. G . Smerchanski, and C. E. Burchill, Canad. J . Chem., 1976,54, 512. H. 1. Stonehill and K. P. Clark, Canad. J . Chem., 1976, 54, 516. lZ1 J. 0.Morley, J.C.S. Chem. Comm., 1976, 88. 11'

390

Photochemistry

amines. Eltsov and co-workers have reported that the formation of 1,ddiaminoanthraquinone from irradiation of alcohol solutions of ammonia and l-amino4-halogenoanthraquinone occurs via the triplet state of the quinone.122 Inoue and co-workers, who have previously reported on similar have outlined the mechanism for the photoamination of sodium 1-amino-4-bromoanthraquinone-2-sulphonate (134),124carried out in aqueous aerated isopropyl alcohol solutions containing ammonia or alkylamines. It is suggested that an exciplex is formed between the anthraquinone and oxygen and that this undergoes nucleophilic attack of the amine resulting in peroxide formation and the substituted product (135).

(134) R = Br R = NR1R2,where R1,R2 = H, alkyl

(135)

The irradiation of substituted K-region arene oxides (136) has been previously reported to yield the ring-expanded products (137).126 Photolysis (254 or 350 nm) of the derivative (138), however, is now found to give the substituted phenanthrene (139) via a triplet excited intermediate, and this is suggested to confirm the singlet multiplicity for the rearrangement of (136) to (137).126 12a

lZ3 lZ4 lZ6 126

0. P. Stadzinskii, N. I. Rtishchev, and A. V. Eltsov, Zhut. org. Khim., 1975,11, 1133. H. Inoue and M. Hida, Chem. Letters, 1974, 255, and references therein. H. Inoue, K. Nakamura, S. Kato, and M. Hida, Bull. Chem. SOC. Japan, 1975,48,2872. N . E. Brightwell and G. W. Griffin, J.C.S. Chem. Comm., 1973, 37. G. W. Griffin, S. K. Satra, N. E. Brightwell, K. Ishikawa, and N. S. Bhacca, Tetrahedron Letters, 1976, 1239.

391

Photochemistry of Aromatic Compounds

5 Intramolecular Cyclization Reactions As in past years, examples of a wide diversity of light-induced cyclization reactions have been described. Stilbene-phenanthrene type processes still attract considerable interest, particularly for their use in the synthesis of helicenes. The concept that the sum of the free valence numbers of atoms involved in the cyclization step should exceed unity (i.e. CF* > 1.0) has been widely used to predict the positions of preferred cyclization. However, two groups have reported that Mulliken electronic overlap populations calculated from extended Hiickel (EH) wavefunctions are valuable indications of reactivity in photocyclizations and dimerization~.~~~, lZ8In a combined report,12gthis approach has been used to show that the reactivity of several pentahelicene derivatives can be directly related to the EH first excited state electronic overlap population of the pair of atoms involved in photocyclization, as well as to the difference in electronic overlap population of these atoms resulting from electronic excitation. This approach is likely to be more widely adopted now that its utility has been demonstrated. Many publications have discussed the dihydrophenanthrene intermediate (140) in the stilbene cyclization process from the viewpoint of its stability, stereochemistry (cis or trans), and nature of the reactive state in its formafion.lao Molecular orbital and energy strain studies have now been applied to such photocyclizations, as illustrated by a discussion of the formation of (140), 10b,10c-dihydrodibenzo[c,g]phenanthrene (141), and 14a,l4b-dihydrodibenzo[b,g]phenanthrene (142) and their ground- and excited-state reactions.131 The particular objective was to determine the factors responsible in the (143) --f (142)

(143)

(144)

v

130

W. H. Laarhoven, T. J. H. M. Cuppen, and R. J. F. Nivard, Rec. Trav. chim., 1968,87,687. K. A. Muszkat and S . Sharafi-Ozeri, Chem. Phys. Letters, 1973, 20, 397. A. H. A. Tinnemans, W. H. Laarhoven, S. Sharafi-Ozeri, and K. A. Muszkat, Rec. Trav. chim., 1975, 94, 239. See, for example, T. J. H. M. Cuppen, and W. H. Laarhoven, J. Amer. Chem. SOC.,1972,94,

131

K. A. Muszkat, S. Sharafi-Ozeri, G. Seger, and T. A. Pakkanen, J.C.S. Perkin I, 1975, 1515.

lZ7 12*

lZB

59 14.

392

Photochemistry

and (143) -+(141) conversions for a number of features, including the absence of the cyclized product (144), the large (ca. 42 kJmol-l) energy barrier in the latter process which is not present in the former, the low quantum efficiency of the (141) -+ (143) conversion and its marked temperature dependence, the relatively slow thermal reopening of (141), and the reason for the strong fluorescence of (141) (0= 0.7) when other dihydrophenanthrenes are nonfluorescent. Stilbene moieties held in a cis configuration by the molecular structure undergo facile light-induced cyclizations, although tetraphenylcyclopentadienone and phenyl-substituted furans are e ~ c e p t i 0 n s . l ~The ~ complex reversible photochemistry of the rigid cis-stilbene system, dixanthylidene (145), has now been studied in great detail, and temperature and external spin-orbit perturbation effects have been The investigation indicates the existence of three labile photoisomers all of which revert thermally to the starting isomer. One of the photoisomers is light-stable and thermally stable below - 140 "C. The quantum efficiency of the conversion of (145) into this isomer decreases with temperature, but is enhanced (up to 220-fold) by the spin-orbit coupling perturbers molecular oxygen, CS2, and ethyl iodide, and hence the conversion is considered to arise from the triplet state. This isomer has a lifetime of 0.05 ms at 0 "C and is the photochromic isomer previously described 134 and observed in practically all dianthrone and dixanthylidene derivatives : its structure involves torsional twist of about 50" about the central bond in (145). The other two isomers are photolabile, are formed from the singlet state of (145), and one is possibly the

precursor of the other. Both are cyclization products of the 4a,4b-dihydrophenanthrene type and one of them, which is photochemically converted back into (145), is deduced to have structure (146). The precursor of this isomer is suggested to be a conformer having a lower thermal stability than (146). Helixanthen (147) is obtained by thermal dehydrogenation of (146) by either molecular oxygen or iodine.133 Full details have been published of the cyclization of the rigid chromophore in substituted 2,3-biphenylbenzo[b]furans (148) in the absence and presence of 13a

133

134

W. M. Horspool, J. Chem. SOC.( C ) , 1971,400; D. T. Anderson and W. M. Horspool, Chem. Comm., 1971, 615; W. H. Laarhoven, T. J. H. M. Cuppen, and R. J. F. Nivard, Rec. Trau. chim., 1968, 87, 687. R. Korenstein, K. A. Muszkat, M. A. Slifkin, and E. Fischer, J.C.S. Perkin ZZ, 1976, 439. R. Korenstein, K. A. Muszkat, and E. Fischer, Mol. Photochem., 1972, 3, 379.

Photochemistry of Aromatic Compounds

393

aliphatic arnine~.l~~g 136 In the former case, the use of a variety of solvents gives only the fully aromatic compound (149; 52%), whereas in the presence of n-propylamine, the 1,4-dihydro derivative (1 50) (65%) results with only relatively minor amounts (12%) of (149). It is concluded from deuterium-labelling experiments that hydrogen atoms from the n-propyl chain of the amine are incorporated I

in the product: hindered amines give only the aromatic product (149). From the observation that acenaphthylene is reduced when incorporated into the irradiation of (148) in the presence of n-propylamine, it is proposed that the hydrogens are eliminated during cyclization not as atoms but in a ‘reductive form’. There have been several publications describing the formation of a phenanthrene by stilbene cyclization as a secondary photoreaction : three are mentioned here. Photolysis (254 nm) of benzpinacol carbonate in methanol yields C 0 2 , Ph2C0, diphenylrnethoxymethane, and tetraphenylethylene : the last compound subsequently yields 9,lO-diphenylphenanthrene in a separate photochemical Two groups have studied the photochemistry of 1,l-diphenylsubstituted vinyl halides in benzene 138 and ether 139 solutions. Both report the formation of tolan and its derivatives, but from the former system 9-phenylphenanthrene also results by reaction of the solvent with the intermediate biradical (151) and subsequent cyclization. The authors of ref. 139 report the formation of phenanthrene from experiments involving 1,2-disubstituted ethylenes which were designed to elucidate the mechanism of the formation of the acetylenes from the 1,l-diphenylethylenes. The use of stilbene cyclization in the synthesis of helicenes is very well known,32 and further examples have been reported. A systematic study of the reaction 136

la8

13? 13*

A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron Letters, 1974, 2497. A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron, 1975,31, 2023. G. W. Griffin, R. L. Smith, and A. Manmade, J . Org. Chem., 1976, 41, 338. T. Suzuki, T. Sonoda, S. Kobayashi, and H. Taniguchi, J.C.S. Chem. Comni., 1976, 180. B. Sket, M. Zupan, and A. Pollak, Tetrahedron Letters, 1976, 783.

394

Photochemistry

Photochemistry of Aromatic Compounds

395

with the compounds (152) and (153) using circularly polarized light has been described.140 The optical yields from the two series follow similar trends, and no asymmetric syntheses are observed in cases of higher benzologues of [lo]-helicene using the 290-370 nm photoband circularly polarized at 313 nm. The same group have also reported on the photosynthesis of [11]-, [12]-, and [14]-helicenes by use of double c y ~ l i z a t i o n s ,a~ ~procedure ~ previously employed for [131helicene.lP2 Two routes to each helicene were designed in order to compare yields and allow structural assignments. Such structural proofs are based on the fact that in each case only the desired helicene can be formed as a common isomer by each procedure. Thus [lll-helicene is formed from (154) and (155) in 54 and 84% yields, respectively, and similar approaches are described for the other two helicenes with yields between 10 and 45% depending on the precursor. All irradiations were in benzene solution in the presence of iodine using Pyrexfiltered radiation. Laarhoven and Kuin have examined the photocheinistry of 2-(2-benzo[c]phenanthrylethenyl)-l,6-methano[l01annulene (156).143 The interest here is in the fate of the bridge on the annulene ring on photocyclization of the two aromatic moieties. Irradiation (360 nm) of an ethanol solution of the cis-trans mixture of (156) in the presence of iodine gives an isolated yield of (157) of 60%. It is not known at present whether the methano-group is eliminated before or after cyclization, but the absence of methano[ l01annulene derivatives from the reaction mixture indicates that the decomposition is probably faster than the cyclization. The present type of cyclization process has also been examined for polymersupported 1,2-diarylethylene~.l~~ Irradiation of a suspension in benzene of l-(4-formylphenyl)-Z(Zbenzo[c]phenanthryl)ethylene (158) and 1-(4-formylphenyl)-2-(2-naphthyl)ethylene (159) attached to a styrene-divinyl benzene copolymer yields (160) and (161), respectively, after hydrolysis. The latter conversion was also noted when the polymer was irradiated in the absence of solvent.

(158)

R=

\

/

A. Moradpour, H. Kagan, M. Baes, G. Morren, and R. H. Martin, Tetrahedron, 1975,2139. R. H. Martin and M. Baes, Tetrahedron ,1976, 31, 2135. lP2 R. H. Martin, G. Morren, and J. J. Schurter, Tetrahedron Letters, 1969, 3683. 143 W. H. Laarhoven and N. P. J. Kuin, Rec. Trau. chim., 1975,94, 105. 144 J. M. Vanest, M. Gorsane, V. Libert, J. Pecher, and R. H. Martin, Chimia, 1975, 29, 343. 140

141

396

Photochemistry CHO

+

It is known that irradiation of o-divinylbenzene yields (162) by a (4 2) addition, plus traces of tetralin, dihydronaphthalene, and naphthalene: 2,3-divinylnaphthalene (but not the 1,2-isomer) also gives 5% of a (2 + 2) cyc10adduct.l~~Such reaction has now been investigated with 2-vinylstilbene where a stilbene-type cyclization is also very pr0bab1e.l~~Indeed, 15% of l-vinylphenanthrene was formed, but the major product (70% yield) was exo-5-phenylbenzobicyclo[2,l,l]hex-2-ene (163), and only 2% of the endo compound was

obtained. The differences in behaviour between o-divinylbenzene and 2-vinylstilbene are attributed, at least in part, to conformational differences. The authors also reported the formation of (164) and the product corresponding to (163) by photolysis of 2,2’-divinylstilbene. The conversion of azobenzene into benzo[c]cinnoline has been the subject of two reports. The essential presence of intramolecular hydrogen bonding for photochemical cyclization of azobenzene-o-carboxylic acids has been proved by and the process has been the lack of reaction of the esters in neutral observed to occur with some other azobenzene derivatives (165) in 95% concenProlonged trated sulphuric acid, and in CH,Cl, in the presence of Lewis irradiation of (165) in CH2C12alone does not yield products. Cyclizations of 1,4-diaryIbuta-1,3-dienesystems which are rigidly held in the cisoid-diene configuration have been well studied by Heller and co-workers for a number of years, and further reports have appeared. Many of these compounds display phot ochromism. The (22,3Z)-isomer of 2- benzylidene-2,3-dih ydro3-mesityl-3-phenylmethylenebenzofuran (166) yields the (22,3E)-isomer on irradiation; this in turn undergoes photocyclization via a conrotatory mode to 145

146

14’ 1 4

J. Meinwald and P. H. Mazzochi, J . Amer. Chem. SOC.,1967, 89, 696; M. Pomerantz and G. W. Gruber, J . Amer. Chem. SOC.,1971, 93, 6615; J. Meinwald, J. W. Young, E. J. Walsh, and A. Courtin, Pure Appl. Chem., 1970, 24, 509. M. Sindler-Kulyk and W. H. Laarhoven, J . Amer. Chem. SOC.,1976,98, 1052. C. P. Joshua, V. N. R. Pillai, and P. K. Ramdas, Indian J. Chem., 1975,13, 290. ~C. P. Joshua and V. N. R. Pillai, Indian J . Chem., 1975, 13, 1018.

397

Photochemistry of Aromatic Compounds Me

II

(165) R1,R2 = H, Me

(166)

H (167)

the trans-6,7a-dihydro intermediate (167),149which yields the trans-5a,6-dihydrobenzonaphthofuran (168) at 80°C and above by a 1,5-hydrogen shift, and is photo-oxidized to (169). Similar studies have been made with succinic anhydride and N-phenylimides (170) 150 and furanones (171).151 From the former class of compound (170), a thermally stable photochromic system has been developed and the potential of a multiphotochromic system investigated; but only one of the expected two cyclizations in fact occurred. The quantitative photorearrangement of derivatives of (171) to (172) provides a convenient synthesis of apolignan derivatives. Me

Me

H

Ph (169)

(170) X = 0 or NPh

I

H

(172)

(171)

This year many examples have been reported which involve cyclization of an ethylene onto an aryl group. The 1,l-diarylethylenes (173), (174), and (175) should all undergo facile cyclization at the positions indicated if (see ref. 129) is the most important feature which controls the p h o t o p r o c e ~ s .All ~ ~ ~these compounds are photoreactive but the products are dependent upon the reaction

zF*

149

150 151 152

J. S. Hastings, H. G. Heller, and K. Salisbury, J.C.S. Perkin I, 1975, 1995. R. J. Hart, H. G. Heller, R. M. Megit, and M. Szewczyk, J.C.S. Perkin I, 1975, 2227. H. G. Heller and P. J. Strydom, J.C.S. Chem. Comm., 1976, 50. R. Lapouyade, R. Koussini, and J. C. Rayex, J.C.S. Chem. Comm., 1975,11, 676.

14

398

Photochemistry

conditions. In degassed cyclohexane solution, irradiation (300nm) of (173) yields 85% of 9,1O-dihydro-9-phenylphananthrene,and (1 74) behaves ~imilar1y.l~~ The reaction is sensitized by xanthone and Michler’s ketone and quenched by oxygen without yielding 9-phenylphenanthrene, so triplet states are considered to be involved. Irradiation of (173) under the normal conditions for the stilbene -+ phenanthrene conversion does induce a slow reaction to yield the phenanthrene. The light-induced reaction of compound (176),which is formed by photolysis of cannabinol (177),yields the phenanthrene by dehydration and

(ys’,”b@

C6H4R1

+-

Ph (173)

+-

Ph (174)

R2

(175) R’, R2 = H, OMe

r i n g - c l o s ~ r e . In ~ ~contrast ~ with (173)and (174), (175)is not affected by direct or sensitized irradiation, but in the presence of oxygen, iodine, or CuBr,, acenaphthylenes (178)are formed, probably by singlet excited stafes.ls2 The mechanism of photocyclization of substituted o-allylphenols to benzofuran and benzopyran derivatives has been studied, and the role of intramolecular hydrogen-bonding between the hydroxy-group and the n-electrons of the allylic of 3-(2-hydroxybenzylidene)group has been d e m o n ~ t r a t e d . ~Irradiation ~~ 4,5-dihydrofuran-2(3H)-one (179) results in intramolecular acylation and formation of (180).166 lS3 lb4 166

lS8

See also P. H. G . Op Het Veld, J. C. Langendam, and W. H. Laarhoven, Tetrahedron Letters, 1975, 231, and references therein. A, Bowd, D. A. Swann, and J. H. Turnbull, J.C.S. Chem. Comm., 1975,797. S . Geresh, 0. Levy, Y. Markovits, and A. Shani, Tetrahedron, 1975,31,2803. I. R. Bellobono, L. Zanderighi, S. Omarini, and B. Marcandalli, J.C.S. Perkin 11, 1975, 1529.

Photochemistry of Aromatic Compounds 399 The stereochemistry of the known aryloxyenone photocyclization has been studied by reference to the compounds (181)-(183).15' In all cases a high-yield reaction occurred to give specifically the cis-fused decalone ring product [e.g. (184) from (18l)l. The relatively strain-free carbonyl ylide (185) is suggested as an intermediate.

p

0

Jo 0

Ph (181) R = Me (182) R = COZCHZCH,

% O

/

\

Ph

(183)

M&$33 0-

H'o / \

Comments on the use of the photocyclization of N-benzoylenamines in the synthesis of berberine alkaloids continue to appear, and the process has been studied for 11 derivatives of (186), when the berbin-8-ones (187) and (188) are formed.168 The same group has demonstrated the use of this reaction by the first total synthesis of ( rf: )-cavidine.lS9 Cyclization of simple a/3-unsaturated anilides is a known process,lSoand has now been reported for benzo[b]thiophen-2-carboxanilide (189).161 The unsubstituted compound (189) yields 40% of

(186) R s = H, OMe, -OCH,O-,

NOz, C0,Me

A. G. Schultz and W. Y . Fu, J. Org. Chem., 1976,41, 1483. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1721. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1791. See, for example, I. Ninomiya, S . Yamauchi, T. Kiguchi, A. Shinohara, and T. Naito, J.C.S. Perkin I, 1974, 1747. Y. Kanaoka, K . Itoh, Y.Hatanaka, J. L. Flippen, 1. L. Karle, and B. Witkop, J. Org. Chem.,

lK7

lS8 16s

160 lS1

(

1975, 3001.

400

Photochemistry

A

(190) and 15% of (191) via oxidative cyclization, but under non-oxidative conditions (191) is the major product and only traces of (190) are formed. Under similar anaerobic conditions, the N-methyl derivative yields the trans-fused isomer of (191); and whereas irradiation of (189) in D 2 0 yields (191) with deuterium in the 14-position, similar reaction of the N-methyl compound yields both the non-deuteriated trans-form of (191) and the corresponding N-methyl cis-16deuteriated (191) derivative. From these results it is deduced that the cis-14-hydrogen comes almost exclusively from the media whereas the trans14-hydrogen originates from an internal source. The occurrence of cyclization of 3-aroylchromones is found to be very dependent on substituents.ls2 Thus whereas 3-benzoyl-2-methylchromone (192) is seemingly stable under the reaction conditions, its isomer 3-(o-toluoyl)- chromone (193) readily forms benzoxanthenone (195) via the enol (194). A novel intramolecular cyclization of the thioketone group onto aryl moieties was reported four years ago by Lapouyade and de Mayo,ls3 and further details

0

162

lo4

OH

0

OH

P. G . Sammes and T. W. Wallace, J.C.S. Perkin I, 1975, 1845. R. Lapouyade and P. de Mayo, Canad. J. Chem., 1972,50,4068. A. Cox, D . R. Kemp, R. Lapouyade, P. de Mayo, J. Joussot-Dubien, and R. Bonneau, Canad. J. Chem., 1975,53,2386.

401

Photochemistry of Aromatic Compounds

R I

pfyR

\

/

S (196) R

\

II

=

-CPh

/

have now been p~b1ished.l~~ Polycyclic aromatic thiones (196) having free peri positions cyclize on nr* excitation to yield thiophen derivatives [e.g. (197)l. The formal 1,3-hydrogen migration was demonstrated to be intermolecular in one case by incorporation of deuterium from D 2 0 during irradiation. With the a-naphthyl derivative, the excited state responsible for the reaction was shown to be the nn* singlet. Each year many accounts appear describing the cyclization of aryl groups separated by heteroatoms, and comparable cyclizations which involve the loss of HHal. Details of the reaction and mechanism of the conversion of diphenylamine into carbazole have previously been noted,32 and the synthetic scope of the process with the substituted triphenylamines has now been reported.ls5 For many derivatives the reaction failed, and only with R1 = R2 = H, F, or OMe was the corresponding carbazole formed. With PhN(Me)-p-C6H4N(Me)Ph, however, a 10%yield of the indolo[3,2-b]carbazole (198) was obtained. Two groups have commented on the cyclization of diphenyl ethers leading to dibenzofurans, and one paper also described a process with Me

QJ-pJAyJI

M@oD OMe

OMe Me Me

Me (198)

(199)

thiodiphenyl ethers leading to dibenzothiaphens with yields in the range 4 0 60%.le6 The second group are concerned with the reactions of polyfunctional 2-methoxyphenyl phenyl ethers which were obtained by degradation of lichen lE5 188

W. Lamm, W. Jugelt, and F. Pragst, J. prakt. Chem., 1975,317,284. K. P. Zeller and H. Petersen, Synthesis, 1975,8, 532.

402

Photochemistry

depsidones.lB7 Such compounds as (199) and three of its more complex derivatives bearing 2-methoxy groups yield the corresponding dibenzofurans by the now well-known procedure (see ref. 32) involving loss of the elements of methanol. In contrast, other methoxyphenyl phenyl ethers photoisomerized to hydroxybiphenyls. The mechanism of these processes and the structural factors which favour the cyclization are discussed on the basic assumption that irradiation of the ethers results in initial formation of an uncleaved biradical species which undergoes various reactions leading to biphenyls, cleavage products, or dibenzofurans: the most favourable pathway is that which involves the most stable biradicals, and the direction of cleavage in asymmetrical ethers to give the hydroxybiphenyl is of course determined by the stability of the intermediate radicals. In a comparison of the photoreactions of Ph2CH-X-CHPh2 systems (X = NH, CH2, 0, and S), the transition state leading to products (e.g. biphenyl) is suggested to resemble the cyclized product Incorporation of a halogen atom in one of the rings naturally causes a considerable change in the photochemistry, and the irradiation of aqueous solutions of 2-iododibenzylamine hydrochlorides has provided a convenient synthesis of 6,7-dihydro-SH-dibenz[c,e]azepines (201).ls@Similarly, N-(2-halogenobenzyl)-~-phenethylaminehydrochlorides yield the corresponding 5,6,7,8-tetrahydrodibenz[c,e]azocines (202).

(201) Iz = 1 (202) n = 2

Me0

OH (203)

OH (205)

A further example in natural product synthesis of the use of photocoupling of two aryl rings by loss of HX has been reported, and involves cyclization of the ( f)-bromodiphenyl (203) to the (+)-spirodienone (204) as a key step in the first synthesis of the aporphine alkaloid ( + )-boldine (2O5).l7O 16' 168

170

J. A. Elix and D. P. Murphy, Austral. J. Chem., 1975,28, 1559. R. W. Binkley, S. C. Chen, and D. G . Hehemann, J. Org. Chem., 1975,40,2406. P. W. Jeffs, J. F. Hansen, and G . A. Brine, J . Org. Chem., 1975,40,2883. S . M. Kupchan, C. K. Kim, and K. Miyano, J.C.S. Chem. Comm., 1976,91.

Photochemistry of Aromatic Compounds

403

Irradiation of the meta-bridged bromo-compound (206) yields the three transannular products (2O7)-(209).l7l Light-induced cyclization of 2,6-dichlorocinnamates (210) and loss of hydrogen chloride yields 5-chlorocoumarin by

(206) n = 2 or 3

(207) ?TI = 3, IZ = 2 (208) nt = IZ = 3 (209) nt = 4,IZ = 2

reaction from the singlet excited state of the cis cinnamate isomer and formation of an unstable ortho-quinomethylketen (211) as the product precursor.172Consistent with this proposal, irradiation of the cinnamate at - 190 "C yields a new red species with structured absorption out to 640 nm, and on warming to - 170 "C this spectrum is replaced by that of the coumarin: the i.r. spectrum of the red species is also consistent with structure (211). Among the products from the photolysis of the dienone (212) is the dehydrobrominated cyclized product (213),173and irradiation of 9-a-bromopropionylanthracene has been reported to yield 2-methyl-1-aceanthrenone(214) and 9-vinylanthryl ketone.17* N-Phenylpyrrole undergoes simple substitution by loss of hydrogen bromide with dibromomaleic anhydride, and the product (215) is ideally constructed for a further intramolecular photoprocess and indeed yields (216) as the final Both dehydrobrominations are considered to arise from triplet states. 171

178

173 17*

175

S. Hirano, H. Hara, T. Hiyama, S. Fujita, and H. Nozaki, Tetrahedron, 1975,31,2219. R. Arad-Yellin, B. S. Green, and K. A. Muszkat, J.C.S. Chem. Comm., 1976,14. C. W. Shoppee and Y. S. Wang, J.C.S. Perkin I, 1976, 695. T. Matsumoto, M. Sato, and S. Hirayama, Bull. Chem. SOC.Japan, 1975,48, 1659. T. Matsuo and S. Mihara, Bull. Chem. SOC.Japan, 1975,48,3660.

404

Photochemistry Br

Ph

Br

Ph

//

0

Me

Further details of the earlier reported 176 photocyclization of N-chloroacetyl2,5-dimethoxyphenethylamine have been but the products and suggested intermediates are unchanged and were reviewed two years The common feature in all these examples of this type of cyclization is intramolecular electron-transfer from the S1aromatic chromophore to the chloroacetyl moiety.179 The exciplex so formed undergoes C-Cl homolysis and the resulting radical couples with the aryl radical cation to yield the cyclized products. Such photocyclizations of the seven isomeric N-chloroacetylindolylethylamines (217) have been examined in attempts to correlate the reactivities of the positions with frontier electron densities calculated by unrestricted Hartree-Fock molecular orbitals.lso A variety of azepinoindoles and azocinoindoles are formed by cyclization at the ortho- and peri-positions. With the 3-, 4-, and 6-isomers of CH,Cl

CHzCHR

o=c'

"HCOCH,CI

R (217)

(218)

(217) high reactivity is observed, whereas the other positional isomers are less reactive and no cyclization at position 1 is detected. The mechanism suggested involves radical intermediates for the unsubstituted N-compounds and radicalcation species with N-alkyl derivatives. This reaction has also been examined by other workers with 2-(N-chloroacetylpiperidylalkyl)indoles, e.g. (21 8).lE1 In general, cyclization occurs at the indole 3-position, i.e. to yield (219) from (218), when the indolylalkyl group is attached to the 2- or 3-position of the piperidine 176 177 178 l7@

lS1

Y. Okuno and M. Kawamori, Tetrahedron Letters, 1973, 3009. Y. Okuno, M. Kawamori, K. Hirao, and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 2584. See Vol. 6, p. 487. Y. Okuno and 0. Yonemitsu, Tetrahedron Letters, 1974, 1169. S. Naruto and 0. Yonemitsu, Tetrahedron Letters, 1975, 3399. R. J. Sundberg and F. X. Smith, J . Org. Chem., 1975,40,2613.

405

Photochemistry of Aromatic Compounds

ring : methanol is preferable to benzene as a solvent. Intramolecular cyclization onto the indole ring system has been noted in other cases. The irradiation of (220) in ethanol and in the presence of iodine yields both (221) and (222) in respective yields of 20 and 5O%.ls2 I n the absence of iodine, but the presence of air, only (222) is formed. With the isomeric derivative (223) of (220), only one product (224) is formed in oxidizing media, but the three products (225)-(227) result under non-oxidizing conditions.ls3 The indole (228) likewise yields (229) and (230) under oxidizing and non-oxidizing conditions, respectively. CN

(220) R

(219)

=

(223) R =

(221) X = N, Y = Z = CH (222) Y = N, X = Z = CH (224) X = Y = CH, Z = N

(228) R =

(229)

(230)

A novel route to N-bridgehead compounds by cyclization of l-styrylimidazoles has been described.ls4 For example, irradiation of the imidazoles (231) in methanol leads to cyclization at the 2-position of the imidazole ring and the formation of imidazo[2,l-a]isoquinolines(232). The reverse mode of cyclization involving 2-styrylbenzimidazoles in C-N bond formation has also been demonstrated,ls4and sterically hindered N-vinyliminopyridiniumylides (233) have been reported to yield a variety of products on photolysis, including the cyclized ylide (234).lS6 C . Dieng, C . Thal, H. P. Husson, and P. Potier, J . Heterocycl. Chem., 1975, 12, 455. C. Riche and A. Chiaroni, Tetrahedron Letters, 1975, 4567. la4 G. Cooper and W. J. Irwin, J.C.S. Perkin I, 1976, 75. Ia6 A. Kakehi, S. Ito, T. Funahashi, and Y . Ota, J . Org. Chern., 1976,41, 1570. 182

lS3

406

Photochemistry

(231) R1, R2, R3 = H, CO,Me, Me, Ph

(234)

(232)

(233) R

=

Me or Et

(235)

The cyclization arising from light-induced loss of sulphur from compounds (235) 186 and (236) is revieved in Part 111, Chapter 6. 6 Dimerization Reactions Work prior to 1975 on the light-induced dimerization of anthracenes has been very well reviewed by Cowan and Drisko.l** It has previously been suggested that the photodimerization observed with certain compounds in the crystalline state is the result of exciton trapping at dislocation sites in the Such exciton trapping properties of an idealized plane defect core have been studied theoretically, as has the macroscopically strained region around the core, and the theory has been applied to the case of crystalline anfhracene.lQo It is suggested that the core trapping initiates the dimerization, but that the compressive strains which are set up in the dimerization region following the formation of some dimer are responsible for the subsequent trapping to produce further dimer in these regions. The photochemistry and photophysics of a number of 9-substituted anthracene sandwich pairs have been studied in the corresponding photo-dimer crystal matrices, and in methylcyclohexane matrices at 6 K.lQ1The photodimerization of 9-methyl-, 9-chloro-, and 9-cyano-derivatives of anthracene in the dimer matrix at 6 K occurs with unit quantum yield, but the presence of excimer fluorescence from sandwich pairs indicates that the topochemical orientation is not perfect. Activation processes which lead to reaction involve molecular reorientation from more stable groundstate configurations, and these are achieved within the constraints imposed by the T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Perkin I, 1975, 1964. K. Praefcke and C. Weichsel, Tetrahedron Letters, 1976, 1787. lR8 D. 0.Cowan and R. Drisko, 'Elements of Organic Photochemistry', Plenum Press, New York, 1975, Chapter 2. lRe For reviews of the subject see: M. D. Cohen and B. S. Green, Chem. in Britain, 1973,9,490; J. M. Thomas and J. 0. Williams, Prog. Solid State Chem., 1971,6, 121. ln0 P. E. Schipper and S. H. Walmsley, Proc. Roy. SOC.,1976,348,203. lB1 J. Ferguson and S. E. H. Miller, Chem. Phys. Letters, 1975,36, 635. lR6 lR7

Photochemistry of Aromatic Compounds

407

solvent or crystalline cage. Other workers have also reported on the luminescence of the sandwich dimer of anthracene produced by photocleavage of dianthracene in methylcyclohexane at 77 K.lQaTopochemical dimerization has been used as a new method for enantiomeric purification.lQ3 The basic principle is to attach chemically a photodimerizable molecule to an enantiomerically enriched sample of an alcohol, amine, etc. The photodimerizable molecules containing the chiral group may crystallize in either the photoactive a-form or in the light-stable y-form, as the short distance for /3-packing is precluded by the bulky chiral group. Irradiation of the mixture then yields the meso photodimer from the a-form whereas the y-form is unaffected and easily separated from the reaction mixture. This

Me

I

oYo-z-Ar approach has been applied to the enantiomeric separation of three 1-arylethanols which were condensed with 9-anthroic acid to yield the corresponding anthroates (237). The crystalline esters were exposed to U.V. light, and the unaffected monomer was extracted simply from the sparingly soluble dianthracenes (238) which reverted to the monomers at their melting points. The chemical yield is quoted as >80% and the enantiomeric purities of the recovered unreacted monomer > 90%. It is to be hoped that this novel method of optical purification will be extended to other systems. The quantum yields of photodimerization and fluorescence have been measured for 9-anthroamide, and methyl, ethyl, n-butyl, t-butyl, and cyclohexyl9-anthroates, as a function of c o n ~ e n t r a t i o n .From ~ ~ ~ these efficiencies and data of fluorescence lifetimes, rate ratios and individual rate constants have been evaluated for several mechanistic schemes. Concentration quenching of the monomer fluorescence, formation of excimers, and photodimerization studies have been reported for anthracene derivatives which have substituents in the side rings of the anthracene nucleus.1Q5Such investigations have shown, not surprisingly, that steric constraints have profound effects on the formation of the excimers and photodimers. Somewhat similar studies have been made by Castellan, who has also examined Isa

lgs

Io4 lS5

P. C. Subudhi, N. Kanamaru, and E. C. Limy Chem. Phys. Lett., 1975,32,503. M. Lahav, F. Laub, E. Gati, L. Leiserowitz, and Z. Ludmer, J. Amer. Chem. Suc., 1976, 98, 1620. R. S. L. Shon, D. 0. Cowan, and W. W. Schmiegel, J. Phys. Chem., 1975,79,2987. I . E. Obyknovennaya, T. M. Vember, T. V. Veselova, and A. S. Cherkasov, Optika i Spektroskopiya, 1975,38,1127.

408

Photochemistry

the effect of solvent on the efficiency of the The quantum yields for dimerization of anthracene and some 2,4-substituted derivatives have been recorded, and it has been reported that disymmetry of charge on the meso positions, the presence of halogen and groups capable of inducing nn* transitions, and again steric constraints all hinder the process; but the effects of solvents (benzene, diethyl ether, acetonitrile, and ethanol) are only weak. The structures of the photodimers of tetracene produced from irradiation of M solutions in benzene have been determined by X-ray diffraction.lg7 2 x Two dimers are formed: structure (239) is assigned to the one which is soluble in organic media, and (240) to the insoluble isomer.

(240)

Three accounts have described various examples of the well-known 9,lO9’, 10’-intramolecular photodimerization of bianthryl derivatives. Applequist and Swart reported a new improved synthesis of 9,9’-dianthrylmethane derivatives and have re-examined their light-induced reactions as the previous studies 19* were apparently made ‘on the wrong Whereas (241a and b) gave (242a and b), (241c and d) were inert, an observation consistznt with the fact that no dianthracene with vicinal bridgehead halogens has yet been reported. The intramolecular dimerization of the dianthracene (243) to yield (244) has been studied in some The quantum yield of the reaction is wavelengthdependent and at 450-470 nm is reported to be 0.70 & 0.06, whereas at wavelengths shorter than 420 nm, it is lower at 0.45 & 0.04. The results are interpreted lB6

lB7

A. Castellan, Compt. rend., 1975, 281, C , 221. J. Gaultier, C. Hauw, J. P. Desvergne, and R. Lapouyade, Cryst. Structure Comm., 1975, 4, 497.

D. E. Applequist, M. A, Lintner, and R. Seale, J . Org. Chem., 1968,33,254. D. E. Applequist and D. J. Swart, J . Org. Chem., 1975,40, 1800. H. Shizuka, Y. Ishii, M. Hoshino, and T. Morita, J . Phys. Chem., 1976,80, 30.

IB8

lB8

aoo

409

Photochemistry of Aromatic Compounds

to mean that direct excitation to the transannular excited state at the longer wavelength is much more fruitful of reaction than excitation to the locally excited state [lLa or lBb] of an anthracene moiety. X

Y (241) a; X = Y = H b; X = H , Y = Br c; X = Y = B r

(242) a; X b; X

= =

Y =H H , Y = Br

d;X=Y=Cl

(243)

(244)

All previously reported intramolecular dimerizations of such systems have involved the anthracene 9,1@9’,1O’-positions : this year, however, an exception to this has been described. Thus Bouas-Laurent and his co-workers, who are well known for their studies on the intermolecular process, have observed that irradiation of bis-(g-anthryl)-l, 1,3,3-tetrarnethyldisiloxane (245) leads to unsymmetrical dimerization of the anthracene moieties with the formation of (246).201This is the first example of photodimerization involving the 1,4-positions and is rationalized by the steric hindrance of the bulky Me,Si groups preventing closure between the 9,lO- and 9’,1O’-positions. The dimers from phenanthrenes had been previously deduced to have head-totail structures (247) and cis configurations about the cyclobutane ring.202X-Ray structure analysis has now shown this assignment to be correct for the photodimer of 9-cyano-10-methoxyphenanthrene and, as with anthracene, an exciplex intermediate is 201

aoa a03

G. Felix, R. Lapouyade, H. Bouas-Laurent, and B. Clin, Tetrahedron Letters, 1976, 2277. R. Galante, R. Lapouyade, A. Castellan, J. P. Morand, and H. Bouas-Laurent, Compt. rend., 1973,277, C, 837. C. Courseille, A. Castellan, B. Busetta, and M. Hospital, Cryst. Structure Comm., 1975,4, 1.

410

Photochemistry

SiMe,

I I

0

(245)

(247)

Photodimerization in the naphthalene series is currently restricted to the p-cyano- and p-alkoxy-derivatives : thermal reactions of the trans-photodimer of 2-methoxynaphthalene have been reported this year.2o4 Although many examples of photodimerization of polynuclear aromatic compounds are known, uncondensed benzenoid aromatic rings do not yield such products and until recently the only example of the reaction with a monocyclic heteroaromatic compound involved the sunlight dimerization of 2-aminopyridine

(yoSJ.

bq cr 0

+

N

Ph

I

0

Ph

N

Ph (249)

R

1

R

=

H, 7-Me, and %Me

(251) 204

T. Teitei, D. Wells, P. J. Collin, G. Sugowdz,and W. F. H. Sasse, Austral. J. Chem., 1975,28, 2005.

41 1

Photochemistry of Aromatic Compounds

h y d r o ~ h l o r i d e .Katritzky ~~~ and Wilde have now reported that 3-oxido-l-phenylpyridinium (248) undergoes both light-induced valence bond tautomerism and dimerization.206 Thus photolysis (350 nm) of (248) in ethyl acetate leads to formation of (249), for which there is no precedent in pyridine chemistry, and (250) as primary products: the exo- and endo-isomers of (251) are formed thermally by addition of (249) to (248). Reversible photodimerization has also been noted with 2-methyl-sym-triazolo[1 ,5-alpyridines (252).207 7 Lateral-nuclear Rearrangements The mechanism of the photo-Fries reaction of phenyl acetate has been established in both the vapour and solution phases, and the involvement of radical intermediates has been demonstrated.208 The rearrangement has now been studied in the presence of p-cyclodextrin when the reaction showed some selectivity with the formation of the o- and p-hydroxyacetophenones in a 1 : 6.2 ratio; phenol production was decreased. Methyl-a-glucopyranoside was reported to have little effect.20gThe rearrangement has also been studied with a number of aryl esters and amides (253) in which the acyl part is derived from a-amino-acids.210 Although some variations were reported, approximately equal yields of the 1,2,3- and 1,2,4-isomers (254) were generally obtained. R

(253) R = H, Me, CHMe,, CH2Ph, 3-indolylmethyl, or (CH,),NHCO,CH,Ph X=OorNH

(255) R = H (256) R = Me

(257) (258)

O

(254)

R1 = NHCOPh, R2 = H R1 = H, K' = NHCOPh

Acetanilides have been known for many years to undergo the photo-Fries rearrangement, and work in this area is generally related to Shizuka and Tanaka's fundamental studies reported in 196tL211 The reaction has now been studied with the four fully aromatic amides (255)-(258) in ethanol solution with 254 nm radiation, and (255) has been subjected to a detailed investigation at various 206 208 207

2os 210

*11

E. C. Taylor and R. 0. Kan, J. Amer. Chem. SOC.,1963,85,776. A. R. Katritzky and H. Wilde, J.C.S. Chem. Comm., 1975,770. T. Nagano, M. Hirobe, M. Itoh, and T. Okamoto, Tetrahedron Letters, 1975, 3815. J. W. Meyer and G. S. Hammond, J. Amer. Chem. SOC.,1972,94,2219; C . E. Kalmus and D. M. Hercules, ibid., 1974,96,449. M. Ohara and K. Watanabe, Angew. Chem., 1975,87,880. H. Keroulas, C. Ouannes, and R. Beugelmans, Bull. SOC.chim. France, 1975, 793. H. Shizuka and I. Tanaka, Bull. Chem. SOC.Japan, 1968,41,2343.

412 Photochemistry wavelengths in a variety of solvents, and in the presence and absence of oxygen.212 It was reported that quantum yields for formation of 2- and 4-aminobenzophenones and products from free-radical precursors decreased as the solvent polarity was increased, and also with increase in the wavelength of the exciting radiation. The presence of oxygen had apparently no effect on the rearrangement, but benzoic acid was formed, it is suggested, from free radicals which escaped from the solvent cage. The authors tentatively interpret their results in terms of an energy-dependent radiationless transition to a reactive singlet state of the carbonyl group. Cleavage of the N-C bond follows to yield free radicals, as outlined in ref. 211. A type of photo-Fries reaction has been observed on photolysis of the aromatic enol-esters (259) to give the p-diketones (260).213 The well-known light-induced rearrangement of diphenyl ethers to hydroxy214 The work described in ref. 167 biphenyls 32 is the subject of two reports.le7~ followed from a study of the cyclization of such molecules to dibenzofurans, and the other report outlines the selective rearrangement of p-phenoxyphenol to phenylhy droquinone.

(259) R = Me, Ph, 2-naphthyl, or 2-anthryl

(260). 0-

Three years ago, a report on the photochemical rearrangement of azoxybenzene to hydroxyazobenzene apparently substantiated the proposal that the reaction proceeded by way of the cyclic intermediate (261),216but it has now been reported that the two isomers (262) and (263) photorearrange to the same o-hydroxyazo-compound (264).21a The mechanistic problem is effectively resolved by the further observation that (263) photoisomerizes to (262), so there is no need to disturb the earlier conclusion. 212 213

214

215

D. J. Carlsson, L. H. Gan, and D. M. Wiles, Canad. J. Chem., 1975,53,2337. D. Veierov, T. Bercovici, E. Fischer, Y. Mazur, and A. Yogev, Helv. Chim. Acta, 1975, 58, 1240. A. Ehrl, Atomkernenergie, 1975, 25, 293. D. J. W. Goon, N. G. Murray, J. P. Schoch, and N. J. Bunce, Canad.J. Chem., 1973,51,3827. N. J. Bunce, Canad. J. Chem., 1975, 53, 3477.

5 Photo-reduction and -oxidation ~

~~

BY H. A. J. CARLESS

1 Conversion of C=O into C-OH This year has seen a growing realization of the difficulties, which have sometimes been overlooked, when making quantitative measurements of the photoreduction of carbonyl compounds. Steel and co-workers1 have investigated the well known role of transient light-absorbing photoproducts [e.g. (2) and (3) in Scheme 11 believed to be Ph2C0 3Ph2C0

'Ph2C0

+ RH

2Ph2kOH

2Ph$OH

+ 'Ph2C0 Ph2kOH

+ R.

Ph,C(OH)C (OH)Ph2

-

(1)

Ph OH

P h 2 6 0 H -t R(3) Scheme 1

formed during the photoreduction of benzophenone in the presence of hydrogen donors. Both U.V. absorption spectra and benzophenone triplet lifetime measurements show the presence of transients which are sensitive to oxygen. The decay of two species with half-lives of 1.7 and 27 h in iso-octane is noted. The major photoproduct, benzpinacol (l), has a low quenching constant (4 x lo6 1 mol-1 s-l) for triplet benzophenone, but the unstable compounds appear to be diffusioncontrolled quenchers. This complicates any measurements of the quantum yield of photoreduction in hydrogen donors, because the transients can affect J. Chilton, L. Giering, and C. Steel, J. Amer. Chem. Soc., 1976, 98, 1865.

413

Photochemistry the measured values both by competing light absorption and by their tripletquenching effect. Steel et a1.l have used Fourier-transform n.m.r. as a useful method for the investigation of these light-absorbing transients. The advantages of this method are that solutions of relatively low concentrations (ca. moll-l) can be quickly analysed in sealed tubes, free from the complication of exposure to oxygen. The n.m.r. spectra show that benzpinacol is formed immediately as the major photoproduct from benzophenone in propan-2-01 or cyclohexane; it is not formed by a slow dark reaction of the light-absorbing transients. In fact, transients such as (2) and (3) can amount to only a small fraction of the total product (perhaps 2%). Schuster and his co-workers have raised some interesting general points concerning photoreduction in their study of light-intensity effects in the photochemistry of cyclohexadienone (4). The main products from irradiation of (4) in propan-2-01 are the reduction product p-cresol (3,the cyclopentenone ether (6), chloroform, and acetone (Scheme 2). The most important observation is a 414

0 6 kfMe2 0

OH

0

+ CHCl3 + Me,CO

-I-

Me CC1,

Me

Me

(4)

marked dependence of the quantum yield of p-cresol ( 5 ) formation on the light intensity, whereas formation of ( 6 ) is negligibly affected. Relevant values for formation of (5) in propan-2-01 are shown in Table 1, and similar trends are

Table 1 Eflect of 366 nm light intensity on the quantum yield of p-cresol(5) from dienone (4) Solution deoxygenated No No No No No a Yes

Yes

Light intensity / 10lephotons cm-2 min-l 6.89 12.4 27.7 106.5 104.3 6.9 108.3

Total photons absorbed1 1018 6.08 5.58 6.29 6.23 6.88 6.13 6.48

(5)

0.02 0.05 0.06 0.12 0.01 1.91 1.54

LSolutionstirred during irradiation.

observed in diethyl ether or cyclohexane. The differences between aerated and deoxygenated solutions are obvious, and are attributed to the interception of radical intermediates such as (7) by oxygen, thereby inhibiting the formation of ( 5 ) . Stirring lowers the quantum yield, indicating that the diffusion of oxygen in a

D. I. Schuster, G. C. Barile, and K. Liu, J . Amer. Chem. SOC.,1975, 97,4441.

415

Photo-reduction and -oxidation

the solution is important. At high light intensities, oxygen in the solution is rapidly depleted, and reaction proceeds by the mechanism outlined in reactions (1)-(5). In the absence of oxygen, the quantum yield for formation of ( 5 ) (4)

- hu

3(4)*

1(4)*

+ Me,CHOH

(7)

eCls

+ Me,CHOH + (4)

___+

Me,eOH

3(4)*

+ Me,eOH ( 5 ) + CCI, CHCl, + M&OH (7) + Me,CO (7)

(1) (2)

(3) (4) (5)

actually decreases slightly with increasing light intensity. This may be because the steady-state concentration of free radicals increases with light intensity, and radical-radical reactions such as reaction ( 6 ) (discussed later in this section) serve to terminate the free-radical chain of reactions (3)-(5). The triplet (4) 2Me,dlOH

Me,CHOH

+ MeC(OH)=CH,

(6)

is the precursor of the zwitterion (8), which leads to (6), and this pathway is not noticeably affected by oxygen. Formation of ( 5 ) is quenched by cyclohexa1,3-diene and by di-t-butyl nitroxide, although these quenchers cannot be intercepting the triplet state of (4) because the formation of (6) is little quenched. Consequently, it is proposed that the quenchers can act not only by tripletenergy quenching, but also by scavenging intermediate radicals, as previously suggested by other workers.a, Certainly the Stern-Volmer plot of ( 5 ) quenching against cyclohexadiene concentration is non-linear, with an initially steep portion at low diene concentrations. OH

0-

Schuster considers that more attention should be paid to light-intensity effects, especially in reactions with radical intermediates. He points out that some of the observed changes in the course of photochemical reactions on varying the wavelength of the incident light could be due to light-intensity effects in such systems. A further publication has reinforced the view 6, that decafluorobenzophenone (9) is unsuitable for use as an actinometer. Not only do fast dark reactions occur when irradiating low concentrations of (9) in propan-2-01, but also the reaction becomes more complex with increasing concentration of (9). a

ti

P. J. Wagner, J. M. McGrath, and R. G. Zepp, J. Amer. Chem. SOC.,1972, 94, 6883. D. R. Charney, J. C. Dalton, R. H. Hautala, J. J. Snyder, and N. J. Turro, J. Amer. Chem. SOC.,1974, 96, 1407. G. Gauglitz and U. Kolle, J. Photochem., 1975,4, 309. J. Dedinas, J. Amer. Chem. SOC.,1973, 95, 7172. P. Margaretha, J. Gloor, and K. Schaffner, J.C.S. Chem. Comm., 1974, 565.

41 6 Photochemistry Murai and Obi have investigated the photochemistry of benzophenone, acetophenone, and benzaldehyde in alcoholic solvents at 77 K under high light intensities (although the actual fluxes used are not mentioned). Whereas the lowest triplet excited state of benzophenone does not produce the ketyl radical (Scheme 3; X = Ph) below 100 K and at normal light intensitie~,~ reactions do N

OH

0

II

(Ph-C-X)"

+ RH

I

Ph-C-X

+

Rm

Scheme 3

appear to take place at high light intensities via a biphotonic process involving a higher excited triplet state of the carbonyl compound.8 This hydrogen abstraction from solvent produces ketyl radicals and solvent-derived radicals (Scheme 3 ; X = Ph, Me, or H), both of which were detected by e.s.r. In the case of benzaldehyde, competing a-cleavage to produce benzoyl radicals is also important. Laser photolysis of benzophenone in benzene and cyclohexane leads to the ketyl radical (Ph&OH),1° which could be made to fluoresce by simultaneous irradiation with a beam of electrons.ll An e.s.r. study has been made of the ketyl radical produced on irradiation of benzophenone labelled at the carbonyl by 13C.12 Nanosecond flash photolysis measurements suggest that transient hydrogen abstraction by triplet benzophenone provides a pathway in its deactivation route, even in the absence of photoprodu~ts.~~ In the presence of the hydrogen donors NN-dimethyltoluidine or 3-methylindole, xanthone reacts to give the xanthone ketyl radi~a1.l~Quenching of this ketyl radical by oxygen is very rapid ( k = 2-3 x lo91 mol-l s-l). Using a specifically deuteriated steroid, Breslow and co-workers l5 have confirmed the earlier proposal l6for the mechanism of the stereospecific functionalization of the steroid nucleus by means of the photochemical hydrogen abstraction of attached benzophenone esters. The photoreduction of the bridgehead phenyl ketones (10)-(12) has been rep0rted.l' Both pinacol and carbinol are formed in the yields shown in Scheme 4,the remainder of the products arising from a-cleavage and subsequent reactions. A re-investigation of the photochemistry of methyl 2-naphthyl ketone shows that dilute solutions in propan-2-01 do appear to undergo slow reduction on irradiation at 350 nm, although the reaction products could not be isolated.18 In more concentrated solutions, triplet self-quenching leads to a reduction in product formation. H. Murai and K. Obi, J. Phys. Chem., 1975,79,2446. T. S. Godfrey, J. W. Hilpern, and G. Porter, Chem. Phys. Letters, 1967, 1, 490. l o 0. Brede, W. Helmstreit, and R. Mehnert, 2.phys. Chem. (Leipzig), 1975, 256, 505. l1 B. W.Hodgson, J. P. Keene, E. J. Land, and A. J. Swallow, J . Chem. Phys., 1975,63, 3671. la H. Murai, M. Jinguji, and K. Obi, J. Phys. Chem., 1976, 80, 429. l3 M. R. Topp, Chem. Phys. Letters, 1975, 32, 144. l4 A. Garner and F. Wilkinson, J.C.S. Faraduy 11, 1976, 72, 1010. l 5 R. L. Wife, D. Prezant, and R. Breslow, Tetrahedron Letters, 1976, 517. l6 R. Breslow, S. Baldwin, T. Flechtner, P. Kalicky, S. Liu, and W. Washburn, J. Ainer. Chem. SOC.,1973, 95, 3251. l 7 H.-G. Heine, W. Hartmann, F. D. Lewis, and R. T. Lauterbach, J. Org. Chem., 1976, 41, 1907. l 8 D.I. Schuster and M. D. Goldstein, Mol. Photochem., 1976, 7 , 209.

417

Photo-reduction and -oxidation

b4b

Ph

OH OH

0

II

Ph-c-R

I

Mc,CHOHp

1

OH

Ph-C-C-Ph I I

+ Ph-C-RII

36% 26% 74%

9% 11% 17%

I1v

R R

H

Scheme 4

Photoreduction of the benzanthrone (13) occurs on irradiation in hexane or alcohol ~ o l u t i o n s .Presumably, ~~ the 7 ~ nature 7 ~ ~of the lowest excited states accounts for the low quantum yield (ca. at 365 nm) of reduction. Irradiation

of 3-acetylcoumarin (14) in propan-2-01 produces a C-4-linked dihydro-dimer in high yield.20 Hydrogen abstraction by the acetyl group, followed by dimerization of the resulting coumarinyl radicals, is thought to be responsible for this reaction. Thus, a similar irradiation of 3-methoxycarbonylcoumarin in propan-2-01 does not lead to reduction. An interesting paper has appeared concerning the detection by CIDNP spectroscopy of enols formed during the photoreduction of aliphatic aldehydes and ketones.21 The enols (with lifetimes of 1@ 20s) are formed by disproportionation reactions of ketyl radicals bearing a hydrogen atom on the carbon adjacent to the radical centre (e.g. as shown for acetone in Scheme 5). These enols seem to be a general feature of aliphatic aldehyde and ketone photoreduction. Consequently, the enolization reaction presents an important route to carbonyl deactivation, because slow thermal ketonization of the enol regenerates starting materials. A study of [2H6]acetone photoreduction by propan-2-01 enables the ratio of rate constants to be found for the reactions in I @ P. Bentley, J. F. McKellar, and G. 0. Phillips, J.C.S. Perkin ZZ, 1975, 1259. 2o

21

K.-H. Pfoertner, Helv. Chim. Acta, 1976, 59, 834. B. Blank, A. Henne, G . P. Laroff, and H. Fischer, Pure Appl. Chem., 1975,41,475.

418

Photochemistry Me,CO

+ Me,CHOH

k

hv

-4 Me,C(OH)C(OH)Me,

2Me$OH kb

I Me,CHOH Scheme 5

+ MeC(OH)=CH,

Scheme 5. Disproportionation of the ketyl radicals (kd) predominates over combination (kc),kd/kc = 3.4, and over the back-reaction which involves OH hydrogen abstraction (kb), kb/kd = 0.3, in acetonitrile at 26 "C. Irradiation of acetone-propan-2-01 mixtures at room temperature produces pinacol as the sole isolated product. However, irradiation at low temperature ( - 70 "C) increases the lifetime of the enol so much ( 2 5000 s; cf. 15 s at room temperature) that the major product becomes an oxetan (15), formed by photocycloaddition of acetone to its eno1.22 MeC(OH)=CH,

+ Me,CO

'lv

H O Q , M e Me Me

CIDNP studies on benzaldehyde photolysis in hexane solution have shown that hydrogen abstraction by triplet benzaldehyde occurs from another molecule of ground-state benzaldehyde [reaction (7)] rather than from hexane, giving a ketyl and a benzoyl radical.2s Further CIDNP studies now reveal a rate constant 3PhCH0

+ PhCHO

-

PhCHOH

+ PhkO

(7)

of 1 x lo61mol-1 s-1 for any subsequent exchange reaction between the ketyl radical and ground-state benzaldehyde [reaction (8)].24 PhCHOH

+ PhCHO

PhCHO

+ PhCHOH

(8)

A full account of the CIDNP spectra arising from irradiation of aliphatic aldehydes in a variety of solvents has been p~blished.,~The relative importance of hydrogen abstraction us. a-cleavage in different solvents varied only for propionaldehyde; acetaldehyde gave self-abstraction, whereas isobutyraldehyde and pivalaldehyde appeared to give only a-cleavage in all the solvents studied. A CIDNP study of photoreactions of formaldehyde in solution showed that the primary process was hydrogen abstraction by triplet formaldehyde from another ground-state formaldehyde molecule.2s A similar self-abstraction step can be the initiating process in aldehyde photo-oxidation in the liquid phase for but-2-ena1, heptanal, and ben~aldehyde.~' 22

2s 24 26

as

A. Henne and H. Fischer, Helv. Chim. Acta, 1975, 58, 1598. P. W. Atkins, J. M. Frimston, P. G . Frith, R. C. Gurd, and K. A. McLauchlan, J.C.S. Faraday ZZ, 1973, 69, 1542. P. G. Frith and K. A. McLauchlan, J.C.S. Faraday ZI, 1975, 71, 1984. H. E. Chen, M. Cocivera, and S. P. Vaish, Canad. J. Chem., 1975,53,2548. J. A. Den Hollander and J. P. M. Van der Ploeg, Chem. Phys. Letters, 1976,37, 149. J. C. Andre, M. Bouchy, and M. Niclause, J. Photochem., 1976, 5, 1.

Photo-reduction and -oxidation

419

Funke and Cerfontain2* have examined in detail the photoreduction of cyclopropanecarbaldehyde (16) and cyclobutanecarbaldehyde. For example, (16) irradiated in propan-2-01 led to the nine products shown in Scheme 6. The

Me,CO

Me,C(OH)C(OH)Me,

Me(CH,),CHO

0 OH

0 0

II

+

II

0 OH0 II I II

A

H OHOH

+

[t-CH,OH

+

PC-C I -M / e

I

I

0

+

pC-(C II H , ) , C H O

H Me Scheme 6

first step would be expected to be formation of the cyclopropylhydroxymethyl radical (17): the structures of all the observed products are understandable in terms of three competing reactions of (17), viz. radical combinations, hydrogen abstraction from solvent and from aldehyde, and rearrangement to 4-oxobutyl radicals [(lS) in Scheme 71. Relatedly, Davies and Muggleton 29 report that the

(19)

Scheme 7

same radical (17) ring-opens and also rearranges via enolic hydrogen abstraction to give (19), so that some other reaction products might possibly have been expected in Funke and Cerfontain's work. Photoreduction of cyclopropyl methyl ketone in the presence of 1-cyclopropylethanol leads to a similar ringopening and rearrangement of the 1-cyclopropyl-1-hydroxyethyl 28

29

C. W. Funke and H. Cerfontain, J.C.S. Perkin II, 1976, 669. A. G. Davies and B. Muggleton, J.C.S. Perkin II, 1976, 502.

Photochemistry

420

whereas photoreduction of cyclobutanecarbaldehyde gives no evidence for ring-opening of the cyclobutylhydroxymethyl Irradiation of the ?&unsaturated carbonyls (20) in hydrogen-donating solvents such as pentane, propan-2-01, or toluene led to photoreduction of the carbonyl group of (20) as one of the observed reactions.30 0

(20)

R

=

Me or H

McKelvey has begun a series of experiments aimed at understanding photochemical hydrogen abstraction from carbohydrates, using ketone sensitizers and Abstraction occurs model compounds such as 2-metho~ytetrahydropyran.~~ from C-2, and the methoxytetrahydropyranyl radical then undergoes the further reactions of methyl loss or ring-breaking. A further paper 32 extends this work to the 2-methoxy-4-methyltetrahydropyrans(21) and (22), and there is an interesting conformational effect on abstraction. Both isomers (21) and (22) give the products shown in Scheme 8, as might have been expected from the earlier

M&OMe,

0

Do +

Ph &CO,Me

However, (21) reacts eight times faster than (22), showing a preference for axial hydrogen abstraction. These results can be explained by an anomeric effect: if oxygen non-bonding orbitals antiperiplanar to the C-H bond being broken stabilize the transition state for abstraction, (21) (two interactions) would be more reactive than (22) (one interaction). Irradiation of 1,4-dioxan leads, by hydrogen abstraction, to two pairs of diastereoisomers, (23) and (24).33 It is postulated that a dioxyl radical (25) gives ring-breaking similar to that mentioned above,31 leading to ethoxyacetaldehyde, then (photochemically) to acetaldehyde, and the photoreduction of this in dioxan gives the diastereoisomeric alcohols (24). It is not obvious whether absorption by impurities or direct absorption by dioxan provides the original source of dioxyl radicals (25). M. P. Zink, H. R. Wolf, E. P. Miiller, W. B. Schweizer, and 0. Jeger, Helv. Chim. Acta, 1976, 59, 32. a1 3a

33

R. D. McKelvey, Carbohydrate Res., 1975, 42, 187. K. Hayday and R. D. McKelvey, J. Org. Chem., 1976,41,2222. P. H. Mazzocchi and M. W. Bowen, J. Org. Chem., 1975,40, 2689.

42 1

Photo-reduction and -oxidation H

Ledwith 34 has reviewed the photoinitiation of polymerization by aromatic carbonyl compounds, quoting examples which involve photochemical hyd,rogen abstraction as the radical-generating step. The diphenylketyl radical (Ph,COH), generated photochemically by irradiation of benzophenone in propan-2-01, gives a novel radical substitution reaction on the 4-cyanopyridinium ion, probably as a result of an electron-transfer reaction from ketyl to 4-cyanopyridinium Previtali and Scaiano 38 have continued their theoretical study of the photoreduction of carbonyl triplets, applying it this time to the rates of hydrogen abstraction from bonds other than C-H. The furan-2,3-dione (26) reacts photochemically at the C-3 carbonyl group with cyclohexene or 2-methylbut-2-ene, to give mixtures of oxetans and hydrogen-abstraction Choo and Wan 38 have made a comparative study of CIDNP and CIDEP (electron polarization) spectra in the photoreduction of pyruvic acid in hydrogen-donating solvents. Ammonia has been found to give an unusual electron-transfer catalysis in the photochemical hydrogen abstraction of anthraquinone (AQ) (27) from the 0

0

relatively poor hydrogen donor t-butyl Irradiation of (27) in the presence of t-butyl alcohol gives the adduct (28)) but the quantum yield of reaction is greatly enhanced by ammonia (@ = 0.0058 becomes @ = 0.10 in the a4

36 3*

37 38 9B

A. Ledwith, J. Oil Colour Chemists' ASSOC.,1976, 59, 157. B. M. Vittimberga, F. Minisci, and S. Morrocchi, J . Amer. Chem. SOC., 1975, 97, 4397. C. M. Previtali and J. C. Scaiano, J.C.S. Perkin ZI, 1975, 934. W. Friedrichsen, Annalen, 1975, 1545. K. Y.Choo and J. K. S. Wan, J. Amer. Chem. SOC.,1975,97,7127. G. G. Wubbels, W. J. Monaco, D. E. Johnson, and R. S. Meredith, J. Amer. Chem. SOC., 1976,98, 1036.

422

Photochemistry

presence of 0.6M NH3). Quenching experiments suggest that the reaction goes through a triplet intermediate of (27), which interacts more rapidly with ammonia than with t-butyl alcohol. A mechanism is proposed [and outlined in reactions (9)-(13)] which involves exciplex formation between triplet AQ as acceptor and ammonia as donor [reaction (lo)]. Hydrogen-atom abstraction by the exciplex, 3AQ

[AQ'-NH,'+] AQ'-

+ Me,COH 3AQ + NH3

''

>

'lo

k

[AQ'-NH,'+] ---+ Me,COH

+

+ *CH,C(OH)Me,

(H+) _I___,

AdH

+ *CH,C(OH)Me,

[AQ'-NH;+]

+

(10)

AQ NH3 NH4+ AQ*-

+

(9)

(1 1)

+ *CH,C(OH)Me, (12)

(28)

(13)

or possibly by free ammonia radical ion (NH3'+), leads eventually to (28). A kinetic analysis shows that k , = 7.1 x lo4 lmol-ls-l, whereas klo = 2 x lo7Imol-ls-l, thus showing the important role of ammonia in complex formation. McLauchlan and Sealy 40 have questioned the assumption that triplet quinones always react with alcohols (e.g. R,CHOH) to produce hydroxyalkyl radicals (e.g. R&OH). E.s.r. spin-trapping experiments have led to the detection of alkoxy-radicals (e.g. R,CHO*) which may have been formed directly or else by initial electron transfer [reactions (14) and (15); Q = quinone]. Although these workers were unable to obtain a precise estimate of the quantum yield of

+ RzCHOH RZCHOH" + RZCHOH 3Q

___+

+ R,CHOH*+ R,CHO* + RzCHOHz+

Q*-

(14)

(15)

alkoxy-radical production, they do provide indications that it is a significant process in the photochemistry of quinone-alcohol systems. Combined CIDNP and CIDEP studies have been made of the photoreduction , ~ ~CIDNP studies of the photoof tetrafluoro-p-benzoquinone in d i ~ x a n and reduction of a series of 1,4-benzoquinones in propan-2-01 have been r e p ~ r t e d . " ~ The much-researched photoreduction of duroquinone (D) (29) has received further attention.43 The triplet state of duroquinone is formed with unit efficiency following excitation in cyclohexane, ethanol, or water. Quantum 0

0 (29) 40 41 42

4a

OH (30)

K. A. McLauchlan and R. C. Sealy, J.C.S. Chem. Comm., 1976, 115. H. M. Vyas and J. K. S. Wan, Canad. J. Chem., 1976, 54, 979. D. A. Hutchinson, H. M. Vyas, S. K. Wong, and J. K. S. Wan, Mol. Phys., 1975,29, 1767. E. Amouyal and R. Bensasson, J.C.S. Faraday I, 1976,12, 1274.

423

Plioto-reduction and -oxidation

yields of photoreduction to the semiquinone radical (*DIP) are 0.4 5 0.1 in ethanol, 0.09 k 0.03 in cyclohexane, and 0.00 in water. Triplet-triplet annihilation [reaction (16)] is the only pathway for photoreduction of (29) in water, and obviously becomes appreciable at high 3D concentrations, producing 3D + 3D

-

Do+

+ Do-

(1 6 )

the radical anion (D*-) which leads on to reduction product. Formation of the duroquinone methide (30) is not a major pathway for deactivation of the 3D state in The quantum yield of photoreduction of 1,4-naphthoquinone is equal at two wavelengths (0ca. 0.90 at 334 nm and 436 nm), corresponding to transitions into the m* and nr* states, which implies unit efficiency for the m* -+ nrr* c o n v e r ~ i o n . ~ ~ Electron transfer from hydroxide ion produces the quinone radical-anion on photoreduction of p-benzoquinone or 1,4-naphthoquinone in water,46 or of anthraquinone in ethanolic potassium hydro~ide.~' The photoreduction efficiency of quinone sulphonates in water is affected by the presence of cationic surf act ant^.^^ Cohen and his co-workers have continued their extensive studies of the photoreduction of ketones by amines, devoting their attention this time to the effect of amine concentration and solvent on the photoreducti~n.~~ The generally accepted pathway (Scheme 9) for reduction involves rapid formation of a chargetransfer complex (31) between triplet ketone and amine. Then, either hydrogen

c 0

II

C,

0

+

>N'

I

CH

' \

Scheme 9

transfer to produce radicals (a) or quenching (6) occurs. Such a mechanism would predict a linear plot of (quantum yield)-l us. (amine concentration)-l. However, plots for photoreduction of benzophenone by cyclohexylamine in either benzene or t-butyl alcohol are curved. Higher quantum yields are obtained at higher amine concentrations (> 0.02 moll-l) than would be expected 4p 46 46

47 4a

49

D. Creed, J.C.S. Chem. Comm., 1976, 121. J. Rennert and P. Ginsburg, J. Photochem., 1975, 4, 171. S. Hashimoto, H. Takashima, and M. Onohara, Nippon Kagaku Kaishi, 1975, 1019. V. Ya. Oginets, Khim. vysok. Energii, 1975, 9, 190. K. Kano, Y. Takada, and T. Matsuo, Bull. Chem. SOC.Japan, 1975,48, 3215. A. H. Parola, A. W. Rose, and S. G. Cohen, J. Amer. Chem. SOC., 1975, 97, 6202.

Photochemistry from extrapolation of the results at low amine concentrations. The effect is less marked in aqueous pyridine (at pH 12) for the reduction of 4-benzoylbenzoate ion by triethylamine, and is not evident in the reduction by 2-butylamine under these conditions. Explanations for these results are based on two related arguments: (i) the amine catalyses the transfer of a proton from radical-cation to radical-anion (a) in the compIex (31), (ii) a ground-state complex of ketone and amine is formed which, after excitation, interacts with amine in solution to produce (31). It seems that much more work is required before these proposals could be proven. Arimitsu and co-workers 50 have extended their earlier laser photolysis studies 51 of the quenching of triplet benzophenone by various amines. Tertiary aromatic amines (e.g. NN-diethylaniline and NN-dimethyl-p-toluidine) give rise to electron transfer in acetonitrile as solvent, producing benzophenone radical-anion and amine radical-cation. However, hydrogen abstraction is the observed process in benzene, producing the benzophenone ketyl radical. Using a range of solvents, it has been shown that the two processes of electron transfer and hydrogen abstraction compete according to the polarity of the solvent. The sum of the quantum yields for ionic dissociation and photoreduction is unity for both the benzophenone-diethylaniline and the benzophenonedimethyltoluidine systems. Primary and secondary aromatic amines or aliphatic amines produce the benzophenone ketyl radical in all the solvents used. Roth and Manion have been able to distinguish the spectra of the neutral aminoalkyl radicals (32) and the aminium radical-cations (33) by means of their CIDNP hyperfine coupling p,p’-Disubstituted benzophenones (X = Cl, Me, or MeO) irradiated in acetonitrile in the presence of triethylamine 424

(33)

(34)

gave CIDNP signals assigned to diethylvinylamine which implied the intermediacy of neutral radicals (32). On the other hand, irradiation of decafluorobenzophenone (9) in acetonitrile in the presence of NN-diethyl-p-toluidine suggested the radical-cation (33) as an intermediate. The amine NN-diethylp-toluidine reacted with several other triplet aromatic ketones in acetone to give evidence for both (33) and (32), i.e. an electron-transfer process and a net hydrogen abstraction. These results are certainly in agreement with those of Arimitsu et aLY5Oand illustrate the importance of solvent in determining the species formed in such systems. Unfortunately, Roth and Manion52could not tell whether species (33) and (32) were formed consecutively or independently. A full account has appeared of the a-diketone (triplet) sensitized decomposition of the acetoin derivatives (34) where R is a nitrogen-containing substituent such as pyrrol-Zyl or ind01-3-yl.~~Electron transfer from nitrogen to photoexcited 60

61 s2 63

S. Arimitsu, H. Masuhara, N. Mataga, and H. Tsubomura, J. Phys. Chem., 1975,79,1255. S. Arimitsu and H. Masuhara, Chem. Phys. Letters, 1973, 22, 543. H. D. Roth and M. L. Manion, J. Amer. Chem. SOC.,1975,97, 6886. H.-S. Ryang and H. Sakurai, J.C.S. Perkin I, 1975, 1590.

Photo-reduction and -oxidation

425

a-diketone leads to a further fragmentation through the N-containing radicalcation (see Vol. 6, p. 521). A study of the photoreduction of 4-benzoylbenzoate ion by methionine (35) and related compounds (36)-(43) shows the relative importance of chargetransfer interaction of the excited ketone with either sulphur or nitrogen as the electron-donor atom.54 For example, triplet ketone interacts with the methionine anion to produce the triplet exciplexes shown in Scheme 10. Later steps lead I\

3(

,C = 0) -k MeSCH,CH2CH(NT3,)CO2-

Scheme 10

on to decarboxylation and hydrogen abstraction which yields ketyl radicals. Quantum yields of ketyl radical production (@)ketyl) and rate constants for complexation (kh) are shown in Table 2. For the S-containing compounds Table 2 Photoreduction of 0.003 mol 1-1 4-benzoylbenzoate ion by 0.04 mol 1-1 methionine and related compounds pH 12 7

Reducing agent MeSCH,CH,CH(NH,)CO,MeSCH,CH,CH(NHCOMe)C02MeSCH2CH2CH2NH, MeSCH,CH,CH,CO,MeSCH,CH,CH,NHCOMe MeCH(NH,)CO2MeCH(NHCOMe)CO,MeCH(NHC0Me)Me MeOCH,CH,NH,

PH7

7 7

kirl

kir/

@)k&pl

0.90 0.43

0.28 0.09 0.09 0.93 0.03 0.07 0.67

1 mol-l s-l 1.6 x log 1.5 x lo9 2.0 x 109 1.3 x 109 1.3 x lo9 1.6 x lo8 2.1 x lo6 1.0 x lo6 1.8 x lo8

@ketsrl

0.55 0.12 0.11 0.13 0.07 0.26 0.05 0.05 0.21

1 mol-l s-l 1.2 x 109 1.5 x 109 1.7 x 109 1.1 x 109 1.1 x 109 -2 x 105 -2 x 105 -3 x 105 1.1 x 106

(35)-(39), values of ki, are high (1-2 x lo9 1 mol-1 s-l) and independent of pH or other functional groups. The amino-compounds (40) and (43) show values of kh about an order of magnitude smaller, at pH 12. In contrast, these amines are largely protonated at pH 7, and the values for ki, are small. The b4

S . G . Cohen and S . Ojanpera, J. Amer. Chern. SOC.,1975,97, 5633.

426 Photochemistry amides (41) and (42), as expected, show quite low values of kir. Consequently, it can be deduced that the initial charge-transfer interactions for (35) and (37) are ca. 90% at sulphur and 10% at nitrogen (because kir = lOki,,), and for (36) >99.9% at sulphur. The quantum yield for reduction arising from interaction at sulphur may be quite low (< 0.09), so that the larger values observed for (35) and (37) may be taken as evidence for electron transfer (kxin Scheme 10) within the triplet exciplexes. A related mechanism is responsible for the synthesis of medium- and large-sized rings which occurs by cyclization on irradiation of sulphur-containing phthalimides (44).66 Charge-transfer interaction of excited 0

0 (44) IZ = 5, 6, 8, 9, 10, or 12

carbonyl with thioether may lead to easy removal of protons from C-H bonds adjacent to the sulphur atom. The resulting biradicals subsequently undergo ring closure, and reaction is therefore regioselective. A variety of 3-aminopropiophenone derivatives gave photopinacolization products (2-25% yield) on U.V. irradiation.66 Other p-amino-ketones such as N-methyl-4-piperidone7 tropinone, or 1-diethylaminobutan-3-one apparently underwent photoreduction to the corresponding /I-amino-alcohols on irradiation at 238 or 313 nm in hexane, although no reaction products were Interaction of singlet excited alkanones with diethylamine and triethylamine probably generates an exciplex.68 Singlet reaction rates, as measured by alkanone fluorescence quenching, show a strong dependence on the steric accessibility of the alkanone carbonyl group. 2 Reduction of Nitrogen-containing Compounds Dopp has published an interesting review of the triplet-state reactions of aromatic nitro-compounds, covering hydrogen abstractions and photoreductions of nitrobenzenes, nitronaphthalenes, and nitropyridine~.~~ Cu and Testa 6o have reported the photoreduction of 5-nitroquinoline in 50% aqueous propan-2-01 in the presence of hydrochloric acid. These authors (see Vol. 7, p. 401) have outlined the reaction mechanism as electron transfer from chloride ion to triplet nitrocompound, leading to 5-amino-6,8-dichloroquinoline.61 Photoreduction of 1-,2-, 3-, and 4-nitro-9-acridones to the corresponding amino-9-acridones occurs in high yield on irradiation in alcoholic m 66

b7 68

6Q 6o 6a

Y. Sato, H. Nakai, T. Mizoguchi, Y. Hatanaka, and Y. Kanaoka, J , Amer. Chem. SOC.,1976, 98, 2349. H. J. Roth, A. Abdul-Baki, and T. Schrauth, Arch. Pharm., 1976, 309, 2. A. M. Halpern and A. L. Lyons, J. Amer. Chem. Soc., 1976,98,3242. J. C. Dalton and J. J. Snyder, J . Amer. Chem. SOC., 1975,97, 5192. D. 0. Dopp, Topics Current Chem., 1975, 55, 49. A. Cu and A. C. Testa, Mol. Photochem., 1974, 6, 473. A. Cu and A. C. Testa, J. Phys. Chem., 1975,79,644. V. Zanker and E. Cmiel, Annalen, 1975, 1576.

427

Photo-reduction and -oxidation

Several further examples of the photoreduction of nitrogen-containing heterocyclic compounds have been reported during the year. The pyridine (45) upon irradiation in aqueous acetonitrile in the presence of diethylamine gives the reduction products (46) and (47), rather than an adduct of amine with heterocycle.63 It is possible that (47) is a secondary photolysis product, formed from (46).

RaR RaR hv, EtpNH

R

$R

+

H

R

H

(45) R = C0,Et

H

(47)

(46)

CIDNP Spectroscopy has again been applied to the photoreduction of acridines in the presence of hydrogen donors (see Vol. 7, p. 402). Libmans4 has examined the role of a singlet radical pair in the hydrogen abstraction by excited acridine (48) from carboxylic acids. The generalized reaction products and the intermediate radical pair are shown in Scheme 11. French workers 65

+

RC0,H

(48)

H

H Scheme 11

have likewise studied the singlet radical pair formed by abstraction of benzo[a]and benzo[c]-acridines from dioxan or tetrahydrofuran. Six-membered monoaza-aromatics such as pyridine, quinoline, 4-methylquinoline, isoquinoline, and 9-phenylacridine are able to produce radicals analogous to (49) on irradiation in methanol.66 The reaction is believed to be 83 84 66

E8

K. Kano and T. Matsuo, Tetrahedron Letters, 1975, 1389. J. Libman, J.C.S. Chem. Comm., 1976, 198. G. Vermeersch, N. Febvay-Garot, S. Caplain, and A. Lablache-Combier, Tetrahedron, 1976, 32,935. .A. Castellano, J. P. Catteau, and A. Lablache-Combier, Tetrahedron, 1975, 31, 2255.

428 Photochemistry one of hydrogen abstraction by the heterocycle nn* excited state. Although these same radicals are produced on irradiation in HC1-acidified methanol, a different biphotonic process pertains. In this latter case, it is proposed that electron transfer occurs from alcohol to an upper excited triplet state of the protonated heterocycle. An e.s.r. investigation of the U.V. irradiation of acridine and quinoline in 2-methyltetrahydrofuran at low temperatures has been published,67 and the effect of [Cr(CN)J3- ion on the photoreduction of acridine in ethanol noted.gs The photoreduction of benzo[c]cinnoline in aqueous acidic alcohols involves the protonated species (50) and yields the protonated 5,6-dihydro-derivative (51).69 Further reduction of (51) on irradiation in ethanol at wavelengths greater than 400 nm produces 2,2'-diaminobiphenyl, possibly through the participation of MeeHOH radicals. The curious formation of carbazole from (50) on ,~~ irradiation at wavelengths less than 380 nm has been reported p r e v i o ~ s l y and is thought to represent an alternative pathway for (51) reaction, although the exact route remains unknown. The photoreduction of porphyrins to chlorins by tertiary amines has been studied by e.s.r. and flash phot~lysis.~l

(52) R = Me or Ph

A full account has appeared of the photoreductive ring cleavage of 3,5-disubstituted isoxazoles (52) to amino-enones (53), from which it now seems that reaction can be catalysed by Cu" The photoreduction of imines still attracts attention. The reduction of some N-aroylimines [e.g. (54)to (55)] occurs cleanly on irradiation in propan-2-01.7~ Ph2C=NCOPh (54)

v Me,C11HOHr Ph,CHNHCOPh

(55)

87

A. Castellano, J. P. Catteau, and A. Lablache-Combier, Photochem. and Photobiol., 1976, 23,

6*

K. Nakamaru and H. Murakami, Sci. Reports Hirosaki Univ., 1975,22,31 (Chem. Abs., 1976,

135. 84, 42 868).

72

H. Inoue, T. Sakurai, and F. Tanaka, Bull. Chem. Soc. Japan, 1975,48,924. H. Inoue and Y. Matsuka, Chem. Letters, 1972, 713. Y.Harel, J. Manassen, and H. Levanon, Photochem. and Photobiol., 1976, 23, 337. T. Sato, K. Yamamoto, K. Fukui, K. Saito, K. Hayakawa, and S. Yoshiie, J.C.S. Perkin I,

ps

A. Padwa and W. P. Koehn, J. Org. Chem., 1975,40,1896.

89 70

71

1976, 783.

429 However, there are puzzling differences in the mechanisms of reaction, according to the substitution pattern. Thus, the compound (54), like alkylimines, apparently reacts by the ‘chemical sensitization’ route, in which traces of sensitizer (e.g. benzophenone) generate the ketyl radicals which are the effective reducing agent. On the other hand, the N-aroylimines (56)-(58) are photoreduced to (59)-(61)

Photo-reduction and -oxidation

Me Ph I I Me(CH,),C-C=NCOAr

Me Ph

I I &Me (CH,),?-CHNHCOAr

I

Me (56) n (57) n (58) n

= = =

Me

1, Ar = Ph 2, Ar = Ph 1, Ar = p-MeOC,H,

respectively, with reaction shown to occur from the nn* triplet state of the imine. The failure of (56)-(58) to give a Norrish Type I1 reaction suggests that the initially occurring hydrogen abstraction is by the carbonyl oxygen atom rather than by the imine nitrogen. Triplet quenching experiments show a low rate of reduction for (56)-(58) (k N 1 x lo31 mol-l s-l in propan-2-01), and a high rate of triplet decay, which account for the low quantum yields of reduction (a ca. lo+). Irradiation of the N-acetylimine (62) in toluene leads to the products of solvent addition (63) and reduction (64).74175 Again, evidence is presented for a mechanism of hydrogen abstraction by the excited imine.

Ph,C=NCOMe

PhMe

+

Ph,b-NHCOMe

+ Ph,CHNHCOMe

Irradiation of the iminolactone (65) in propan-2-01 gives reduction to a mixture of the dl- and meso-dimers of the radical (66).7s This free radical is exceptionally stable, and hence the dimers possess an unusually weak C-C bond.?? A study of the photochemistry of 4-acylpyrimidines has enabled Alexander and Jackson78to estimate the relative reactivities of the C=O and C=N groups in these pyrimidines towards intramolecular hydrogen abstraction. Both C=O and C=N triplets have about equal reactivity towards primary C-H hydrogen abstraction ( k N 7 x lo7s-l). Curiously, C=N triplets are not much more S. Asao, N. Toshima, and H. Hirai, Bull. Chem. SOC.Japan, 1975,48, 2068. N. Toshima, S. Asao, and H. Hirai, Chem. Letters, 1975, 451. 7* T. H. Koch, J. A. Oleson, and J. DeNiro, J. Org. Chem., 1975, 40,14. ‘’ T. H. Koch, J. A. Oleson, and J. DeNiro, J. Amer. Chem. SOC.,1975, 97, 7285. E. C. Alexander and R. J. Jackson, J. Amer. Chem. SOC.,1976,98,1609. 15 74

7L

Photochemistry

430

reactive towards secondary hydrogen abstraction (k = lo8 s-l), whereas C=O triplets are so (k = 1.2 x log s-l). 3 Miscellaneous Reductions The role of transition-metal complexes in photo-assisted hydrogenation continues to be investigated. Schroeder and Wrighton 7g have examined photocatalysed alkene isomerization and hydrogenation using [Fe(CO),]. Hydrogenation occurs on irradiation (300-380 nm) of mixtures of [Fe(CO),] and hydrogen with alkene, the key photochemical step being proposed as generation of [H,Fe(CO),(alkene)]. For simple alkenes, hydrogenation and isomerization occur at comparable rates. Butynediol is photohydrogenated to butenediol by use of the catalyst [IrCI(CO)(PPh,),].80 KroppS1 has continued his work on the photochemistry of alkenes. Unsymmetrically tetrasubstituted alkenes on irradiation in methanol give the corresponding alkanes, besides the products of addition of methanol. Similarly, 2-isopropylidenenorbornane leads to 2-endo-isopropylnorbornaneas a major component of the complex mixture of products. Kropp has now produced evidence in such systems for the ejection of free electrons, formed from the alkene following electron promotion to a Rydberg excited state. The only volatile products from sensitized irradiation of the exo- and endo-isomers of 5-chloronorbornene, 5-hydroxynorbornene, and 5-acetoxynorbornene are the corresponding norbornanes.82 Unexpectedly, no rearrangements or exo-endo isomerizations are noted during these photoreductions. The bicyclic enone (67) 0

is reduced on irradiation in the presence of cyclopentene or cyclohexene as hydrogen donors, yielding (68) amongst the observed products.8s The photoreduction of phenols by sodium borohydride to the corresponding cyclohexenols and cyclohexanols occurs in a surprisingly specific manner.84 By means of n.m.r. shift reagent studies, Barltrop and Bradbury have been able to ascertain the positions of deuterium incorporation on irradiation of p-cresol in aqueous sodium hydroxide solutions in the presence of NaBD,. The 4-methylcyclohexenol product (69) has deuterium incorporated only at the positions shown in Scheme 12. As also shown in this Scheme, a reaction mechanism is proposed to explain these results which involves phenoxide dissociation into phenoxyl radicals and solvated electrons. Subsequent attack by [BDJ- at C-1 of the phenoxyl radical is followed by intramolecular deuterium transfer by a cyclic 79

84

M. A. Schroeder and M. S. Wrighton, J. Amer. Chem. SOC.,1976, 98, 551. W. Strohmeier and K. Gruenter, J. Organometallic Chem., 1975, 90, 0 4 8 . H. G. Fravel and P. J. Kropp, J. Org. Chem., 1975,40,2434. S. J. Cristol, R. P. Micheli, G. A. Lee, and J. E. Rodgers, J. Org. Chem., 1975,40, 2179. A. Kunai, T. Omori, K. Kimura, and Y . Odaira, Bull. Chem. SOC.Japan, 1975,48, 731. D. Bradbury and J. Barltrop, J.C.S. Chem. Comm., 1975, 842.

43 1

Photo-reduction and -oxidation

0-

D

(69)

R

= H or a boron derivative Scheme 12

route. Further protonation and reduction steps lead to (69). Only this kind of pathway can account for both the observed positions and stereospecificity of deuterium incorporation. The 4-methylcyclohexanol also produced may be a secondary product, as it can be formed from 4-methylcyclohexenol on irradiation in the presence of phenol and NaBH,. Attack of hydride ion on excited aromatic compounds can lead to r e d ~ c t i o n . ~ ~ However, a different mechanism for the photoreduction of aromatic hydrocarbons by NaBH, is followed in the presence of an equivalent of an electron acceptor like 1,4-dicyanoben~ene.~~ Photo-Birch reduction to the dihydroderivatives occurs for phenanthrene, anthracene, or naphthalene, on irradiation in aqueous acetonitrile in the presence of NaBH, and 1,ddicyanobenzene. It seems likely that an exciplex of aromatic electron donor (D) with 1,4-dicyanobenzene acceptor (A) dissociates to give aromatic radical-cations, which are then attacked by borohydride ion (Scheme 13). Alternatively, borohydride may D

+ A ---%D'+

+A"

+

HZD A Scheme 13

BH.,-

H,O

>

HD'+X-

HD- + A

attack the exciplex directly. A similar mechanism to that of Scheme 13 no doubt also applies to the photoreaction of cyanide ion with phenanthrene and naphthalene in the presence of 1,4-di~yanobenzene.~~ Formation of an exciplex in which the aromatic molecule is an electron acceptor rather than an electron donor can also lead to photoreduction. Libman 86

J. A. Barltrop, Pure Appl. Chem., 1973, 33, 179. K. Mizuno, H. Okamoto, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975, 839. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975, 553. J. Libman, J. Amer. Chem. SOC.,1975, 97, 4139.

Photochemistry has described examples of this type in the reduction and reductive alkylation of 1-cyanonaphthalene (70) on irradiation in acetonitrile in the presence of methoxyphenylacetic acids (71a) and (71b) or phenoxyacetic acid (71c). The products are shown in Scheme 14. There is good evidence for exciplex formation in such 432

+

RCH,CO,H

(70) a; R = p-MeOC,H, b; R = rn-MeOC,H, .~ C ; R = PhO

'W

>

(71) -t

+ (RCHJ,

RMe

-t CO,

Scheme 14

systems [reaction (17)], and electron transfer from (71) to (70) in the exciplex [reaction (1 S)] would lead to cyanonaphthalene radical-anions. These may then take up a proton from the (71) radical-cation [reaction (19)], leading eventually to the observed products. The reaction of 0- and p-dicyanobenzenes in the ArCN*

+ RCH,CO,H

[ArCN *** RCH,CO,H]* ArCN'-

+ '+RCH,CO,H

-

[ArCN

RCH,CO,H]*

(17)

ArCN'-

+ '+RCH,C02H

(1 8)

+ RkH, + CO,

(19)

HArCN=

presence of triethylamine 8g probably occurs by a similar route. Certainly, the interception of exciplexes by chemical reactions is an increasingly important field in photochemistry. Photoreduction of substituted benzo[b]furans by aliphatic amines [e.g. (72) to (73)] probably involves electron transfer from amine to benzofuran, which

R

(72) a; R = H b;R=Me

(73)

produces the aromatic radical-anion. The reactivities of various substituted benzofurans have been correlated with calculated spin densities on these radicalanions; only when spin density is highest at the 2-position (as opposed to the 4-position) is a stable photoreduction product observed.go Lablache-Combier also reports the reductive photocyclization of some 2,3-diphenylbenzo[b]furans in n-pr~pylamine.~~ Interest in the fate of polychlorinated aromatic compounds such as polychlorinated biphenyls (PCB) and tetrachlorodibenz-p-dioxinin the environment K. Tsujimoto, K. Miyake, and M. Ohashi, J.C.S. Chem. Comm., 1976, 386. C. PBrkAnyi, A. Lablache-Combier,I. Marko, and H. Ofenberg, J. Org. Chem., 1976,41, 151. A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron, 1975, 31, 2023.

Photo-reduction and -oxidation

433

has led to several studies of the relevant photochemistry. Japanese workers O2 have described the photoreduction of 3- and 4-chlorobiphenyl by sodium borohydride in aqueous acetonitrile to yield biphenyl. The authors favour a mechanism of hydride attack on excited chlorobiphenyl, although it is not clear on what grounds they have rejected the previously proposed radical chain mechanism for reductions of this type.g3 A Swedish groupg4has studied the photochemical dechlorination of 1,2,4-trichIorobenzeneas a model compound relevant to PCB. The primary products on irradiation in cyclohexane or propan-2-01 are 1,3- and 1,4-dichlorobenzene, the product ratio of 1,3- : 1,44sorners being significantly different on direct irradiation (0.15) from that on acetone sensitization (4.8). These facts suggest that both singlet and triplet excited states give rise to dechlorination. Reductive dechlorination is the major reaction on irradiation of hexachlorobiphenyls in of unsymmetrically substituted tri- and tetra-chlorobiphenyls in c y c l o h e ~ a n e , and ~ ~ of polychloronaphthalenes in methanol The tranquillizer chlorpromazine gives a similar reductive dechlorination on irradiation in propan-2-01.~~ The photochemistry of polybromobiphenyls, which are used as plasticizers and flame retardants, has also been in~estigated.~~,Reductive debromination is the main reaction of bromobiphenyls on irradiation at ca. 300 nm in cyclohexane ~ 0 1 u t i o n .As ~ ~for PCB,loO2-substituted biphenyls show enhanced quantum yields of reaction over the 3- or 4-substituted isomers. The presence of triethylamine assists such reductions, and an electron transfer from triethylamine to excited bromo-compound may be responsible. The photoreduction of esters has aroused some interest. PBte lol has explored the reaction mechanism of the previously reported photoreduction of esters to alkanes in wet hexamethylphosphoramide (HMPA) [reaction (2O)].lo2 It appears R1C02R2

hv

RICO,H

+ R2H

that HMPA is the source of hydrogen, and that reaction goes through a radical intermediate. Moreover, absorption of light by either HMPA or ester can initiate reaction. One possibility is that an exciplex is formed in which an electron can be transferred from HMPA to ester. Subsequent reaction of the ester radical anion with water leads to reduction. Photoreductive removal of the toluene-p-sulphonyl group from tosylate esters of steroids occurs on irradiation in the presence of sodium borohydride, yielding alcohols.103 B2 93 84

g8 g7

K. Tsujimoto, S. Tasaka, and M. Ohashi, J.C.S. Chem. Comm., 1975, 758. J. A. Barltrop and D. Bradbury, J. Amer. Chem. SOC.,1973,95,5085. B. Akermark, P. Baeckstrom, U. E. Westlin, R. Gothe, and C. A. Wachtmeister. Acta Chem. Scand., 1976, B30, 49. L. 0. Ruzo and M. J. Zabik, Bull. Enuiron. Contam. Toxicol., 1975, 13, 181. L. 0. Ruzo, S. Safe, and M. J. Zabik, J. Agric. Food Chem., 1975, 23, 594. L. 0.RUZO,N. J. Bunce, S. Safe, and 0. Hutzinger, Bull. Enuiron. Contam. Toxicol., 1975,14, 341.

A. K. Davies, S. Navaratnam, and G. 0. Phillips, J.C.S. Perkin II, 1976,25.

N. J. Bunce, S. Safe, and L. 0. RUZO,J.C.S. Perkin I, 1975, 1607. L. 0. Ruzo, M. J. Zabik, and R. D. Schuetz, J. Amer. Chem. SOC.,1974,96, 3809. H.Deshayes, J. P. Pete, and C. Portella, Tetrahedron Letters, 1976, 2019. ln2 H. Deshayes, J. P. Pete, C. Portella, and D. Scholler, J.C.S. Chem. Comm., 1975,439. lo3 Y . Kondo, K. Hosoyama, and T. Takemoto, Chem. andPharm. Bull. (Japan), 1975,23,2167. 89

loo Io1

434 Photochemistry de Mayo lo4has summarized the hydrogen abstraction reactions given by aliphatic and aromatic thiones. Irradiation of organic disulphides in aldehyde solvents results in reductive fission of the S-S linkage, producing equimolar amounts of the corresponding thiol and acylated thiol.lo6 Lastly, attempts have been made to reach a better understanding of the photoreduction of thiazine dyes in aqueous solution.lo6 4 Singlet Oxygen The purpose of this section is to mention some of the organic aspects of the chemistry of singlet molecular oxygen (lo2, lAg). Ohloff lo7has reviewed the use of singlet oxygen as a reagent in organic synthesis, with emphasis on the preparation of important flavours and fragrances. New or modified sources of continue to be developed. Most dyes used as sensitizers of lo2production are anionic compounds and thus insoluble in aprotic solvents; consequently, this problem must be circumvented for work in such solvents. Schaap and co-workers lo*have published a full account of their work on the use of polymer-based dye sensitizers for generation.loB Such sensitizers have the advantages of being able to function heterogeneously, being more stable towards bleaching than unbound dye, and being easily removed after reaction. For example, polymer-bound Rose Bengal functions quite efficiently, giving a quantum yield of lo2of 0.43 in dichloromethane. It is also possible to prepare polymer-bound sensitizers such as eosin-Y, ffuorescein, chlorophyllin, and haematoporphyrin (the last two possibly being of importance in biological oxidation studies). As an alternative to heterogeneous sensitization, Boden 110 has described the use of homogeneous photosensitization. The dyes Rose Bengal and eosin-Y can be made soluble in carbon disulphide or dichloromethane by the use of crown ethers (e.g. 18-crown-6) or quaternary ammonium salts (e.g. tricaprylmethylammonium chloride), and can then function as sensitizers of lo2production. Other workers 111 have examined the generation in aqueous micellar systems. Singlet oxygen, generated by irradiation of of oxygenated aqueous solutions of methylene blue, can diffuse into sodium dodecyl sulphate micelles, where it may react with solubilized organic substrates (1,3-diphenylisobenzofuran,in this case). This mechanism of transfer may be important in relation to the known effects of lo2in causing damage to biological systems. Peroxides are providing several new sources of lo2.Phthaloyl peroxide (74) in benzene appears to generate lo2at room temperature or on gentle heafing.ll2 P. de Mayo, Accounts Chem. Res., 1976, 9, 52. M. Takagi, S. Goto, and T. Matsuda, J.C.S. Chem. Comm., 1976,92. lo6 R. Bonneau, J. Pereyre, and J. Joussot-Dubien, Mol. Photochem., 1974, 6, 245. lo' G. Ohloff, Pure Appl. Chem., 1975, 43, 481. lo8 A. P. Schaap, A. L. Thayer, E. C. Blossey, and D. C. Neckers, J. Amer. Chem. Soc., 1975,97, lo'

lob

lop ll0 ll1

3741. E. C. Blossey, D. C. Neckers, A. L. Thayer, and A. P. Schaap, J. Amer. Chem. Soc., 1973,95,

5820. R. M. Boden, Synthesis, 1975, 783. A. A. Gorman, G. Lovering, and M. A. J. Rodgers, Photochem. and Photobiol., 1976, 23, 399.

K.-D. Gundermann and M. Steinfatt, Angew. Chem. Znternat. Edn., 1975,14, 560.

435

Photo-reduction and -oxidation

PP

0

II

\

c/o II

0

Ozonization of benzaldehyde, 2-methyltetrahydrofuran, and isopropyl methyl ether produces hydrotrioxide intermediates [e.g. (75) from benzaldehyde], which decompose at or below room temperature to generate 102.113 From a study of the products of cholesterol oxidation, Smith and Kulig 114 have found evidence that the base-catalysed disproportionation of hydrogen peroxide [reactions (21) and (22)] does produce lo2in addition to ground-state oxygen (302). A ratio of

+ HO- 7 HOO- + HzO HzOz + HOOHZO + HO- + H,O,

_I__,

(21) 0 2

(22)

: production of at least 1 : 3 is suggested, although this estimate is based only on the product yields. There is spectroscopic evidence for generation of (both lAg and lC,+) in the self-reaction of secondary peroxy-radicals derived from linoleic acid.lls Such a process could be involved in microsomal lipid peroxidation. Pitts and his group had previously recommended the aqueous decomposition of potassium perchromate (K3Cr0,) as a ‘clean’ source of 102,116 but have now gone on to show the occurrence of other oxidative pathways during its deThus, the relative reactivity of purine and pyrimidine bases to perchromate is not as expected for lo, attack, and other unidentified products also arise from the oxidation of certain cycloalkenes. The upper limit for the yield of lo2production is estimated to be ca. 6%. Osmium(I1) and iridium(n1) complexes have been found to sensitize photo-oxidation by a lo2route.l18 The variation of the quantum yield of lo2production as a function of pH has been measured for the dye toluidine blue as sensitizer.lls The quenching of lo2by organic molecules continues to receive attention. The rate constants for physical quenching of lo, by bilirubin lZo and biliverdin lZob are large and similar (2.3-2.5 x los and 3.3 x los 1 mol-1 s-l, respectively). However, the rate constant for chemical reaction with lo, is much greater for 118 114 116

F. E. Stary, D. E. Emge, and R. W. Murray, J. Amer. Chem. Soc., 1976, 98, 1880. L. L. Smith and M. J. Kulig, J. Amer. Chem. Sac., 1976, 98, 1027. M. Nakano, K. Takayama, Y.Shimizu, Y.Tsuji, H. Inaba, and T. Migita, J. Amer. Chem. SOC.,1976, 98, 1974.

117

J. W. Peters, J. N. Pitts, I. Rosenthal, and H. Fuhr, J. Amer. Chem. SOC.,1972,94,4348. J. W. Peters, P. J. Bekowies, A. M. Winer, and J. N. Pitts, J. Amer. Chem. SOC.,1975, 97,

11.9

J. N. Demas, E. W. Harris, C. M. Flynn, and D. Diemente, J. Amer. Chem. SOC.,1975,97,

116

3299. 3838. 119

120

R. Pottier, R. Bonneau, and J. Joussot-Dubien, Photochem. and Photobiol., 1975,22, 59. (a) B. Stevens and R. D. Small, Photochem. and Photobiol., 1976,23, 33; (b) C. S. Foote and T.-Y. Ching, J. Amer. Chem. Soc., 1975, 97, 6209.

436 Photochemistry bilirubin (1.7-4 x lo8, cf. < 3 x los 1 mol-ls-l for biliverdin). The azodioxide (76) appears to exist in equilibrium with its dinitroso-isomer (77).121 Although azo-dioxides such as (76) are efficient triplet quenchers, they do not quench lo2significantly. On the other hand, nitroso-compounds such as (77)

n

c1

C1 'NO

do quench at approaching diffusion-controlled rates (e.g. 9.3 x lo9 1 mol-1 s-l for 2-methyl-2-nitrosopropane). There has been dispute as to whether the dismutation of the superoxide radical anion (027produces or ground-state oxygen [reaction (23)]. 202'-

+ 2H+

----+

H202

+ 02(lAg or 3Xs-)

(23)

Guiraud and Foote122have now established that lo, is rapidly quenched by 0;-, with a rate constant of 1.6 x lo91 mol-1 s-l in DMSO, which may help to clarify the situation. The bimolecular emission of lo2in aqueous solutions [reaction (24)] is enhanced by cyclic tertiary diamines such as 1,4-diazabicycloO,pA,)

+ O2pA,)

-

+ hv

202(3Zu-)

(24)

[2,2,2]octane and NN'-dimethylpiperazine, which is surprising because tertiary amines are generally observed to be quenchers of 102.123

5 Oxidation of Aliphatic and Alicyclic Unsaturated Systems The controversy continues as to whether the reaction of singlet oxygen with olefins to give allylic hydroperoxides involves an ene-reaction or a perepoxide intermediate. Two theoretical studies of the reaction of singlet oxygen (l&) with ethylene have lent some support to the feasibility of a perepoxide intermediate [(78), Scheme 151 as a shallow minimum on the potential surface.124,126 Dewar and Thie1125have calculated that the transition state for reaction with lo2is reactant-like, and that the perepoxide (78) can rearrange with a higher activation energy to 1,2-dioxetan (79); a concerted route to dioxetan is not favourable. The same method applied to the reaction of propene with lo2 describes the most favoured mechanism for allylic hydroperoxide formation as a two-step process via a perepoxide intermediate. The perepoxide readily rearranges to the product of an ene reaction, For electron-donating substituted olefins, such as vinylamine or 2,3-dihydropyranYformation of a zwitterion [e.g. (SO)] is the preferred addition pathway (Scheme 16). Ring closure of zwitterion to 121 123

la4 126

P. Singh and E. F. Ullman, J. Amer. Chem. SOC.,1976,98,3018. H. J. Guiraud and C. S. Foote, J. Amer. Chem. SOC.,1976, 98, 1984. C. F. Deneke and N. I. Krinsky, J. Amer. Chem. SOC.,1976,98, 3041. S. Inagaki and K. Fukui, J. Amer. Chem. SOC.,1975,97, 7480. M. J. S. Dewar and W. Thiel, J. Amer. Chem. SOC.,1975, 97, 3978.

437

Photo-reduction and -oxidation

0

0-0 I I CH2-CH,

+

,0.-P

J

CH,-kHz

(79)

Scheme 15

dioxetan, or to perepoxide, occurs easily, If the perepoxide can in turn rearrange to ene product (Sl), there should be competition between the ene reaction and formation of a dioxetan. Dewar and co-workers126have also sought to explain the formation of the epoxides which have sometimes been observed in the reaction of olefins with loz.Calculations of the three possible reactions (25)-(27) suggest that for

Lo+-0co+-o[=O+-O-

+ +

-O,('h,,

CH2=CH2

Lo

4

+

0 3

2 LO

sterically hindered olefins, reaction (27) is unfavourable. Reactions (25) and (26) would then compete to produce a mixture of epoxide and dioxetan (as is indeed observed in some cases). However, for sterically unhindered olefins, reaction of the perepoxide with olefin [reaction (27)] would be the dominant pathway, leading to epoxide. Hence, ethylene should react with lo2to yield oxiran. Since this prediction appeared, it has been demonstrated that la6

M. J. S.Dewar, A. C.Griffin, W. Thiel, and I. J. Turchi, J. Amer. Chem. SOC.,1975,97,4439.

Photochemistry formaldehyde chemiluminescence can be detected from the gas-phase reaction of singlet oxygen (lAg) with eth~1ene.l~'Formaldehyde chemiluminescence would arise from fragmentation of a vibrationally excited dioxetan, and is therefore evidence for the formation of 1,Zdioxetan in this system. Arrhenius parameters have been determined for reactions of lo, with some substituted butenes, cyclopentenes, and cyclohexenes in the gas phase.12* Differing reactivities are mainly due to varying activation energies, rather than preexponential factors. An investigation of the kinetic aspects of dye-sensitized photo-oxygenation of olefins in a gas-liquid reactor has concentrated on masstransfer pro blems.12a The conformationally fixed cyclohexylidenecyclohexanes (82) and (83) react with lo, to give, in each case, a mixture of the two stereoisomeric allylic hydroperoxides (84) and (85) (Scheme 17).130 The ratio of (84) : (85) from (82) was 438

OOH

OOH

Scheme 17

60 :40 and from (83) was 33 : 67. Such a result cannot be explained on the basis of a concerted ene mechanism, but supports the formation of an intermediate, which could possibly be a perepoxide, in the reaction. Oxidation of several unconjugated cyclic dienes by lo2gave a normal ene reaction to yield allylic hydroperoxides, and there was no evidence for transannular reaction.lsl cis,cisCyclodeca-l,6-diene appeared to be inert towards loz. Dye-sensitized photo-oxidation of terpenes can sometimes produce complex mixtures. Thus, the Rose Bengal-sensitized oxidation of linalool in methanol allowed the isolation and identification of eight whilst a-terpineol gave six compounds on photosensitized Magnesium phthalocyanine has been used as a photosensitizer in the oxidation of a- and P - ~ i n e n e .Photo~~~ la@ lSo lS1

Isa lSa

D. J. Bogan, R. s. Sheinson, and F. W. Williams, J. Amer. Chem. SOC.,1976,98, 1034. R. D. Ashford and E. A. Ogryzlo, J. Amer. Chem. SOC.,1975,97,3604. D.Brkic, P. Forzatti, I. Pasquon, and F. Trifiro, J. Photochem., 1976,5, 23. R. M. Kellogg and J. K. Kaiser, J. Org. Chem., 1975, 40, 2575. A. Horinaka, R.Nakashima, M. Yoshikawa, and T. Matsuura, Bull. Cham. SOC.Japan, 1975, 48, 2095. K. h a , Nippon Shokuhin Kogyo Gakkai-Shi, 1973, 20,43 (Chem. Abs., 1975,83, 162 382). Y. S. Cheng, M. D. Tsai, J. M. Fang, and S. S. Hsu, Hua Hsueh, 1975, 8 (Chem. Abs., 1976, 84, 105 816).

H. Kropf and B. Kasper, Annalen, 1975, 2232.

Photo-reduction and -oxidation

439

sensitized oxidation of the sesquiterpene lactone lipiferolide (86) produces peroxyferolide (87), which has also been isolated from a plant It is unusual to find an allylic hydroperoxide from a naturally occurring source, and (87) may well arise in the leaves of the plant by chlorophyll-mediated lo2 addition to (86). Singlet oxygen reacts with germacratriene (88) to yield allylic

HOQ

hydroperoxides at the isopropylidene double bond ca. nine times more rapidly than at the endocyclic double bonds, in contrast to the relative reactivity of these double bonds on epoxidation of (88).136 Germacrone (89) reacts with lo2to give a complex mixture, from which only one allylic hydroperoxide (arising by C-5 attack of oxygen) could be isolated in 2.5% yield.131 There have been further reports of the reactivity of cholesterol l l 413' ~ and some fatty acids 13' towards lo,. The text of an interesting lecture by Bartlett has been published, in which the formation of dioxetans from '0, and alkenes is reviewed.13* Excited states can be generated photochemically, or by intermolecular or intramolecular energy transfer. Zimmerman has published another route to photochemical rearrangement without light, which involves a dioxetan in the intramolecular generation of an excited As shown in Scheme 18, the dioxetans (91) prepared by low-temperature photosensitized oxidation of the methylenecyclohexadienes (90) decompose on heating to produce cyclohexadienone (92) and its known photorearrangement product (93). Curiously, the rearranged product (93) is still observed even when the other carbonyl fragment produced is methyl 2-naphthyl ketone, despite the fact that this latter ketone has a lower triplet energy (ET 59 kcal mol-l) than the cyclohexadienone (92) (ET 68.5 kcal mol-l). This observation may support theoretical predictions of the preferential production of nr* excited triplets in such dioxetan fragmentations, rather than the rn* triplets of methyl 2-naphthyl ketone. Further examples of the reaction of alkylthio-substituted alkenes with lo2 have been reported. For the ethylthiocycloalkenes (94) 140 and (95),141 the dioxetans formed decompose by C-C or C-S bond cleavage. The ratio of the two pathways depends on the ring size of the substrate 140 and the conformation ls6

R. W. Doskotch, F. S. El-Feraly, E. H. Fairchild, and C.-T. Huang, J.C.S. Chem. Comm.,

lS6

T. W. Sam and J. K. Sutherland, J.C.S. Perkin I, 1975, 2336. F. H. Doleiden, S. R. Fahrenholtz, A. A. Lamola, and A. M. Trozzolo, Photochem. and Photabiol., 1974,20, 519. P. D. Bartlett, Chem. Sac. Rev., 1976, 5, 149. H. E. Zimmerman and G. E. Keck, J. Amer. Chem. SOC.,1975,97,3527. W. Ando, K. Watanabe, and T. Migita, Tetrahedron Letters, 1975, 4127. W. Ando, K. Watanabe, and T. Migita, J.C.S. Chem. Comm., 1975,961.

1976,402. 13'

lS8 lS8 140

141

4 40

Photochemistry

(90) a; R = Ph b; R = m-MeOC,H,

c; R

=

(91)

2-naphthyl

kG8O

OC

i-

Ph

&Ph

i- MeCOR

Ph

Ph

(92) 8 3 4 8 %

(93) 17-12%

Scheme 18

SEt

(94) n

=

5, 6, 7, 8, 10 or 12

n

(95) a; R1 = R2 = H b; R1 = Me, R2 = H c; R1 = Pri, R2 = Me d; R1 = Ph, R2 = H e; R1 = H, R2 = But

of the dioxetan. 141 Photochemical oxidation of 1-ethoxy-2-ethylthioethylene has been ~ e p 0 r t e d . l ~ ~ It has been suggested that the formation of glyoxal in the mercury-photosensitized reaction of oxygen with acetylene may proceed via attack of lo2 (lAg) on the a~ety1ene.l~~ The charge-transfer complex formed between ethylene and oxygen at low temperatures has been irradiated at 206 nm.144 Cyclic conjugated dienes generally react with lo2in a [4 21 addition reaction to yield 1,4-endo-peroxides. For example, dye-sensitized photo-oxygenation of the cyclopentadiene (96) produces a mixture of 1,4-endo-peroxide (97) together with products arising from endo-peroxide ~earrangernent.~~~ Mention must be

+

m (96)

Ira

143

144 146

(97)

R. I. Shekhtman, V. A. Krongauz, V. Yu. Borovkov, and E. N. Prilezhaeva, Izuest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1139. S. L. N. G. Krishnamachari and T. V. Venkitachalam, Mol. Photuchem., 1976, 7 , 75. H. W. Buschmann, Ber. Bunsengesellschaftphys. Chem., 1974, 78, 1344. W. Skorianetz and G . Ohloff,Helv. Chim. Acta, 1976, 59, 1.

Photo-reduction and -oxidation

441

made here of the fact that Barton and co-workers148have used the triphenylmethyl cation and other electrophiles as catalysts in the photo-oxygenation of ergosteryl acetate to the peroxide. These catalysts allow the ‘spin-forbidden’ addition of triplet (ground-state) oxygen to cisoid conjugated dienes, yielding endo-peroxides. A reaction route involving excitation of a diene-catalyst complex to a triplet state and spin-allowed reaction with triplet oxygen could give ground-state peroxide. Ergosterol is converted into the peroxide in fungi by simultaneous photo-oxidative and enzymic The photo-oxidation process is probably sensitized by known pigments in the fungi. Cyclohexa-l,3-dienes generally react with lo2to form endo-peroxides which may rearrange thermally to lY3-diepoxides.Such reactions have been used in the synthesis of crotepoxide, an anti-tumour agent, from sensitized photooxygenation of the cyclohexadiene (98).14* The diacetate (99), however, was inert towards lo2. Singlet oxygen reacts with the oxepin-benzene oxide (100) CH20CH,Ph

$:: (98) R = H

(99) R = COMe

(102) R = H (103) R = Me

system and the arene oxide (101) to form a l,.l-endo-peroxide in each case.149 The presence of an angular methyl group causes differences in the observed lo, reaction with dienes (102) and (1O3).l6O Thus, the diene (102) yields mainly (> 80%) endo-peroxides, whereas (103) produces a mixture of the two allylic hydroperoxides (104) (10-15%) and (105) (85-90%) resulting from an ene reaction. Kondo and Matsumoto lS1 have examined the relative reactivity of acyclic ene and diene systems towards lo2.p-Myrcene (106), which has both isolated and conjugated double bonds, gives reaction at the isolated double bond more readily, yielding allylic hydroperoxides (107) and (108). These products may react more slowly with lo2at the diene system to form lY4-endo-peroxides(109) and (110) respectively. From a study of (106) and other acyclic monoterpenes, D. H. R. Barton, R. K. Haynes, G. Leclerc, P. D. Magnus, and I. D. Menzies, J.C.S. Perkin I, 1975, 2055. 14’ M. L. Bates, W. W. Reid, and J. D. White, J.C.S. Chem. Comm., 1976, 44. 148 M. R. Demuth, P. E. Garrett, and J. D. White, J. Amer. Chem. SOC.,1976, 98, 634. l*@C. H. Foster and G. A. Berchtold, J. Org. Chem., 1975,40, 3743. lS0 I. Sasson and J. Labovitz, J. Org. Chem., 1975, 40, 3670. M. Matsumoto and K. Kondo, J. Org. Chem., 1975,40,2259. lP6

442

Photochemistry 0OH

OOH

p. (110)

(109)

the double-bond reactivity towards lo2appears to be trisubstituted alkene > 2-substituted lY3-diene> 1,l-disubstituted or lY2-disubstituted alkene. The same authorslS2have gone on to make use of this order of reactivity in the synthesis of some furanoterpenes by photosensitized oxidation of p-myrcene. For example, reduction of the endu-peroxide ring of (109) and dehydration produces a synthesis of the furan ring, with subsequent steps leading to the frrranoterpene perillene (1 11). Furans can similarly be produced following the

(1 11)

lo2photo-oxygenation of polyaryl-substituted cyclopentadienols.lSS Sensitized photo-oxidation of the s-trans-diene grouping of the steroid oestra-4,g-dien17/%01-3-onegives an allylic hydroperoxide,16*and direct photo-oxidation of the s-trans-diene grouping in the diterpene abietic acid has been noted.lss Dye-sensitized photo-oxidation of the furan ring of the furanolactone (112) to give the products shown in Scheme 19 is a key step in the synthesis of the alkaloid camptothecin.ls6 Photosensitized oxidation of the furanoid ring of petasalbin has been reported.lS7 Et OC0,Me

g YI 031 OH

10,

+

0

lSa IS3

K. Kondo and M. Matsumoto, Tetrahedron Letters, 1976, 391. J. J. Basselier, J. P. Le ROUX,F. Caumartin, and J. C. Cherton, Bull. SOC.chim. France, 1974, 2950.

M. Maumy and J. Rigaudy, Bull. SOC.chim. France, 1975, 1879. us A. Enoki and K. Kitao, Mokuzai Gakkaishi, 1975, 21, 101. lS6 E. J. Corey, D. N. Crouse, and J. E. Anderson, J. Org. Chem., 1975, 40, 2140. lS7 K. Naya, R. Kanazawa, and M. Sawada, Bull. Chem. SOC.Japan, 1975,48,3220. 16'

Photo-reduction and -oxidation

443

Addition of lo2at -70 "C to the s-cis-diene system of the polyarylfulvenes (1 13) gives reasonably stable 1,4-endo-peroxides [except (1 14g)l which undergo an interesting rearrangement at room temperature in the presence of methanol or ethanol to form 1,Z-dioxetans (115) (Scheme ZO).168

R1

R1

Ph

Ph

Ph

Ph H

R3

R2 = R3 = Ph (114) R1 = R2 = Ph, R3 = Me C; R1 = R2 = Ph, R3 = p-ClC,H, d; R1 = R2 = Ph, R3 = p-NO&H, e; R1 = R2 = Ph, R3 = p-MeOC,H, f ; R1 = R3 = Ph, R2 = H g; R1 = R2 = H, R3 = Ph

(113) a; b;

R1 =

Scheme 20

o-Alkyl-substituted aromatic carbonyl compounds undergo intramolecular hydrogen abstraction on irradiation to form an enol, which can react with oxygen to produce an endo-peroxide. This process allows a regioselective hydroxylation at the C-8 methyl group of (116).159 Oxidation of the C=C AcO

0

Me

Me (116)

functional group occurs in several other photoreactions. Photo-oxidation of 2methyl-ly4-naphthoquinone (vitamin K3) in ethanol yields the 2,3-epoxide.ls0 The dye-sensitized photo-oxidation of 2',4',6'-trihydroxychalcone to the corresponding flavonol has been reported,l6l and photolysis of bis-(3-hydroxyflavenylidene)and bis-(3-epoxyflavenylidene) in air leads to oxidative cleavage of the 2,3-double bond of one half of the bisflavenylidene molecule.lsa 6 Oxidation of Aromatic Compounds This year has seen several extensive reports of the 1,4-cycloaddition of singlet oxygen to compounds having a double bond in conjugation with an aromatic system. Thus, the photosensitized oxidation of l-vinylnaphthalenes (117)in carbon tetrachloride results in 1,4-attack by lo2to produce the endo-peroxides (1 18),le3 Ib8

lLD

160

la%

J. P. Le Roux and C. Goasdoue, Tetrahedron, 1975, 31, 2761. W. A. Ayer and D. R. Taylor, Canad. J. Chem., 1976,54, 1703. J. M. L. Mee, C. C. Brooks, and K. H. Yanagihara, Biochem. Biophys. Res. Comm., 1975, 65, 228. H. M. Chawla and S. S. Chibber, Tetrahedron Letters, 1976, 2171. R. J. Molyneux, H. Aft, and P. Loveland, Chem. and Ind., 1976, 68. M. Matsumoto and K. Kondo, Tetrahedron Letters, 1975, 3935.

444

Photochemistry

(117) a; R1 = Ph,R2 = H b; R' = R2 = H c; R1 = R2 = Me

d; R1 = Me,R2 = H R1 = H,R2 = Me

e;

The reaction is stereospecific, as shown by the exclusive formation of (118d) from trans-l-propenylnaphthalene(117d) and only (1 18e) from the cis-isomer (1 17e). a-Substituents on the side-chain inhibit the 1,4-cycloaddition, and no endo-peroxide could be isolated. Instead, allylic hydroperoxides were found, no doubt arising from a normal ene-reaction of lo2 on the side-chain of the aromatic compound. For such (a-substituted-vinyl)naphthalenes, it seems possible that steric interactions with the naphthalene peri-hydrogen hinder a conformation in which the side-chain is coplanar with the naphthalene ring, and hence 1,4-attack by lo2 is prevented. Kondo and co-workers 164 have also examined the behaviour of 2-vinylthiophens (119) on photosensitized oxidation.

(119) a; R1 = b; R' =

c; d; e; f; g;

R1 = R1 = R1 = R1 = R1 =

R2 = Me,R3 = H R3 = H Me,R2 = R3 = H Ph,R2 = R3 = H R2 = H,R3 = Ph R2 = H,R3 = Me R2 = R3 = Me R2 =

(120)

Again, endo-peroxides (120) are formed as a result of 1,4-attack of lo2on the aromatic and vinyl double bonds, However, in the case of the (a-substitutedviny1)thiophens (119f) and (119g), endu-peroxide formation does compete with production of allylic hydroperoxide. This observation, when compared with those described above for l-vinylnaphthalenes, becomes understandable in terms of a much reduced steric interaction between the substituted side-chain and the thiophen ring. Foote et alls6 have published a full account, with further examples, of their earlier work lB6on the formation of diepoxy-endo-peroxides (122) in the photooxidation of indene and substituted indenes (121) in acetone at -78 "C. The probable mode of formation of the adducts (122) involves initial lY4-additionof lo4

M. Matsumoto, S. Dobashi, and K. Kondo, Tetrahedron Letters, 1975,4471. P. A. Burns, C. S. Foote, and S. Mazur, J. Org. Chem., 1976,41, 899. C. S. Foote, S. Mazur, P. A. Burns, and D. Lerdal, J . Amer. Chem. Sac., 1973,95,586.

lE6

lo6

Photo-reduction and -oxidation

445

mR3 R2

R2

lo*

Me,CO, - 78 'CO

lo2to the indene, followed by rearrangement of this endo-peroxide to a diepoxide, and attack of a second molecule of lo2. A similar reaction of 1,Zdihydronaphthalenes (123) with lo2occurs at -78 "C in acetone, although generally in these examples both 1,4-~ycloaddition and ene-reaction compete to yield mixtures of diepoxy-endo-peroxides and allylic hydroperoxides.le7 The reaction pathway is quite sensitive to the substitution pattern of (123), since (123a) gives

(123) a; R1 = H,R2 = Ph b; R1 = Ph, R2 = H C; R' = R2 = Ph d; R1 = Me,R2 = Ph e ; R1 = Me, R2 = H f; R1 = R2 = H

only allylic hydroperoxide whereas (123b) gives only diepoxy-endo-peroxide. The products from lo2attack on l-phenylcycloalkenes depend upon the cycloalkene ring size.le8 l-Phenylcyclopentene yields only an allylic hydroperoxide, l-phenylcyclohexene gives a 3 : 1 mixture of allylic hydroperoxide and a double endo-peroxide, and 1-phenylcyclobutene gives a more complex mixture (Scheme 21), the composition of which is solvent-dependent. The authors claim that their results can be rationalized on the basis of the formation of three intermediates, perepoxide, dioxetan, and endo-peroxide.ls8 0

0

II

a

Ph

'02

+ cC-H QooH Ph C-Ph I1

0

II

+

C-H

V" \=/

Scheme 21 lo'

lb8

P. A. Burns and C. S. Foote, J. Org. Chem., 1976,41, 908. C. W. Jefford and C. G . Rimbault, Tetrahedron Letters, 1976, 2479.

446

Photochemistry

Naphthalene does not react with lo, to form a 1,4-endo-peroxide. Nevertheless, this endo-peroxide (126) is reasonably stable and has been produced indirectly by dye-sensitized photo-oxidation of the [l01annulene (124).ls0 Treatment of the resultant amine (125) with nitrosyl chloride leads to elimination of

N20 at low temperatures, and consequent production of (126). Gentle warming of the peroxide (126) quantitatively produces naphthalene and loa,rather than forming a 1,3-diepoxide. However, the intermediate peroxide (125) can rearrange to 1,3-diepoxide and therefore allows the synthesis of syn-naphthalene 1,2:3,4d i e p ~ x i d e . ~1,4,5,8,9-Pentamethylanthracene '~ is readily photo-oxidized to the expected 9,10-end~-peroxide,l~~ and the photo-oxidation of anthracene on an alumina catalyst has been r e p 0 ~ t e d . l ~The ~ effects of solvent and reactant concentrations on the self-sensitized photo-oxidation of rubrene have been investigated in 1,3-Diphenylisobenzofuran(127) has proved a popular substrate for photooxidation studies, being oxidized by lo2to o-dibenzoylbenzene. It has now been shown that (127) is also very susceptible to autoxidation by free-radical initiators at 30 "C and that this process does also yield some o-dibenz~ylbenzene.~~~ Caution is therefore required when deducing lo2as an intermediary from the conversion of (127) into o-dibenzoylbenzene. Photo-oxidation of (127) in aromatic solvents leads to the known endo-peroxide, which is fairly stable at 20 "C,but photo-oxidation of (127) in carbon tetrachloride gives o-dibenzoylbenzene quantitatively. The peroxy-acid oxidation of (127) also yields o-dibenzoylbenzene, but evidence has been produced against the involvement of '0% in this ~eacti0n.l~~ The photolysis of pyridine N-oxide in benzene is well known to lead to phenol in competition with the formation of intramolecular rearrangement products of the N-oxide. The former conversion has been studied as one which mimics the biological hydroxylation of aromatic rings. It has now been found that the internal oxygen rearrangements can be blocked by protecting the N-oxide as the boron trifluoride complex, when much increased yields of phenol are Stein and co-workers have reported further on the photochemical oxidation of benzene in aerated aqueous solutions, but they have not been able to provide any more evidence on the structure of the unisolated p h o t o p r o d u ~ t . ~ ~ ~ M. Schafer-Ridder,U. Brocker, and E. Vogel, Angew. Chem. Znternat. Edn., 1976, 15,228. E. Vogel, H.-H. Klug, and M. Schiifer-Ridder, Angew. Chem. Znternat. Edn., 1976,15,229. 171 H. Hart, J. B.-C. Jiang, and R. K. Gupta, Tetrahedron Letters, 1975, 4639. li2 S. Nakanishi and K. Ito, Nippon Kagaku Kaishi, 1975, 687. 173 H. D. Brauer and H. Wagener, Ber. Bunsengesellschaftphys. Chem., 1975, 79, 597. 17* J. A. Howard and G . D. Mendenhall, Canad. J. Chem., 1975,53,2199. 17s R. F. Boyer, C. G. Lindstrom, B. Darby, and M. Hylarides, Tetrahedron Letters, 1975,4111. lie G. Serra-Errante and P. G. Sammes, J.C.S. Chem. Comm., 1975, 573. 17' Y . Ilan, M. Luria, and G. Stein, J. Phys. Chem., 1976, 80, 584.

170

447

Photo-reduction and -oxidation Ph

(128) a; n = 1, X = COCO2H b; n = l , X = COzH c; n = 2 , X = COzH d ; n = 2 , X = OH

Ph (127)

e; n

=

2,X

=

Me0 (129) a; X = Me

b; X C;

NHAc

X

= =

CH,OMe CHO

A full paper has appeared describing the dye-sensitized photo-oxidation of g-hydroxyphenylpyruvic acid (128a) reported last year (see Vol. 7, p. 416), and this work has now been extended to the oxidation of other p-substituted phenols (128b--e).178 Interest in the yellowing of wood pulp has led to studies of the products from the participation of lo2in the photo-oxidation of lignin model compounds containing phenolic 17g or styryl functional groups such as (129).lS0 Photoexcited aromatic nitro-compounds are able to give oxidative cleavage of the aromatic ring of aromatic methoxy-compounds (e.g. see Vol. 6, p. 543).181 7 Oxidation of Nitrogen-containing Compounds Photo-oxidation of amines has again been the subject of several reports this year. Davidson and Tretheweylea have shown from kinetic data that, in the photooxidation of triethylamine sensitized by Rose Bengal in aqueous methanol, both singlet oxygen [reactions (28) and (29)] and radical intermediate pathways [reaction (30)] are involved. The relative importance of each pathway depends Dye (TI)

Amine Dye (TI)

-

+ 30a

+ lo2

+ Amine

___+

____+

Dye (So)

-

+ lo,

[Complex]

[Complex]

Products

(28) (29)

Products

(30)

upon amine concentration: at low concentration of triethylamine (< 0.1 moll-1) the singlet-oxygen route contributes significantly, whilst at higher concentrations (0.4 mol I-l) a radical mechanism dominates. In continuation of their studies on the dye-photosensitized oxidation of tertiary amines, French workers have reported the oxidation of alkaloids which contain an N-methyl heterocyclic ring, such as An iminium ion is apparently a general intermediate in the formation of products, and the transformation of iminium ion to enamine has been used in the synthesis of some indole alkaloids.184 Secondary and tertiary amines such as (130) undergo dyesensitized photo-oxidation with the production of nitroxyl radicals [e.g. (131)].lS6 I. Saito, Y.Chujo, H. Shimazu, M. Yamane, T. Matsuura, and H. J. Cahnmann, J. Amer. Chem. Soc., 1975, 97, 5272. 17@ G. Brunow and M. Sivonen, Paperi Puu, 1975, 57, 215, 219. lB0 G. Gellerstedt and E. L. Pettersson, Acta Chem. Scand., 1975, B29, 1005. 181 I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC. Japan, 1975, 48, 2865. ls2 R. S. Davidson and K. R. Trethewey, J.C.S. Chem. Comm., 1975, 674. Y.Hubert-Brierre, D. Herlem, and F. Khuong-Huu, Tetrahedron, 1975, 31, 3049. R. Beugelmans, D. Herlem, H.-P. Husson, F. Khuong-Huu, and M.-T. Le Goff, Tetrahedron Letters, 1976, 435. V. B. Ivanov, V. Y.Shlyapintokh, 0.M. Khvostach, A. B. Shapiro, and E. G. Rozantsev, J. Photochem., 1975, 4, 313.

178

448

Photochemistry OH

OH

Singlet oxygen is implicated in this process, which occurs in low quantum yield (10-2-10-4) but high overall chemical yield. The direct photo-oxygenation of the aromatic amines p-phenylenediamine and NN-dimethylaniline has been noted.lsB The photo-oxygenation of p-phenylenediamine in cyclohexane leads to p-benzoquinonediimine (Q = 0.01) and 4,4'diaminoazobenzene (Q = 0.003), perhaps via a charge-transfer complex of singlet excited amine with oxygen. Indian workers have again investigated the ketone-sensitized photo-oxidation of diphenylamine which yields diphenyl nitroxide,ls7,lS8whilst a similar photo-oxidation sensitized by methylene blue instead produces N-phenyl-p-benzoquinonirnine.les Enamines generally behave as electron-donating substituted olefins and are attacked by lo2 (see Section 5 ) to yield 1,Zdioxetans or their thermal decomposition products. Sensitized photo-oxidation of the enamines (1 32) in pyridine at room temperature is believed to proceed through a 1,Zdioxetan intermediate.1go According to substitution pattern, the dioxetan decomposes

R'

\

R3

I

c=c, A

R4

N

X

eo

W 0 (132) a; R1 = H, R2 = Me,R3 = Ph, X = 0 b; R1 = R2 = Me,R3 = Ph, X = 0 c; R1 = R2 = Me, R3 = Ph, X = CH, d ; R1 = H, R2 = Me, R3 = Et, X = 0 e ; R1 = H,R2 =: Me,R3 = Et,X = CH, f; R1 = R2 = Me, R3 = Pri, x 0

H

(133)

either by ring fission to two carbonyl fragments (C-C fission) or by C-N bond cleavage. There is thus a resemblance to the competitive C-C or C-S bond cleavages noted for the dioxetans from lo2attack on vinyl s ~ l p h i d e s141 .~~~~ The products of direct photo-oxidation of the enamide (133) include the dioxetan from lo2addition to the olefinic bond, and compounds arising by rearrangement of the dioxetan.lQ1 Photo-oxidation of an enamine has been reported in the synthesis of the aporphine alkaloid, cepharadione B.lS2 Rose Bengal-sensitized photo-oxidation of steroidal etiojervane derivatives which are ap- or K. Maeda, A. Nakane, and H. Tsubomura, Bull. Chem. Soc. Japan, 1975,48,2448. W. R. Bansal, S. Puri, and K. S. Sidhu, J. Indian Chem Soc., 1975, 52, 308. lS8 N. R. K. Raju, M. Santhanam, B. Sethuram, and T. N. Rao, Indian J. Chem., 1975,13,493. lS9 W. R. Bansal, N. Ram, and K. S. Sidhu, Zndian J. Chem., 1975, 13, 987. ln0 W. Ando, T. Saiki, and T. Migita, J, Amer. Chem. Soc., 1975, 97, 5028. F. Abellb, J. Boix, J. Gbmez, J. Morell, and J.-J. Bonet, W e b . Chim. Acta, 1975, 58, 2549. lea J. M. Sah, M. J. Mitchell, and M. P. Cava, Tetrahedron Letters, 1976, 601. lS6

Photo-reduction and -oxidation

449

jly-unsaturated enamines produces the corresponding a/3- or jly-unsaturated ketones.lg3 In both reports,lg2,lg3there is a critical dependence on solvent polarity: a polar solvent such as methanol assists the photo-oxidation of enamine to ketone at the expense of the formation of other products. The photo-oxidation of 2-methyl-2-nitrosopropane in the gas phase proceeds by a dissociative mechanism, as outlined in reactions (31)-(33).lg4 The identified BdNO

+ O2 + NO

But*

ButO,*

hv

ButNO*

But.

+ NO

But02* ButO*

__I_,

+ NOz

(31) (32)

(33)

reaction products are t-butyl nitrate (62%), acetone (18%), 2-methyl-2-nitropropane (14%), t-butyl nitrite (2%),and isobutene (2%). Most of these products are derived by combination and fragmentation of the t-butoxyl radicals formed in reaction (33). Photo-oxidation of [2H,]azomethane produces deuteriated methanol, methyl hydroperoxide, and dimethyl peroxide.lgs The methylene blue-sensitized photo-oxidation of a nitrone (134) has been reported by Ching and F 0 0 f e . l ~ ~Quantitative conversion of (134) into the

hydroperoxide (135) is observed on oxidation at - 63 "C in deuteriochloroform, and it seems most likely that a normal ene-reaction of lo2(see Section 5 ) is involved in this conversion, rather than a 1,3-dipolar cycloaddition. Singlet oxygen, generated by dye-photosensitization, is able to cleave the C=N bond in benzophenone oxime (136a), its anion, or the O-methyl ether (136b), producing benzophenone and nitrite (Scheme 22).lg7 An unstable dioxazetan, from lo2 Ph2C=N- OR

lo'

> Ph.,C=O

+

O=N-OR

(136) a; R = H b; R = Me Scheme 22

addition to the C=N bond, would be a plausible intermediate in this cleavage react ion. A new method has been reported for the conversion of lactams (137) into imides (138) by irradiation in the presence of benzophenone and oxygen.1g8 A. Murai, C. Sato, H. Sasamori, and T. Masamune, Bull. Chem. SOC.Japan, 1976, 49,499. J. Pfab, J.C.S. Chem. Comm., 1976, 297. lP6 J. Weaver, R. Shortridge, J. Meagher, and J. Heicklen, J. Photochem., 1975, 4, 109. lP6 T.-Y. Ching and C. S. Foote, Tetrahedron Letters, 1975, 3771. In7 C. C. Wamser and J. W. Herring, J. Org. Chem., 1976, 41, 1476. 19* J.-C. Gramain, R. Remuson, and Y. Troin, J.C.S. Chem. Comm., 1976, 194. lD3

ln4

450

Photochemistry

(137) a; R = Me,n = 1 b; R = H, n = 1 c; R = Me,n = 2

The lactams are inert towards lo2,and evidence is put forward to support a hydrogen abstraction mechanism in which triplet benzophenone abstracts hydrogen specifically from the methylene group adjacent to the nitrogen atom. The radical produced then reacts with oxygen, eventually yielding (138). Lightner et al. have continued their series of publications on the dyesensitized photo-oxidation of pyrroles with studies of N-phenylpyrr~le,~~~ 2,3,5-trimethylpyrr0le,~~~ and t-butylpyrroles.201In each case it is believed that the initial steps are lo2attack via 1,4-addition to form an endo-peroxide and possibly 1,2-addition to form a dioxetan. The suggestion is made that the endo-peroxide may in part rearrange to dioxetan below room temperature (a rearrangement also observed for polyarylfulvenes 168). The thermally unstable endo-peroxides can in fact be observed by n.m.r. at ca. -80 "C in [2H6]acetone or Freon 11, and are precursors to the isolated photoproducts.201s202 There is continuing interest in the photochemistry of bilirubin, which is the pigment responsible for neonatal jaundice. Rates of reaction of bilirubin with lo2have been calculated (see Section 4).120 The self- or dye-sensitized photo-oxygenation of some oxopyrromethenes (139) and other monopyrroles related to bilirubin has been investigated.203 R3

R2

(139) a; R1 = R3 = R4 = Et, R2 = Me b; R1 = R2 = H, R3 = R4 = Et c; R1 = Et, R2 = Me, R3 = R4 = H

Dye-sensitized photo-oxidation at room temperature of N-substituted indoles leads to cleavage of the 2,3-double bond [e.g.:(140) produces (141)], perhaps via a d i o ~ e t a n .However, ~~~ when the irradiation of (140) is carried out at -70 "C a peroxidic intermediate, possibly (142), can be intercepted by functional groups of the side-chain to give a 3-hydroperoxyindoline (143) in high yield.206 Analogously, other Japanese workers have provided full details of their earlier report 206 on the involvement of a 3-hydroperoxyindoline in the photo-oxidation Ips 2oo 201 202

204 205

208

D. A. Lightner, D. I. Kirk, and R. D. Norris, J. Heterocyclic Chem., 1974, 11, 1097. D. A. Lightner and L. K. Low, J . Heterocyclic Chem., 1975, 12, 793. D. A. Lightner and C . 4 . Pak, J . Org. Chem., 1975,40,2724. D. A. Lightner, G. S. Bisacchi, and R. D. Norris, J. Amer. Chem. Soc., 1976, 98, 802. D. A. Lightner and Y.-T. Park, Tetrahedron Letters, 1976, 2209. I. Saito, M. Imuta, S. Matsugo, H. Yamamoto, and T. Matsuura, Synthesis, 1976, 255. I. Saito, M. Imuta, S. Matsugo, and T. Matsuura, J. Amer. Chem. SOC.,1975, 97, 7191. M. Nakagawa, T. Kaneko, K. Yoshikawa, and T. Hino, J. Amer. Chem, SOC.,1974,96,624.

451

Photo-reduction and -oxidation

rn 00-

'02

- 70°C

H2CH20

Me

I

1 0 ,

20°C

COCH2CH20H N-CHO Me

OOH

OLD Me

(141)

of the N-methyltryptamine (144a), and now describe the isolation of this species (145a).207Such compounds as (145) are also isolable from the photo-oxidation of the tryptamine and tryptophan derivatives (144b) and (144~).~O*

(144) a; R1 = H, R2 = Me b; R1 = H, R2 = C0,Me

(145)

c; R1 = R2 = C02Me

Other reported examples of the photo-oxidation of nitrogen-containing heterocycles include the reaction of substituted pyrazines such as (146) and pyrimidines with lo2to form endo-peroxides (Scheme 23),200and the photooxidation of the reduced lumiflavin cation.210 Dioxetans are produced in some

(146) a; R

= PhCH2 b;R=Me Scheme 23

heterocyclic oxidations. Thus, a stable dioxetan is formed from attack of lo2 at - 50 "C on the 4,5-double bond of the 1,2'-dimer of 2,4,5-triphenylimidazole (lophine).211 Oxidations of 3-ben~ylidenepiperazine-2~5-diones such as (147) ao7

aoD

alo

M. Nakagawa, K. Yoshikawa, and T. Hino, J. Amer. Chem. SOC.,1975, 97, 6496. M. Nakagawa, H. Okajima, and T. Hino, J. Amer. Chem. SOC.,1976, 98, 635. J. L. Markham and P. G. Sammes, J.C.S. Chem. Comm., 1976, 417. N . Lasser, H. Levanon, and J. Feitelson, Photochem. and Photobiol., 1975, 22, 7 . G. Rio and B. Serkiz, J.C.S. Chem. Comm., 1975, 849.

452

Photochemistry

(147) a; b;

R1 = H, R2 = Ph R1 = Ph, R2 = H

(148)

with lo2at 25 "C give cleavage of the benzylidene group, via formation of fairly stable dioxetans.212 Photo-oxidation of either isomer (147a) or (147b) leads to the same dioxetan (148), which suggests that a non-concerted formation of dioxetan may be involved. A zwitterionic intermediate would be in accord with calculations for lo2attack on electron-donating substituted 01efins.l~~ There have been several reports of the photo-oxidation of amino-acids. Riboflavin-sensitized photo-oxidation of methionine, which leads to several products, has been noted at various pH values.213 The photo-oxidation of methionine by 4-benzoylbenzoate ion 64 has already been discussed (see Section 1 and Scheme 10). The oxidation product, methional (MeSCH,CH,CHO), is formed in 100% yield at pH 7. Irradiation of flavin mononucleotide in the presence of sulphur-containing amino-acids causes deamination by a closely related mechanism.214 Electron abstraction by flavin mononucleotide from sulphur is followed by intramolecular electron transfer from carboxylate anion to the sulphur radical centre, which leads on to decarboxylation and deamination. Photo-oxidation of tryptophan produces N'-formylkynurenine, which may then act as a sensitizer for further photodynamic action, resulting in the degradation of proteins. The pathway by which N'-formylkynurenine acts as sensitizer of tryptophan degradation has been shown to involve partly lo2production and partly triplet-state hydrogen abstraction with consequent production of oxygen radical-anion (02'-)from ground-state oxygen.215 Adenine accelerates the lumiflavin-sensitizedphoto-oxidation of tryptophan, histidine, met hionine, and guanine, without altering the reaction products.216 The photosensitized oxidation of tyrosine derivatives in the presence of sodium alginate has been investigated.217,218 The photo-inactivation of enzymes is often a result of the specific photooxidation of certain amino-acid residues which they contain. Papain, for example, on methylene blue-sensitized photo-oxidation, loses one histidine residue and 220 Destruction of histidine residues is also reported to becomes be involved in the Rose Bengal-sensitized photo-oxidation of a-glucan phosphorylases,221rabbit haemopexin,222and the visible-light-induced photo212

213 21r 21s 216

217 218

21D 220

a21

2a2

P. J. Machin and P. G. Sammes, J.C.S. Perkin I, 1976, 628. H. Nakamura, Koshien Daigaku Kiyo, 1975,4, 13 (Chem. Abs., 1975, 83,59 239). J. R. Bowen and S. F. Yang, Photochem. andPhotobiol., 1975, 21, 201. P. Walrant, R. Santus, and L. I. Grossweiner, Photochem. and Phorobiol., 1975, 22, 63. A. Yoshimura and S. Kato, Bull. Chem. SOC.Japan, 1976, 49, 813. G. R. Seely and R. L. Hart, Phorochem. and Photobiol., 1976, 23, 1. G. R. Seely and R. L. Hart, Photochem. and Photobiol., 1976, 23,7. K. Okumura and T. Murachi, J. Biochem. (Japan), 1975, 77, 913. A. Ohara, S. Fujimoto, and H. Kanazawa, Chem. and Pharm. Bull. (Japan), 1975,23,967. A. Kamogawa and T. Fukui, Biochim. Biophys. Acta, 1975, 403, 326. V. L. Seery, W. T. Morgan, and U. Muller-Eberhard, J. Biol. Chem., 1975, 250, 6439.

Photo-reduction and -oxidation

453

oxidation of dinitrophenylhistidine-200 human carbonic anhydrase B.223 Brief reviews have appeared which cover various aspects of the photosensitized oxidation of amino-acids 224 and proteins.224,225 Applied to proteins, such techniques can give valuable information on their three-dimensional (tertiary) structure.22s In the photo-oxidation of lanthanide ion-lysozyme complexes, the lanthanide ion can convey some protection from photo-oxidative attack to nearby tryptophan and methionine residues, which again allows useful 'mapping' Photodynamic sensitization of the of the tertiary structure of inactivation of lysozyme by 8-methoxypsoralen is a result of lo2production.22s 8 Miscellaneous Oxidations Interest in the fate of chlorinated hydrocarbons in the environment continues. The photolysis of chlorofluoromethanes in the presence of oxygen or ozone 229 and the photo-oxidation of vinyl chloride in air 230 have been studied. Heicklen and co-workers have reported the mercury-photosensitized oxidation of 1,l-dichloroethene 231 and t r i c h l o r ~ e t h e n e . ~ ~ ~ Several examples of the photo-oxidation of organic molecules catalysed by metallic ions have been published. Copper@) salts have been shown to play an essential role in the photo-oxidation in methanol of @-unsaturated ketones such as dypnones ( 1 4 9 a - - ~ )and ~ ~ ~mesityl oxide (149d). It now appears234that endo-peroxides may be intermediates in this reaction, because (150) has been R

Me

H. NMe,

hCOR

Me

(149) a; R = Ph b; R = p-MeC,H, c; R = p-BrC,H, d;R=Me

( 1 50)

isolated, and this could arise by oxygen addition to the s-cis-dienol form of (149d). A method for the formation of remote double bonds by cupric acetate-catalysed photolysis of alkyl nitrites depends upon the ability of Cu" salts to oxidize an intermediate alkyl radical to an a1kene.235Irradiation of alkyl nitrites in benzene in the presence of Cu" ions leads to 23-36% of the corresponding &-unsaturated 223 224 226

22e 227

M. Kandel, A. G. Gornall, L. K. Lam, and S. I. Kandel, Canud. J. Biochem., 1975,53,599. G. Jori, Photochem. and Photobiol., 1975, 21, 463. G. Laustriat and C. Hasselmann, Photochem. and Photobiof., 1975, 22, 295. G. Jori, Anais Acad. brasil. Cienc., 1975, 45, Suppl., 33 (Chem. Abs., 1975, 83, 127 573). G. Jori, M. Folin, G. Gennari, G. Galiazzo, and 0 . Buso, Photochem. and Photobiol., 1974, 19, 419.

228 229

230 231 232

233 a34

236

W. Poppe and L. I. Grossweiner, Photochem. and Photobiol., 1975,22,217. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J . Photochem., 1975, 4, 381. T. Kagiya, K. Takemoto, and Y . Uyama, Nippon Kagaku Kaishi, 1975, 1922. E. Sanhueza and J. Heicklen, J. Photochem., 1975, 4, 17. E. Sanhueza and J. Heicklen, J. Photochem., 1975, 4, 161. T. Sato, K. Tamura, K. Maruyama, and 0. Ogawa, Tetrahedron Letters, 1973, 4221. T. Sato, K. Tamura, K. Maruyama, 0. Ogawa, and T. Imamura, J.C.S. Perkin I, 1976, 779. Z. Cekovid and T. Srnik, Tetrahedron Letters, 1976, 561.

Photochemistry alcohols. Iron(1n) is an effective photo-oxidant for a wide range of organic 454

Hydrogen abstraction by an alkoxy-radical, generated from the photolysis of a-peroxynitriles, forms the basis of a method for the introduction of a functional (cyano) group into unactivated C-H The unusual cleavage of a cyclopropane ring by lo2has been reported for (151), which gives mainly ring-opened products that can be accounted for by the formation and subsequent breakdown of a 1,Zdioxolan intermediate.238 The reaction of a-keto-carboxylic acids with lo2appears to generate peroxy-carboxylic acid by oxidative decarboxylation [reaction (34)].239"However, this peroxy-acid reacts immediately with more a-keto-carboxylic acid [reaction (35)], so that the overall reaction product observed is a carboxylic acid. RCOC02H RC03H

+ lo2

+ RCOCOZH

___I*

+ C02 2RCOzH + COa RC03H

(34)

(3 5 )

A study has been made of the mechanism and rates of attack of lo2on cyanine and it has been noted that lo2is able to oxidize selenides to selenoxides in good yield.241Photo-oxidation of the drug phenothiazine has been studied by e.s.r. and photo-oxidation of p-ketoalkylpyridinium iodides leads to or-diketone~.~~~ The photo-oxidation of derivatives of the triterpene lupan-29-01 has been Irradiation of solutions of ozone in saturated hydrocarbons at -78 "C with visible light gives alcohols and ketones derived from ozone-hydrocarbon complexes by oxygen insertion into C-H Irradiation with U.V. light (254 nm), however, leads to competing attack by excited oxygen atoms [O(lD)] on the hydrocarbon. Ground-state atomic oxygen [O(3P)]is often generated by photolytic methods, and its reactions with organic compounds have been reviewed.24s Another report of the products of photo-oxygenation of diethyl ether has appeared,lss and is broadly in agreement with that reported last year.247 2a6

238

240

242 243

244 246 246

ar7

A. Cox and T. J. Kemp, J.C.S. Faraday I, 1975, 71,2490. D. S. Watt, J. Amer. Chem. SOC.,1976, 98, 271. R. H. Rynbrandt and F. E. Dutton, J. Org. Chem., 1975, 40, 3079. C. W. Jefford, A. F. Boschung, T. A. B. M. Bolsman, R. M. Moriarty, and B. Melnick, J. Amer. Chem. SOC.,1976, 98, 1017. G. W. Byers, S. Gross, and P. M. Henrichs, Photochem. and Photobiol., 1976, 23, 37. L. Hevesi and A. Krief, Angew. Chem. Internat. Edn., 1976, 15, 381. I. Rosenthal and R. Poupko, Tetrahedron, 1975,31,2103. T. Mukaiyama, K. Atsumi, and T. Takeda, Chem. Letters, 1975, 1033. A. Vystrcil, V. Krecek, and M. Budesinsky, Coll. Czech. Chem. Comm., 1975, 40, 1593. T. H. Varkony, S. Pass, and Y. Mazur, J.C.S. Chem. Comm., 1975, 709. R. E. Huie and J. T. Herron, Progr. Reaction Kinetics, 1975, 8, 1. C. von Sonntag, K. Neuwald, H.-P. Schuchmann, F. Weeke, and E. Janssen, J.C.S. Perkin ZZ, 1975, 171.

6 Photoreactions of Corn pounds containing Heteroatoms other than Oxygen BY S. T. REID

1 Nitrogen-containing Compounds Rearrangement.-The mechanism of photoisomerization about the C-N double bond has been the subject of a number of separate investigations. Two different pathways have been proposed for the direct and triplet-sensitized syn-antiphotoisomerizations of the 4-nitrophenylhydrazones of benzaldehyde and certain of its p-substituted derivatives.l A twisted triplet intermediate is proposed for the sensitized process, whereas in the direct isomerization the reaction is said to occur either concurrently with or subsequent to internal conversion of electronic into vibrational energy. Evidence for two different pathways in an analogous study of the 4-nitrophenylhydrazone of pyridine-Zaldehyde has also been published.2 The syn-isomer undergoes complete conversion into the antiisomer on direct excitation, but a photoequilibrium between the two isomers is rapidly established in the triplet-sensitized process. Thermally reversible synanti-isomerization has been reported in the 2-phenylhydrazones of 1,2,3-triketones and related 1,2-diketone~,~ and an investigation of the mechanism of tripletstate photochemical isomerization of benzoylacetanilide and pyrazolone azomethine dyes has been de~cribed.~In the latter case, two pathways were distinguished, the first involving torsion about the azomethine bond and the second involving inversion at the nitrogen atom. Electron-donating substituents facilitate the torsion mechanism in contrast with electron-withdrawing substituents, which favour the inversion mechanism. cis-Azoalkanes, unlike the corresponding aryl azo-compounds, undergo photodecomposition with loss of nitrogen at room temperature. In an attempt to obtain stable cis-azoalkanes, the cis-isomers of the azo-compounds (1) derived from adamantane and norbornane were prepared by irradiation of the corresponding trans-isomers (2) in toluene at 0 oC.6 The cis-isomers reverted to the trans-isomers on heating without substantial competing loss of nitrogen. Equilibrium isomer ratios at 320 and 430nm for the photochemical cis-transisomerization of a series of l-aryl-3-(3-methylbenzothiazolin-2-ylidene)triazenes l

a

G. Condorelli, L. L. Costanzo, S. Giuffrida, and S. Pistara, 2.phys. Chem. (Frankfurt) 1975, 96, 97. G. Condorelli, L. L. Costanzo, L. Alicata, and A. Giuffrida, Chem. Letters, 1975, 227. P. Courtot, R. Pichon, and L. Le Saint, Tetrahedron Letters, 1976, 1181. W . G. Herkstroeter, J. Amer. Chem. SOC.,1976, 98, 330. P. S. Engel, R. A. Melaugh, M. A. Page, S. Szilagyi, and J. W . Timberlake, J. Amer. Chem. Soc., 1976, 98, 1971.

455

456

Photochemistry

(3) have been determined;s the reaction proceeds via a pathway similar to that found for azobenzene. Further studies of the photochromism of salicylaldehyde 2-quinolylhydrazone (4) have been described. The coloured form ( 5 ) is remarkably stable in both

H

hv, 250-400 n m L

"b

H-*'

w

A

HI ._....I

N\

N

protic and aprotic solvents at room temperature. Structural requirements for this isomerization have been deduced from an examination of a series of substituted salicylaldehyde 2-q~inolylhydrazones,~ and details of the kinetics and mechanism of the thermal decay of the coloured form have been published.s A photostationary equilibrium is rapidly established on irradiation of either of the isomeric 2-diethylamino-l,3-diphenylprop-2-en-l-ones (6) and (7) in diethyl ether.g The major products of prolonged irradiation are the dihydroisoquinoline (8) and a mixture of cis- and trans-chalcone (9); their formation can best be accounted for in terms of a Norrish Type I1 process involving the biradical species (10). Analogous photoreactions have been reported for other 2-dialkylamino-l,3-diphenylprop-2-en-l-ones, but deconjugation of the apunsaturated ketone appears to be preferred in cases where this is possible, as in a

E. Faughaenel, R. Haensel, W. Ortmann, and J. Hohlfeld, J. prakt. Chem., 1975,317, 631. M. F. Zady, F. N. Bruscato, and J. L. Wong, J.C.S. Perkin I, 1976,2036. J. L. Wong and M. F. Zady, J. Org. Chem., 1975,40,2512. J. C . Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2459.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

ACH=CHPh

Ph

457

*

t

(9)

+ [MeCH=NEt]

Me

-

he (10)

J01

OH

the derivative (11) which affords the isomer (12) in 50% yield on irradiation in diethyl ether or methanol. An alternative cyclization has been observed in dialkylaminocyclohexenones.l0 On irradiation, 2-piperidinocyclohex-2-en-1-one (13) is converted into the azetidine (14) by a process which presumably must involve y-hydrogen abstraction by the carbonyl group to form the biradical(l5); a singlet excited state appears to be involved. In a number of derivatives, and especially those with N-tosyl groups, competing cyclization to the corresponding

0

lo

J. C.Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2463.

458

Photochemistry azetidinol (16) occurs. Surprisingly, no evidence for cyclization has been observed in the photochemistry of the related 2-alkoxycyclohexenones. Photochemically induced electrocyclic processes continue to attract much attention. Quenching studies indicate that the photoisomerization of l-ethoxycarbonyl-lH-azepine (17) occurs via a singlet excited state;ll a triplet state is available but unreactive. Arylated 1,3-dihydr0-2H-azepin-2-ones(18) are

C02Et

(18) R1 = R2 = But R3 = Me, Prn, or C,HI1

converted by an analogous disrotatory process into the isomers (19) in high yield on irradiation in benzene.lz Cyclization is also observed on irradiation of diazepines, as, for example, in the conversion of lH-1,2-diazepines into the corresponding 2,3-diazabicycl0[3,2,O]hepta-3,6-dienes.~~In contrast with their carbocyclic analogues the benzocycloheptatrienes, which react principally by [1,7] hydrogen shift and ring contraction, lH-2,3-benzodiazepines (20) undergo a

'qR3 R

R'

hv, Pyrex

s

'

,

R1'

-qR2

R2 (20)

:

(21)

R1

R2

R3

H OMe

H Me H

H H

H H H H H

Me Me Ph H PhCH2

H Ph H Ph

Ph

facile valence isomerization to give the 2a,7-dihydro[l,2]diazeto[4,1-a]isoindoles (21).l* The reaction, which provides the first example of an isolable 1,2-diazetine, is virtually quantitative, and there is no nitrogen extrusion as might be expected by analogy with the photochemical behaviour of 1,2-benzodiazepines. A more recent report describes the related conversion of 3H-1,2-diazepines into the corresponding 1,2-diazet0[4,1-a]pyr~oles.~~ l1 la

lS

'l lS

G. Jones and L. J. Turbini, J. Photochem., 1976, 5, 61. H.-D. Becker and K. Gustafsson, Tetrahedron Letters, 1976, 1705. J. P. Luttringer, N. PCrol, and J. Streith, Tetrahedron, 1975,31,2435. A. A. Reid, H. R. Sood, and J. T. Sharp, J.C.S.Perkin I, 1976, 362. C. D. Anderson, J. T. Sharp, E. Stefaniuk, and R. S. Strathdee, Tetrahedron Letters, 1976,305.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

459

Reversible cyclization of the benzoazahexatriene intermediate (22) has been proposed to account for the formation of N-acetylbenzoazetines (23) from 1-acetyl-2-cyano-l ,2-dihydroquinolines (24) on Pyrex-filtered irradiation in diethyl ether or ethanol.16 A competing irreversible cyclization of the triene (22; R = Me) to the dihydroindole (25) predominates on further irradiation.

AC

NC

Ac

N I CO,Et

(z8)

p.

0 N I

An aza[l3]annulene (26) of unknown configuration has been obtained by lowtemperature irradiation of the heterocycles (27), (28), and (29).17 The azaannulene (30) is also formed on irradiation of the aziridine (29). The 2-azabicyclo[2,2,l]hexane ring system has been synthesized by irradiation of N-substituted 3-allylamino-5,5-dimethylcyclohex-2-en-l-ones (31) in cyclohexane.18 For the lo

M. Ikeda, S. Matsugashita, F. Tabusa, H. Ishibashi, and Y. Tamura, J.C.S. Chem. Comm.,

l7

1975, 575. G. Frank and G. Schroder, Chem. Ber., 1975, 108, 3736.

Y. Tamura, H. Ishibashi, M. Hirai, Y. Kita, and M. Ikeda, J. Org. Chem., 1975,40,2702.

Photochemistry

460

N-methyl, N-allyl, and N-phenyl derivatives, intramolecular cycloaddition takes place to give exclusively or predominantly the less stable stereoisomer (32). The N-acetyl derivative (31; R = Ac), however, is converted into a 1 : 1-mixture of isomers (32; R = Ac) and (33); this difference in stereospecificity is tentatively

____, cyclo hv, pyrex hexane I

&-Jk HT

Me

Me (34)

(35)

accounted for in terms of different excited states. It is not clear why the corresponding dimethyl derivative (34) takes an unusual course, giving 7-azabicyclo[4,3,0]nonan-2-one (35) as the sole product.l@ Valence tautomerization to the 1,4-diazocine (36) is proposed to account for the photoreaction of the azetidino[3,2-b]pyridine (37).20 The final products of irradiation in diethyl ether at - 78 “C

c Et

I

C-N=C=C, C H’;‘ Et !

Me

l* *O

HI

/

Ph H

(39)

Y. Tamura, H. Ishibashi, and M. Ikeda, J . Org. Chem., 1976,41, 1277. J. W. Lawn, M. H. Akhtar, and W. M. Dadson, J . Org. Chem., 1975,40, 3363.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

461

are the l-(S)-pyrrole (38) and the N-(A)-ketenimine (39). The optical purity of the products corresponds to about 80% retention of configuration for the proposed [1,3] sigmatropic rearrangement. The stereochemistry of the reaction is therefore in agreement with orbital-symmetry predictions. The photochemistry of 1,3-oxazin-6-one (40) appears to be similar to that previously described for a-pyrone. Irradiation in an argon matrix gave two unstable species to which structures (41) and (42) have been tentatively assigned on the basis of spectral evidence.21 Further irradiation led to decomposition and the formation of HCN, C 0 2 ,and acetylene.

(44)

(43)

The study of heterocyclic analogues of the stilbene -+ phenanthrene cyclization continues to attract some attention. Novel pyridocarbazoles have been obtained in the absence by the photocyclization of 1-/3-indolyl-2-pyridylacrylonitrile~;~~ of oxygen, pathways competing with dehydrogenation are sometimes observed.23 4-Methylsulphonylbenzo[c]cinnolines were prepared by photochemical cyclodehydrogenation of the corresponding 2-methylsulphonylazobenzenes in 98% sulphuric A novel route to the imidazo[2,1-a]isoquinoline ring system has been accomplished by irradiation of cis-1-styrylimidazole (43) in methanol in the presence of iodine.25 Unlike the stilbene --f phenanthrene cyclization, an intermediate of the dihydrophenanthrene type is not possible in this case, and a dipolar species (44) has been proposed. Analogous cyclizations occur with substituted l-styrylimidazoles and with l-styrylbenzimidazole. The cyclization of 1-dimethylamino-2,2-bis-(9-fluorenylidenemethyl)ethyleneis followed by elimination of dimethylamineto give benzo[e]fl~oreno[9,1-kZ]acephenanthrylene.~~ Further synthetic applications of the photocyclization of enamides have been reported. Details of the preparation of alkylated benzophenanthridinones (45) in 50% yield by irradiation of the N-cyclohexenyl-l-naphthamides(46) have been published,27and similar cyclizations have been employed in the synthesis 21 22

23

24 25 26

27

A. Kranz and B. Hoppe, J. Amer. Chem. SOC.,1975, 97, 6590. C. Dieng, C. Thal, H. P. Husson, and P. Potier, J. Heterocyclic Chem., 1975, 12, 455. C. Riche, A. Chiaroni, H. Doucerain, R. Besselievre, and C. Thal, Tetrahedron Letters, 1975, 4567. C. P. Joshua and V. N. R. Pillai, Indian J. Chem., 1975, 13, 1018. G. Cooper and W. J. Irwin, J.C.S. Perkin I , 1976, 75. C. Jutz and H.-G. Lobering, Angew. Chem. Internat. Edn., 1975, 14, 418. I. Ninomiya, T. Naito, and A. Shinohara, Japan Kokai, 74 134 679 (Chem. Abs., 1975, 83, 28 125).

16

462

Photochemistry

of protoberberine alkaloids 28 and of ( & ) - c a ~ i d i n e . ~ A~ novel stereoselective synthesis of lycorine-type alkaloids using the key intermediate (47), obtained by photocyclization of the enamido-ketone (48), has also been described.80 Two different mechanisms are believed to be involved in the non-oxidative cyclization of benzo[b]thiophen-2-carboxanilide (49 ; R = H) and its N-methyl derivative (49; R = Me).31 The former affords the cis-fused product (50), whereas the latter is converted into the trans-isomer (51); the formation of the trans-isomer is

(49)

28

2B

31

\

[l,5] shift

I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1720. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1791. H. Iida, S. Aoyagi, and C. Kibayashi, J.C.S. Perkin I, 1975, 2502. Y. Kanaoka, K. Itoh, Y . Hatanaka, J. L. Flippen, I. L. Karle, and B. Witcop, J. Org. Chem., 1975,40, 3001.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

463

presumably the result of a suprafacial 1$5-hydrogen shift in the trans-dihydrointermediate (52). Oxidative cyclization of a different type is observed on irradiation of the fully methylated 6-(benzylidenehydrazino)uracils (53) and this provides a new synthetic route in up to 90% yield to pyrazolo[3,4-d]pyrimidines (54).3a Carbazoles and indolo[3,2-b]carbazoles have been synthesized by photocyclization of substituted triphen~1arnine.s.~~

Me

hie

Me

(53)

(54)

The photochemical generation of nitrile ylides from 2H-azirines has been reviewed.34 These species are now widely used in synthetic chemistry and new applications continue to be reported. The low-temperature irradiation (- 196 "C) of 2,3-diphenyl-2H-azirine (55) has been reinvestigated and leads almost quantitatively to the formation of the dipolar species, benzonitrile-benzylide (56), with = 0.78.35 Irradiation ( A = 345 nm) of the benzylide (56) resulted in almost Ph H

b H Ph

N

-

+

hv

complete reconversion into the 2H-azirine (0= 0.15). The benzylide (56) also underwent a quantitative conversion into 2,5-diazahexa-lY3,5-triene(57) on heating to - 160 "C, thus providing evidence that the triene is formed not only via an indirect route involving the bicycle (58) as previously demonstrated, but also by direct dimerization of the benzylide. Evidence for the intermediacy of dipolar species (59) in the photochemical conversion of ( 5 8 ) into (57) has been presented. Rearrangement via the ylide to give the oxazoline (60) is observed on 32 33 34

3b

F. Yoneda and T. Nagamatsu, Bull. Chem. SOC.Japan, 1975, 48, 1484. W. Lamm, W. Jugelt, and F. Pragst, J. prakt. Chem., 1975, 317, 284. A. Padwa, Angew. Chem. Internat. Edn., 1976, 15, 123. A. Orahovats, H. Heimgartner, H. Schmid, and W. Heinzelmann, Helv. Chim. Acta, 1975, 58, 2662.

464

Photochemistry PhwcHzR

+ Ph-C-N-CH

/,v _ I ,

I

CH,R

N

irradiation of 2-hydroxymethyl-3-phenyl-2H-azirine(61 ; R = OH) in benzene, whereas in the 2-chloro- or 2-bromo-analogues (61; R = C1 or Br) an unexpected rearrangement involving a novel 1,4-halogen shift occurs to give the N-vinylimines (62).3s The ylides derived from spiro-azirines (63) are readily trapped by methanol, giving the imines (64).37 In contrast to these results, however, irradiation of 2-phenyl-l-azaspiro[2,2]pent-l-ene (65) resulted in an unusual photochemical cycloelimination to give products such as the azirine (66) derived from the novel carbene 2-phenylazirinylidene.

\\ f

’

- ..

N CH2’

(63) n = 1, 2, or 3 (65) n = 0

p,, OMe

Intermolecular and intramolecular addition reactions of nitrile ylides have been reported. Irradiation (280-350 nm) of a benzene solution of 3-phenyl2H-azirines in the presence of carboxylate esters leads to the formation of 5-alkoxy-3-oxazolines by regiospecific addition of benzonitrile-methylide to the ester carb0ny1.~~ Intramolecular addition of ylide to a carbonyl is observed on irradiation of 2-formyl-3-phenyl-2H-azirine (67) to give 2-phenyloxazole (68) in 70% yield.39 Analogous transformations were found in 2-vinyl-substituted 2H-a~irines,~~, *O but the major product of irradiation of (2)-3-phenyl-2-styryL 87 88

40

A. Padwa, J. K. Rasmussen, and A. Tremper, J.C.S. Chem. Comm., 1976, 10. A. Padwa and J. K. Rasmussen, J. Amer. Chem. SOC.,1975,97, 5912. P. Gilgen, H.-J. Hansen, H. Heimgartner, W. Sieber, P. Uebelhart, H. Schmid, P. Schonholzer, and W. E. Oberhlnsli, Helv. Chim. Acta, 1975, 58, 1739. A. Padwa, J. Smolanoff, and A. Tremper, J . Amer. Chem. SOC.,1975, 97, 4682. A. Padwa, J. Smolanoff, and A. Tremper, J. Org. Chem., 1976, 41, 543.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

465

2H-azirine (69) is the benzazepine (70). The preference for cyclization to a sevenmembered ring is a consequence of the proposed linear geometry of the dipolar intermediate. Sensitization and quenching experiments indicate that the primary photoreaction of the 2-vinyl-substituted 2H-azirine system occurs from the singlet state.

CHO (67)

phpd" Ph

/

The ylide derived from 2-phenyl-3-methyl-3-allylazirine(71) has a different fate; the major product of irradiation is the 2-azabicyclo[3,1,O]hex-Zene (72), and the reaction is viewed as an intramolecular 1,l-cycloaddition of the carbene (73) to the alkene.41 A more recent study provides support for a stepwise pathway for this addition,42 and similar additions have been reported in related sys As part of a continuing study of phototransposition in aromatic and heteroaromatic systems, the irradiation of 3-, 4-, and 5-methyl-2-cyanopyrroles in acetonitrile has been On the basis of product analysis the pathway outlined in Scheme 1, involving 2,5-bonding followed by a 1,3-sigrnatropic shift 41

43

44

A. Padwa and P. H. J. Carlsen, J. Amer. Chem. SOC.,1975, 97, 3862. A. Padwa and P. H. J. Carlsen, J. Amer. Chem. SOC.,1976, 98, 2006. A. Padwa, A. Ku, A. Mazzu, and S. I. Wetmore, J. Amer. Chem. SOC.,1976,98, 1048. J. Barltrop, A. C. Day, P. D. Moxon, and R. R. Ward, J.C.S. Chem. Comm., 1975, 786.

466

Photochemistry

major product

minor product Scheme 1

of the nitrogen atom, has been proposed for the photorearrangement of 2-cyanopyrroles. A second 1,3-sigmatropicshift is necessary to account for the formation of the minor product. The suggestion that 2,5-bonding in excited pyrrole initiates the transposition is supported by a correlation diagram for the T,T* state of pyrrole. An analogous intermediate is believed to be implicated in the photoisomerization of the azine monoxide (74),46 and reaction of the mesoionic 1,2,4-triazo1-3-ones (75) to give the corresponding benzimidazoles, azobenzenes,

&

h

&

N=N<

N-N:

0-

0-

-

-

Et Et ff'O

(74)

Arl

I

R

A?'

N

-

Ar'

I

R

hv

0

Ar2'

0

and aryl isocyanates is thought to involve initial formation of the bicyclic intermediate (76), followed by rearrangement to the triazolone (77) and subsequent fragmentati~n.~~ A different mechanism is proposed for the photochemical conversion of isoxazoles into oxazoles and keto-ketenimine~.~'The isocyanide (78) has been established as an intermediate in the photorearrangement at -77 "C of isoxazole (79) to oxazole (80). Spectroscopic evidence for the azirine (81) as a O5 O6 O7

R. Paredes and W. R. Dolbier, Rev. Latinoamer. Quim., 1975, 6, 29. H. Kato, T. Shiba, E. Kitajima, T. Kiyosawa, F. Yamada, and T. Nishiyama, J.C.S. Perkin I, 1976, 867. J. P. Ferris and R. W. Trimmer, J. Org. Chem., 1976, 41, 13.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

467

N

(79)

1+

-

(78)

(80)

precursor to (78) has been reported, and the azirine itself is believed to be formed from the vinyl nitrene (82). Conflicting evidence has been described in a study of the rearrangement of 4-carbonyl-substituted isoxaz01es.~~Irradiation of the isoxazole (83) gave the isomeric isoxazole (84) as the only primary photoproduct;

(83)

(84)

(85)

this was converted into the oxazole (85) on further irradiation by a pathway which does not appear to involve an azirine intermediate. Reductive ring cleavage was the principal reaction observed on irradiation of isoxazoles in alcohols in the presence of copper(r1) Earlier studies have shown that irradiation of N-1-alkylated indazoles gives the corresponding alkylaminobenzonitriles, whereas N-Zalkylated indazoles are converted into N-1-alkylated benzimidazoles. Flash photolysis studies have been carried out with the aim of detecting possible photochemical intermediates.60 Radical species have been identified by e.s.r. spectra and are believed to arise by cleavage of the N-N bond. Triplet states do not now appear to be involved in any of these transformations. The previously reported photocyclization of enaminonitriles to imidazoles has been extended to include cyclic five-, six-, and seven-membered systems (86).61 Ketenimine intermediates (87) have been

-

4 - H /-CflC

K

g

y

* --% (CHA

(86) IZ = 3, 4, or 5

dB

I

Lc"N* (87)

c2(H

(88)

A. Padwa, E. Chen, and A. Ku, J. Amer. Chem. Soc., 1975,97,6484. T. Sato, K. Yamamoto, K. Fukui, K. Saito, K. Hayakawa, and S. Yoshiie, J.C.S. Perkin I, 1976, 783.

J. P. Ferris, K. V. Prabhu, and R. L. Strong, J. Amer. Chem. Soc., 1975, 97, 2835. J. P. Ferris and R. W. Trimmer, J. Org. Chem., 1976, 41, 19.

468

Photochemistry

detected spectroscopically at low temperature; the precise mechanism for the conversion of these intermediates into imidazoles (88) is at present uncertain, but it has been clearly demonstrated that the process is a monophotonic one and that pyrazoles are not intermediates. The products of irradiation of 2-isoxazolines have been described in an earlier Report and pathways postulated for their formation. Further work with substituted 2-isoxazolines has now been reported;62 product distribution appears to depend largely upon the nature of the substituents, with excitation of different chromophores being involved, but no definite conclusions can yet be drawn concerning the factors influencing this reaction. Irradiation of cis- (89) and trans-2,4,5-triphenylimidazoline(90) in acetonitrile or benzene leads to the formation of a photostationary mixture of cisand trans-isomers, the latter p r e d ~ m i n a t i n g . In ~ ~ acetone solution, 2,4,5-triphenylimidazole was also formed. Two types of intermediate have been proposed,

namely imidazolinyl radicals (91) or (92) arising by hydrogen abstraction, or a biradical or zwitterionic species resulting from cleavage of the C-4-C-5 bond. Cleavage to the reactive nitrile oxide (93) also results from irradiation (300 nm) of benzofurazan (94);64the nitrile oxide can be trapped as the isoxazole (95) with dimethyl acetylenedicarboxylate. In the presence of triethyl phosphite, the cis,cis-dinitrile (96) is obtained from benzofurazan in high yield.SS

sa 63 64

s5

Y. Ito and T. Matsuura, Tetrahedron, 1975, 31, 1373. T. Matsuura and Y. Ito, Bull. Chem. SOC.Japan, 1975, 48, 3369. I. Yavari, S. Esfandiari, A. J. Mostashari, and P. W. W. Hunter,J. Org. Chem., 1975,40,2880. T. Mukai, S. Nitta, and T. Ohine, Jap. P., 74 28 176 (Chem. Abs., 1975,83,43 086).

Photoreactions of Compounds containing Heteroatoms other than Oxygen

469

Little new work has been reported for six-membered heterocycles. Perfluoroalkylpyridine (97), on irradiation in CF,ClCFCI,, is quantitatively converted into the p-bonded species (98) and the two azaprismanes (99) and Diazabicyclo[2,2,0]hexa-2,5-dienes (101) and (102) have been shown to be intermediates in the formation of fluorinated pyrazines (103) from pyridazines

(97) R' = CF,CF, R2 = CF, R3 = CF(CF3),

(99) R1

+

h v , 254 nm

F

(104).s7 In certain cases, the 1,Zdiaza intermediate (101) can be isolated and is easily converted thermally or photochemically into the 1,4-diaza isomer (102). 2-Alkylcinnolinium-4-olates(105) undergo rearrangement in ethanol to give 3-alkyl-4(3H)-quinazolones (106) in good yield.68 The reaction, which is claimed without any real evidence to proceed via the diazabenzvalene intermediate (107), is analogous to that previously reported for pyridazinium-3-olates. sE ri7

R. D. Chambers, R. Middleton, and R. P. Corbally, J.C.S. Chem. Comm., 1975, 731. R. D. Chambers, J. R. Maslakiewicz, and K. C. Srivastava, J.C.S. Perkin I, 1975, 1130. D. E. Ames, S. Chandrasekhar, and R. Simpson, J.C.S. Perkin I, 1975, 2035.

Photochemistry

470

The study of the photorearrangement of nitrones and N-oxides is one that has again consumed much effort, although little novelty can be attached to many of the reports. The photorearrangement of nitrones to oxaziridines is well documented; the first example of the reverse process, the rearrangement of oxaziridines to nitrones, has been reported and has been used to account for the racemization of certain chiral oxaziridines on irradiation (254nm) in dichloromethane.5B Oxaziridines (108) are postulated to be the primary products of irradiation of the oxoindolinylidenamine N-oxides (109) in benzene or THF, giving ring-expanded products (1 10) and (1 1 l), the deoxygenated products 0

(109) R

=

0

0

CMe,, CHMe,, or 1’11

(108) 0

-

o\+,Me N

C

GI

O

Me

(112), and in low yield the isatin.60 An alternative ring-opening reaction is observed in methanol. In the isomeric nitrone (113), the oxaziridine (114) can be isolated and is further converted into the tetrahydroquinonazoline-2,4-dione (115) and N-methylisatin in both aprotic solvents and in methanol. The photooxidation of nitrones with singlet oxygen has been described.61 Independent support for the intermediacy of oxaziridines in the photochemically induced rearrangement of heteroaromatic N-oxides comes from a

0(116) R = H, Me, MeO, PhCH,O, Ph, C1, or CN 69

6o

(117)

J. Bjprrgo, D. R. Boyd, R. M. Campbell, and D. C. Neill, J.C.S. Chem. Comm.,1976, 162. H. G. Aurich and U. Grigo, Chem. Ber., 1976,109,200. T . Ching and C. S. Foote, Tetrahedron Letters, 1975, 3771.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

471

study of the thermal addition of nitrile oxides with 2-phenylben~azete.~~ An unusual copper salt effect on the photochemistry of pyridine N-oxides has been d i ~ c o v e r e d .Irradiation ~~ of pyridine N-oxides (116) in water containing CuSO, gave 3-substituted 2-formylpyrroles (117) in yields of 33-42%. In the absence of CuSO, or other copper salts, yields were reduced to 2--6%. The intramolecular photorearrangement of pyridine N-oxide can be completely suppressed by The protecting the N-oxide as a complex with boron t r i f l ~ o r i d e . ~ ~ photoisomerization of certain quinoline N-oxides to the corresponding 2-quinolones

"WR' R'

R'

"yyJ--"

I

Thv

+

+ R2

RoI-

(125) Scheme 2 62

C. W. Rees, R. Somanathan, R. C. Storr, and A. D. Woolhouse, J.C.S. Chem. Comm., 1975,

63

740. F. Bellarny, P. Martz, and J. Streith, Heterocycles, 1975, 3, 395. G. Sena-Errante and P. G. Sammes, J.C.S. Chem. Comm., 1975, 573.

64

Photochemistry has been studied as a function of pH.sK A decrease in the quantum yield of reaction for the protonated N-oxide is observed; this together with other supporting evidence suggests that photoisomerization occurs from the n,n* singlet state of the non-protonated molecule. Full details of the products derived from a series of acridine 10-oxides (118) have been published and the proposed pathway is outlined in Scheme 2.66 Irradiation is thought to afford the intermediate oxaziridine (119) in equilibrium with its valence isomer, the 1,2-oxazepine (120). The former is favoured in polar solvent, the latter in non-polar solvent. Products (121) and (122) are derived from (119), presumably by a process involving 1,5- or lY9-oxygenrearrangement, whereas products (123) to (126) are the results of further rearrangement of (120). Products (123) and (124) are not obtained when there is a C-9 substituent. Reactions originating from the state of isoquinoline N-oxide and from both the S, and TI states of acridine N-oxide have been observed on irradiation with green light (540 nm) of a benzene solution containing a suspension of alumina powder dyed with e o ~ i n . ~Electron ' transfer from the excited singlet eosin is implicated in this process. The photochemical and photophysical processes undergone in dilute ethanol solution by 2-benzoyl-3-phenylquinoxaline1,4-dioxide (127) have been studied.68 Direct irradiation (404 nm) yields only one photoproduct, 1,3-dibenzoylbenzimidazolone (128), formed monophotonically with 0 = 0.093. The most plausible pathway is outlined in Scheme 3. The remaining excited singlet reverts 472

0-

" Ph

to the ground state via internal conversion. Fragmentation also occurs on irradiation (313 and 254 nm), leading to the formation of ethyl benzoate. Tripletsensitized experiments show that the triplet state, which is not populated by e5 e6

13' 68

G . G . Aloisi and G . Favaro, J.C.S. Perkin II, 1976, 456. S. Yamada, M. Ishikawa, and C. Kaneko, Chem. and Pharm. Bull. (Japan), 1975,23,2818. N. Hata, Chem. Letters, 1975, 401. N. A. Masoud and J. Olmsted, J. Phys. Chem., 1975, 79, 2214.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

473

direct irradiation, readily eliminates oxygen to give 2-benzoyl-3-phenylquinoxaline. The photoisomerization of substituted phenazine N-oxides has also been examined.69 Deoxygeiiation to the parent 1,2,4-triazines and ring contraction to 1,2,4-triazoles were observed on irradiation of a series of substituted 1,2,4triazine 4-0xides,~Oand photoreactions involving rearrangement, deoxygenation, and ring-opening have been reported for 6-methyl- and 6,9-dimethyl-purine 1 In principle, azoxy-compounds are capable of undergoing reactions analogous to those described for nitrones. The isolation of an oxadiaziridine has recently been accomplished in this way by irradiation of (2)-bis-( 1-methyl ethy1)diazene N-oxide (129).72 Cyclization was the major pathway, but was accompanied by

minor amounts of deoxygenation. Isomerization to the (E)-azoxyalkane was not observed in this case, but did occur in (2)-cyclohexylmethyldiazene l-oxide and in (2)-methylcyclohexyldiazine 1 -oxide in addition to ring closure. Both isomers, (130) and (13 l), of 4-methoxyazoxybenzene undergo photochemically induced Me0 11Y d

EtOH

I

0-

rearrangement to the same o-hydroxyazo-compound (132).73 In the /%isomer (13l), prior photoisomerization to the a-isomer (130) occurs; an oxadiaziridine may be involved in this process, although evidence for this proposal is lacking. Migration of the oxygen atom into the peri-position does not occur on irradiation of 1,l '-a~oxynaphthalene.~*The first synthesis of a 1-alkyl derivative

'I1 'Ia

'I3 74

A. Albini, G . F. Bettinetti, and S. Pietra, Guzzettu, 1975, 105, 15. H. Neunhoeffer and V. Bohnisch, Annulen, 1976, 153. F. L. Lam and J. C. Parham, J. Amer. Chem. SOC.,1975,97,2839. K. G. Taylor, S. R. Isaac, and L. J. Swigert, J. Org. Chem., 1976,41, 1146. N. J. Bunce, Canad. J. Chem., 1975, 53, 3477. N. J. Bunce, D. J. W. Goon, and J.-P. Schoch, J.C.S. Perkin I, 1976, 688.

474

Photochemistry 0-

(134)

(133)

of 1,2,3-benzotriazole 2-oxide (133) has been accomplished by photorearrangement of 1-methyl-l,2,3-benzotriazole3-oxide (134).7s Again an oxadiaziridine may be involved although there is no conclusive evidence for this. Quenching studies indicate that a singlet state is involved in the rearrangement and a triplet state in the accompanying deoxygenation. Pyridinium ylides undergo reactions formally analogous to these reported above for heteroaromatic N-oxides. The formation of pyrazoles (135) from

RtY2 R3

N-N

I

CO,E t

(137)

R2$l

7RTN

N’ I

(139)

i

fyj: R1

(138)

I

C0,Et

(1401

N-ethoxycarbonyliminopyrazinium ylides (136) has been reported;7s a mechanism involving the 1,2,5-triazepine intermediate (137) has been proposed and is supported by the isolation of RlCN in each case. Ring expansion, presumably 76

M. P. Serve, W. A. Feld, P. G. Seybold, and R. N. Steppel, J. HeterocycZic Chem., 1975, 12, 811.

7e

T. Tsuchiya, J. Kurita, and K. Ogawa, J.C.S. Chem. Cumm., 1976,220.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

475

via a diaziridine intermediate, is also observed in some N-(3-quinazolinio)amid ate^,^' and the diaziridine (138) is proposed as an intermediate in the transformation of the cinnolin-6-ium salt (139) into the isomer (140).78 An important study of the photochemically induced valence-bond tautomerization and dimerization of 3-oxido-l-phenylpyridinium(141) has been described.'O Irradiation (350nm) in ethyl acetate yields the isomer (142), the photodimer 0

f--

- N

I

I

Ph (142)

/

Ph (141)

(143)

(143), and two other dimers resulting from thermal cycloaddition of (142) to the starting betaine. The formation of the valence tautomer is a photochemically allowed process. Additional evidence that the o-nitrobenzaldehyde to o-nitrosobenzoic acid rearrangement occurs via an excited triplet state and that the rearrangement is intramolecular comes from a new study of the photoreaction in degassed THF.80 The quantum yield for product formation is 0.5 and is not significantly altered by change in the concentration of o-nitrobenzaldehyde. Radicals produced by irradiation of o-nitrobenzaldehyde in water or methanol were shown by e.s.r. studies to have an aryl nitroxide structure and are believed to arise from o-nitrosobenzoic acid.*l On irradiation, fully protected 2,3-, 3,4-,8a and 4,6-O-o-nitrobenzylidene glycopyranosides83 undergo rearrangement to the corresponding glycoside o-nitrosobenzoates, which are isolated as their oxidation products, the o-nitrobenzoates. Thus, the 2,3-O-o-nitrobenzylidenederivative (144) is converted

--+ MeOH

AcO

ACO (145) R1 = H,

(144) Re,

=

R2 = o-N02C,H4C0

O-NOZCCH,, R,, = H = o-NO,C,H4

R,,, = H, R,,

into the 2-o-nitrosobenzoate (145); as each of these transformations gives almost exclusively one isomer, the reaction constitutes a good synthesis of partially protected pyranose derivatives. The mechanism is thought to involve an initial 77

8a

83

J. Fetter, K. Lempert, J. Mnrller, and G. Szalai, Tetrahedron Letters, 1975, 2775. R. Y. Ning, J. F. Blount, W. Y. Chen, and P. B. Madan, J. Org. Chem., 1975,40,2201. A. R. Katritzky and H. Wilde, J.C.S. Chem. Comm., 1975, 770. W. G. Filby and K. Guenther, 2. phys. Chem. (Frankfurt), 1975, 95, 289. R. G. Green, L. H. Sutcliffe, and P. N. Preston, Spectrochim. Acta, 1975, 31A, 1543. P. M. Collins and N . N. Oparaeche, J.C.S. Perkin I, 1975, 1695. P. M. Collins, N . N. Oparaeche, and V. R. N. Munasinghe, J.C.S. Perkin I, 1975, 1700.

476

Photochemistry

hydrogen abstraction followed by hydroxy-group migration to give the orthoacid. This would be expected to rearrange rapidly to the hydroxynitrosobenzoate with retention of configuration at the pyranosyl carbon atom. On irradiation, o-nitrobenzaldehyde N-acetyl-N-alkylhydrazones(146) are converted in a similar fashion into nitroso-hydrazides (147) which in turn undergo slow thermal rearrangement to the triazenes (148);84 further irradiation of (148) led to 0

(146) R

8 1+

=

(147)

Me, Et, or PhCH,

I RNHAc (149)

+

N.2

+

CO,

f--

'Ac

decomposition and the formation of benzyne, the amides (149), nitrogen, and carbon dioxide. The formation of all but one product from irradiation of crystalline nitrobenzene derivatives (150) has been shown to originate from an intramolecular hydrogen abstraction by the excited nitro-group from the t-butyl group (Scheme 4).85 The major products are 1-hydroxyindolin-2-ones (151)

84

Y. Maki, T. Furuta, M. Kuzuya, and M. Suzuki,J.C.S. Chem. Comm., 1975, 616. D. Dopp and K.-H. Sailer, Chem. Ber., 1975, 108, 3483.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

477

although the primary products appear to be the photolabile indolenine l-oxides (152). It has been experimentally demonstrated that oxygen transfer from the excited nitro-group is implicated in the ready photochemical cleavage of the amide bond in N-a~yl-8-nitrotetrahydroquinolines.~~ These amides and the related light-sensitive 1-acyl-7-nitroindolines87 may serve as useful protecting groups for the carboxyl function. An intramolecular hydrogen abstraction may also be involved in the photoreaction of methyl 1l-o-nitrophenoxyundecanoate, but the details of this process are by no means clear.88 A new reaction of nitro-compound has been found in certain o-nitrophenyl heterocycles. Cyclization by attack at C-5 leading to the nitrosoazo intermediates (153) and the eventual formation of benzotriazole l-oxides (154) is observed on R3

RY3 R4

"N

R2&R4 0

hv ___,

6"" GN'O* /

Rl

(153)

I 0-

irradiation of a series of l-(o-nitropheny1)pyrazoles(155) in Analogous transformations have been reported for 1-(o-dinitropheny1)imidazoles giving 6-nitrobenzimidazole 1-0xides.~~ These products on irradiation not unexpectedly undergo further rearrangement to benzimidazolones. Only reduction products are obtained from the corresponding l-(p-nitrophenyl)pyraz~les.~l A new and potentially useful route to 2-aroyl-lH-quinazolinones(156) has been accomplished by irradiation of 2-o-nitrophenyl-5-aryloxazoles (157).92 A complex pathway is obviously involved in this unusual transformation and a bromoazirine intermediate (158) has been proposed. There is, however, no evidence to support this proposal in preference to others equally persuasive. 88 87

9a

B. Amit, D. A. Ben-Efraim, and A. Patchornik, J.C.S. Perkin I, 1976, 57. B. Amit, D. A. Ben-Efraim, and A. Patchornik, J. Amer. Chem. SOC.,1976, 98, 843. K. Stiller, A. C. Waiss, and W. F. Haddon, Chem. and Ind., 1975, 652. P. Bouchet, C. Coquelet, J. Elguero, and R. Jacquier, Bull. SOC.chim. France, 1976, 184. P. Bouchet, C. Coquelet, J. Elguero, and R. Jacquier, Bull. SOC.chim. France, 1976, 192. P. Bouchet and C. Coquelet, Bull. SOC.chim. France, 1976, 195. 1. A. Silberg, R. Macarovici, and N. Palibroda, Tetrahedron Letters, 1976, 1321.

478

Photochemistry

(157) R

=

HorBr

(1 58)

I 0

0

A re-examination of the photochemistry of substituted /3-methyl-P-nitrostyrenes (159) has shown that competing reactions take place from two different excited The well known conversion into oximino-ketones (160) via the nitrite (161) occurs from a singlet excited state or a higher triplet state T,, whereas the competing formation of aromatic aldehyde (162) and nitrile oxide (163), which is favoured in the presence of electron-withdrawing substituents, is derived from the lowest triplet state. The N-oxide (164) is a possible intermediate in the second transformation.

# - DCH0 +*-,

R

0-

R

+

A+ 111

C I Me

Rearrangements in nitrogen-containing carbonyl compounds originating from excitation of the carbonyl group merit brief discussion in this section. Azetidinols have been prepared from a-amino-ketones by a Norrish Type I1 process involving y-hydrogen a b s t r a c t i ~ n . ~Hydrogen ~ abstraction from the 93 114

1. Saito, M. Takami, and T. Matsuura, Tetrahedron Letters, 1975, 3155. E. H. Gold, U.S.P. 3 898 142 (Chem. Abs., 1975, 83, 178 795).

Photoreactions of Compounds containing Heteroatoms other than Oxygen

479

&carbon atom is observed on irradiation of N-(o-toly1)phthalimide (165) leading to the indolo[2,1-a]isoindoles (166) or (167).86 Phthalimides containing electrondonating groups do not undergo cyclization. Other workers have, however,

(166)

(165)

(167)

successfully cyclized phthalimide Mannich bases g6 and bis(phthalimidomethy1)alkylamines g7 in a similar manner. Photochemically induced cyclization of a series of N-alkyl-substituted succinimides (168 ; n = 2) and glutarimides (168 ; n = 3 ) has been used as a method for ring expansion to the ketolactams (169).g8 0

0 R' R3

+ 11v (CH,), 3 f R 4

Mc i (168)

6

R2 R1 0 ( 1 69)

(170)

The azetidinol(170), arising by a Norrish Type I1 process, is a likely intermediate, and the route has obvious synthetic potential for medium-ring heterocycles. The relative triplet reactivities of C=N and C=O double bonds with respect to intramolecular hydrogen abstraction reactions have been investigated in 4-Propionylpyrimidine 4-propionyl-, 4-butyryl-, and 4-~aleryl-pyrimidines.~~ (171), in which y-hydrogen abstraction is only possible via C=N triplets, rearranges exclusively on irradiation (313 nm) in benzene to the cyclopropanol

(171)

(172)

(172). Norrish Type I1 eliminations compete in 4-butyryl- and 4-valerylpyrimidine. The photoreactions of azetidin-2,4-diones have been compared with those of cyclobutanones.loO Three competing modes of decomposition were recognized Y. Kanaoka, C. Nagasawa, H. Nakai, Y. Sato, H. Ogiwara, and T. Mizoguchi, Heterocycles, 1975, 3, 553. H. J. Roth and D. Schwarz, Arch. Pharm., 1975, 308, 631. O7 H. J. Roth and D. Schwarz, Arch. Pharm., 1975, 308,218. O8 Y. Kanaoka and Y. Hatanaka, J. Org. Chem., 1976, 41, 400. E. C. Alexander and R. J. Jackson, J . Amer. Chem. Soc., 1976,98, 1609. l o o J. A. Schutyser and F. C. De Schryver, Tetrahedron, 1976, 32, 251. nL

480 Photochemistry on irradiation in methanol : (i) cycloreversion to yield keten and isocyanate, which react with methanol to give methyl ester and methylurethane; (ii) decarbonylation to an aziridinone, which undergoes ring-opening in methanol ; (iii) ring expansion to an oxacarbene, which with methanol forms a 5-methoxyisoxazolid-3-one. All products are claimed to arise from the 1,6biradicals formed by cleavage of either a C-N or a C-C bond. Ring-cleavage and ringexpansion reactions have also been reported in 3-NN-diethylaminocyclobut2-en-1-ones.lol Further examples of the photochemically induced ring contraction of 2-acylpyrazolidin-3-ones to N-acylaminoazetidin-2-oneshave been reported. The photoreaction continues to be of interest as an approach to the synthesis of penicillin-like systems. The fused bicycle (173) has been prepared in 50% yield

0

H H

A & n MeOH hv, O

H

I

HN,

a I!

O

H

by this route,lo2 and the synthesis of a number of spiro l-acylaminoazetidin2-ones has been described.lo3 Ring expansion of a spiro[indene-2,l’-fi-carbolin]l-one system has been used in a new total synthesis of yohimbine.lo4

+

,2] cycloadditions in nitrogen-containing heteroAddition.-The study of 2,[ cycles continues to arouse interest, particularly in systems which are related to constituents of nucleic acids. Irradiation (A > 280 nm) of uracil in aqueous acetone produces, together with the cyclobutane dimer, a uracil-acetone adduct identified as an oxetan.lo5 The process is reversed by irradiation (265 nm) of the oxetan in aqueous solution; acetone and uracil are obtained with a quantum efficiency of 0.16. The photodimerization of 1-cyclohexylthymine is completely quenched by the addition of cis-penta-l,3-diene, indicating that the dimerization occurs via an excited triplet sfate.lo6 9-Ethyladenine has been shown experimentally to quench the excited singlet state of l-cyclohexylthymine by formation of a hydrogen-bonded base pair exciplex. Ground and lowest excited singlet and triplet states have been calculated for thymine, uracil, and cytosine by the configuration-interaction method.lo7 The transition to the first excited state mostly affects the C-542-6 double bond and triplet excitation is almost exclusively localized there. T. Nishio, H. Aoyama, and Y . Omote, Heterocycles, 1975, 3, 703. P. Y. Johnson, C. E. Hatch, and N. R. Schmuff, J.C.S. Chem. Comm., 1975,725. lo3 P. Y . Johnson and C. E. Hatch, J. Org. Chem., 1975, 40, 3502. 104 T. Kametani, Y. Hirai, M. Kajiwara, T. Takahashi, and K. Fukumoto, Chem. and Pharm. Bull. (Japan), 1975, 23, 2634. Io5 A. J. Varghese, Photochem. and Photobiol., 1975, 21, 147. 106 T. Nakata, M. Yamato, M. Tasumi, and T. Miyazawa, Photochem. and Photobiol., 1975,22, 97. 1 0 7 V. I. Danilov, Dopouidi Akad. Nauk Ukrain. R.S.R., Ser. A , 1975, 1021 (Chem. Abs., 1976, 84, 058 388).

lol

loa

Photoreactions of Compounds containing Heteroatoms other than Oxygen

48 1

A head-to-head cyclobutane dimer of unspecified stereochemistry was obtained 2-Methylon irradiation of 17~-acetoxy-4a-aza-~-homoandrost-1-en-3-one.~~* s-triazolo[l,5-a]pyridine (174) undergoes a reversible photoaddition to give the (n4+ ,4] dimer (175);lo9 the 7- and 8-methyl derivatives undergo analogous reactions.

NH

v ____, I1

The introduction of methyl, ethyl, and propyl substituents into the 3- and 3’-positions of 1,l’-trimethylenebis-(5-alkyl)uracils decreased the quantum cis,syn-Cyclobu tane yields for intramolecular cyclobu t ane formation. derivatives were, however, obtained in all cases on irradiation in water. Intramolecular [,2 + ,2] addition is also observed in bis-(5H-dibenzo[a,d]cycIohepten-5-y1)amine (176), giving the novel heterocyclic cage ring system (177).l11 Numerous examples of intermolecular addition of nitrogen-containing heterocycles to alkenes have been reported. Quinol-2-one and its 4-methyl and 3,4-dimethyl derivatives add stereospecifically to cyclohexene to give the cisadducts on irradiation in N-Methyl-4-hydroxyquinol-2-one(178) undergoes an analogous addition to cyclohexene to give the adduct (179).113 New examples of the photoaddition of maleimide and maleic anhydride to fivemembered heterocycles have been described, both cyclobutane and [,4 + ,2] adducts being formed.l14 The 2,3-dihydropyridazine (180) differs in its photoreactivity from other diazacyclohexadienes, and on irradiation in the presence J. J. Bonet, I. Protabella, and F. Servera, Afinidad, 1975, 32, 172.

lo8

lo9

T. Nagano, M. Hirobe, M. Itoh, and T. Okamoto, Tetrahedron Letters, 1975, 3815. K. Golankiewicz and A. Zasada-Parzynska, Bull. Acad. polon. Sci., Skr. Sci. chim., 1974, 22, 945.

J. Rokach, Y.Girard, and J. G. Atkinson, J.C.S. Chem. Comm., 1975, 602. 112 0. Buchardt, J. J. Christensen, and N. Harrit, Acta Chem. Scand., 1976, B30, 189. 113 R. G. Hunt, C. J. Potter, S. T. Reid, and M. L. Roantree, TetrahedronLetters, 1975, 2327. n4 C. Rivas, C. Perez, and T. Nakano, Rev. Latinoamer. Quim., 1975, 6, 166. 111

482

Photochemistry

hv

Y o

(180) R

=

cyclohexene

I

Me

Me or Ph

of fumaronitrile is converted into the cyclobutane (181).l15 Novel azocin-2ones (182) are obtained along with cyclobutanes (183) and pyridone dimers from the reaction of pyrid-2-ones (184) with cyanoethylenes;lls the origin of the azocines remains uncertain although they would appear to be derived by ringopening of the alternative cyclobutane. It is perhaps noteworthy that no addition to the C-5-C-6 bond was observed.

hv

Rf (184) R1 =

R1 = R1 =

R4

CN

0 R2 = H R3 = R4 = H H, R2 = Me R3 = Me,R4 = H Me,R2 = I3 R3 = H,R4 = Me

The fluoro-substituent has been found to have a powerful effect in controlling regioselectivity in such additions. In contrast to uracil, thymine, and 6-methyluracil, 5-fluorouracil (185) shows complete regioselectivity in its acetonesensitized reaction with isobutene to give the adduct (186).l17 Similar results are obtained with other alkenes. The photocycloaddition of 5-fluorouracil to

(185) 115 116 11'

(186)

A. Padwa and L. Gehrlein, J. Heterocyclic Chem., 1975, 12, 589.

K. Somehawa, T. Shimou, K. Tanaka, and S . Kumarnoto, Chem. Letters, 1975, 45. A. Wexler and J. S. Swenton, J. Amer. Chem. Soc., 1976, 98, 1602.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

483

enol acetates followed by base-catalysed ring cleavage provides a useful procedure for functionalization at C-5 of unprotected uracils.llS Cycloaddition reactions of the C=N double bond are still relatively rare. Contrary to earlier reports, irradiation of N-(4-dimethylaminobenzylidene)aniline does not appear to give azobenzene and ci~-4,4’-bis(dimethylarnino)stilbene;l19 the isolation of these products was used as evidence for an intermediate diazetidine cycloadduct. Full details of the photocycloaddition of 3-ethoxyisoindolone (187) to a variety of alkenes have now been reported.120 With cyclohexene, for example, a 51% yield of adduct (188) was obtained. In certain instances, competing reactions were observed (Scheme 5). With isobutene, for example, the major product (189) was that derived by a photochemical 0

(188)

I

Iiv cyclohexene

@: \

OEt

hv Me,C=CH,

~

dT] d T > +

\

\

EtO

EtO

Scheme 5

‘ene-type’ reaction, whereas with tetramethylethylene a unique reaction occurred leading to the formation of the azepinone (190). A mechanism which accounts for the formation of all products in terms of a triplet exciplex has been proposed. ,2] photocycloaddition of alkenes Norrish Type I cleavage competes with [,2 to the C=N double bond in 2-0xazolin-4-0nes.~~~ The addition of isopropenyl

+

11* ll@

A. Wexler, R. J. Balchunis, and J. S. Swenton, J.C.S. Chem. Comm., 1975, 601. H. Ohta and K. Tokumaru, BUN. Chem. SOC.Japan, 1975,48,1669. K. A. Howard and T. H. Koch, J. Amer. Chem. SOC.,1975,97, 7288. R. M. Rodehorst and T. H. Koch, J. Amer. Chem. Sac., 1975,97,7298.

484

Photochemistry

R

H, Me, Et, Pryor Bu (192) acetate and cyclohexenyl acetate to 6-azauracil has also been described.122 The first example of an intramolecular photocycloaddition is claimed for the pyrimidine-purine dinucleotide analogues (191);123the major products of irradiation in aqueous solution are believed to be the thermally and photochemically unstable azetidines (192). The previously reported unusual [3 + 21 cycloaddition of s-triazolo[4,3-b]pyridazine to alkenes has been extended to include other cycloalkenes, as shown for example for cyclo-octene in Scheme 6,12* and details of the photoreaction of 2,5-diphenyl-l,3,4-oxadiazolewith indazoles have been published.12s (191)

=

CH-CN

Scheme 6

Addition of a different type is observed in high yield on irradiation of 1,3dimethyluracil (193) in THF to give the 5- and 6-(tetrahydrofuran-2-y1)5,6-dihydrouracils (194)and (195).f2sIdentical products are obtained when the

$: 0

0

c A N' r hv, >260nm, THF I Me

q

x

;

M

Me

e

+

Me

(196) 3. S. Swenton and R. J. Balchunis, J. Heterocyclic Chem., 1974,11, 917. la3 S. Paszyc, B. Skalski, and G. Wenska, Tetrahedron Letters, 1976, 449. lZ4 J. S. Bradshaw, J. T. Carlock, and G . E. Maas, J. Heterocyclic Chem., 1975, 12,931. lZ6 K. Oe, M. Tashiro, and 0. Tsuge, Chem. Letters, 1976, 153. 128 M.D.Shetlar, J.C.S. Chem. Comm., 1975, 653. 122

Photoreactions of Compounds containing Heteroatoms other than Oxygen

485

reaction is induced by photolytic decomposition of di-t-butyl peroxide (A > 290nm), and a radical mechanism involving hydrogen abstraction by excited uracil from THF is therefore proposed. The same process has been reported for certain N-substituted phthalimides in THF lZ7and a similar process for N-methylphthalimide in toluene, substituted toluenes, and dimethylamines.128 The photochemically induced addition of alcohols and cyclic ethers to polyazanaphthalene derivatives such as (196) has been described.lZg The addition is accelerated in the presence of acetone or acetophenone as sensitizer, but no dehydration of the product was observed as is often the case with pyridine and quinoline adducts. The photoalkylation of niketamide and its 2- and 4-isomers presumably proceeds by an analogous pathway.130 Evidence indicating that different mechanisms are involved in the photochemical substitution of sixmembered monoaza-aromatic compounds by methanol in neutral and in HCI acidified medium have been p~b1ished.l~~ Aryl imines undergo reduction and reductive dimerization on irradiation in propan-2-01. This reaction is now known not to involve excited imine, but to proceed via an a-amino-radical generated by hydrogen atom transfer to the imine from a ketyl radical; the ketyl radical is itself derived from carbonyl compounds present in starting material. Recent work with N-acyldiphenylketimines has led to the discovery that an excited state capable of hydrogen abstraction reactions is involved in the photoreduction laZand photoaddition reactions 133 of these species. Irradiation of N-acetyldiphenylmethyleneamine (197), for example, in toluene gave the adduct (198) and the dihydro-derivative Ph DL

+NY (197)

I1v

toluene

Ph

1

+

PhCH,-C-NHK I

Ph

-. Yh

0

0 (199)

(199). The addition was promoted by the use of halogenated solvents la4and by the use of p-chloro- or p - b r ~ m o - t o l ~ e nfurther e , ~ ~ ~indicating that a triplet state of the ketimine is implicated. The acetone-initiated addition of formamide to 2,3,4,6-tetra-O-acetyl-l-deoxy-~-arab~~zo-hex-1-enopyranose has been reported.lS6 The presence of higher concentrations of bivalent anion is found to accelerate as well as to change entirely the photoreactions of riboflavin. Intramolecular addition of the 2’-hydroxy-group at the C-9 peri-position is preferred to dealkylation to yield 1umi~hrome.l~~ The final product of this addition arises by H. J. Roth and D. Schwarz, Arch. Pharm., 1976,309, 52. Y. Kanaoka, K. Sakai, R. Murata, and Y. Hatanaka, Heterocycles, 1975, 3, 719. lZB A. Miyake, Y. Oka, and S. Yurugi, Chem. and Pharm. Bull. (Japan), 1975, 23, 1500. 1 3 0 I. Ninomiya, 0. Yamamoto, and T. Kiguchi, Heterocycles, 1974, 2, 329. lal A. Castellano, J. P. Catteau, and A. Lablache-Combier, Tetrahedron, 1975, 31, 2255. laa A. Padwa and W. P. Koehn, J. Org. Chem., 1975, 40, 1896. 133 S. Asao, N. Toshima, and H. Hirai, Bull. Chem. SOC. Japan, 1975, 48, 2068. 134 N. Toshima, S. Asao, and H. Hirai, Chem. Letters, 1975, 451. lS5 S. Asao, N. Toshima, and H. Hirai, Bull. Chem. Soc. Japan, 1976, 49, 224. 136 A. Rozenthal and M. Ratcliffe, Canad. J. Chem., 1976, 54, 91. M. S. Jorns, G. Schollnhammer, and P. Hemmerich, European J . Biochem., 1975,57, 35. lZ7 lZ8

486

Photochemistry

autoxidation of a dihydroflavin intermediate and has the structure (200). The first step in the photochemical hydroxylation of lumichrome (7,s-dimethylalloxazine) may be the addition of water to the protonated excited singlet The significance of neighbouring-group participation of amides in the photochemical hydration of acetylenic bonds has been revealed. o- Acetamidophenylacetylene (201), on irradiation in hexane or acetonitrile, is converted into the CHzOH

I

(CHOW, I

C H OMe-in

Htfl

ti

0

0' II

'CH2C,H40Me -m

\

NHCOMe (203)

isomeric benzoxazines (202) which readily add water to give the same o-acetamidophenyl ketone (203).139An analogous and unusual addition of water to the C=N function of o-acetamidobenzonitrile to give o-acetamidobenzamide has also been reported, but no cyclic intermediate could be isolated in this case. Triplet states are proposed as precursors in these photocyclizations. Cyclization of NN-bistrifluoromethylaminoacetylene occurs on irradiation to yield 1,3,5-tris(bistrifluoromet hylamino)benzene.lPo The aminium radical-initiated addition of N-nitrosoarnines to alkenes has been the subject of further study. The photoaddition of N-nitrosodimethylamine to norbornene gave cis,exo-(204) and trans-2-nitroso-3-ammonium norbornanes (205) in good yield.lP1 The trans-isomer gave the oxime (206) in the usual R. R. Duren, R. H. Dekker, and J. Verbeek, Rec. Trav. chim., 1975,94, 106. T. D. Roberts, L. Munchausen, and H. Schechter, J. Amer. Chem. Soc., 1975, 97, 3112. lrlo J. Freear and A. E. Tipping, J.C.S. Perkin I, 1975, 1074. 141 K. S. Pillay, S. C. Chen, T. Mojelsky, and Y. L. Chow, Canad. J. Chem., 1975,53,3014. 138 139

&

Photoreactions of Compounds containing Heteroatoms other than Oxygen Me,N-NO /,",H+

487

@>

, &H2Me2+

N=O

NO

+

CH=NMe,

0

CH=NOH

fashion, whereas the cis-isomer underwent rapid ring cleavage to give products derived from the 1,3-bisformylcyclopentane derivative (207). The effects of temperature on the reactivity of the adducts of N-nitrosopiperidine with camphene and pinene have been investigated,142and in the presence of oxygen, addition of N-nitrosoamines to norbornene takes place to give cis,exo- and trans-2-nitrato-3-aminon~rbornanes.~~~ Oxygen is believed to intercept photochemically generated NO to form a nitrogen trioxide radical which in turn is scavenged by a carbon-radical intermediate. Cleavage of the benzene ring of aromatic methoxy-compounds on irradiation in the presence of aromatic nitro-compounds is thought to involve addition of the nitro-group as illustrated for 1,4-dimethoxynaphthalene in Scheme 7.144 Cleavage occurs selectively at the 1,%bond with respect to the methoxy-group, and aromatic nitro-compounds having lowest n , ~ *triplet are more effective f

Ar

OMe

..

___, OMe

OMe

+ ArN: Me0

CHO

i

ArNHz + ArN=NAr Scheme 7 H. H. Quon and Y. L. Chow, Tetrahedron, 1975, 31,2349. K. S. Pillay, K. Hanaya, and Y. L. Chow, Canad. J . Chem., 1975,53, 3022. lP4 I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC.Japan, 1975, 48, 2865. lP2

488

Photochemistry

than those having lowest n,n* triplet. Precedence for this addition is to be found in the reaction of nitrobenzene with cyclohexene. The reported addition of water and benzene to 17/3-acetoxy-4-aza-androst5-en-3-one on irradiation in benzene is not easily e~p1ained.l~~ Miscellaneous Reactions.-The well known synthesis of aldosterone 21-acetate by photolysis of the 1lp-nitrite of corticosterone acetate suffers from the disadvantage that radical attack at c-19 competes with the desired attack at c-18. This problem has now been overcome and an improved route to aldosterone devised by the introduction of extended conjugation into the nitrite, resulting in an increase in the separation between C-19 and the llp-oxygen atom;146thus, on irradiation, the 11-nitrite of 1lfl-hydroxypregna-l,4-dien-3-one(208) affords 0H.

(208)

(209)

the C-18 oxime (209) in 55% yield. The best solvents for this reaction were found to be THF and acetonitrile. Nitrite photolysis has also been employed in the synthesis of ( k )-tetrahydroanhydroaucubigenone from 2-(2-nitrosoxyethyl)7-oxabicyclo[3,3,0]octan-3-one.147 The syntheses of 11-deoxy-18-hydroxycorticosterone and 18-hydroxycorticosterone21-acetates have been accomplished by irradiation of the appropriate steroidal 20-nitrites in the presence of oxygen, leading to functionalization at C-18 and the formation of C-18 nitrates.14* Interception of 8-alkyl radicals, generated by nitrite photolysis, has been achieved with cupric acetate;140 both unsaturated alcohols and cyclic ethers are obtained in this way, as shown, for example, for 2-hexyl nitrite in Scheme 8.

i

Cu(OAc),

Scheme 8 145

146

147

lQ8 149

J. Boix, J. Gbmez, and J.-J. Bonet, Helv. Chim. Acta, 1975, 58, 2545. D. H. R. Barton, N. K. Basu, M. J. Day, R. H. Hesse, M. M. Pechet, and A. N. Starratt, J.C.S. Perkin I., 1975, 2243. H. Obara, H. Kimura, J. Onodera, and M. Suzuki, Chem. Letters, 1975, 221. D. H. R. Barton, M. J. Day, R. H. Hesse, and M. M. Pechet, J.C.S. Perkin I, 1975,2252. Z. CekoviC and T. SrniC, Tetrahedron Letters, 1976, 561.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

489

A process and a reactor for the photo-oximation of cycloalkanes using a gaseous nitrosating agent such as nitrosyl chloride have been described in the patent 1 i t e r a t ~ r e . lThe ~ ~ photochemical nitrosation of chlorocyclododecane with nitrosyl chloride affords syn- and anti-isomers of a-chlorocyclododecanone oxime together with dichlorocyclododecane, chloronitrocyclododecane, chlorocyclododecyl nitrate, and chlorocyclododecanone.lsl Abstracts reporting the analogous nitrosations of cyclododecane 152 and cyclo-octane 153 have been published, but details are not available. New reports provide additional evidence that carbon-nitrogen bond cleavage is the major pathway in the photochemistry of nitrosoalkanes. Substituted hydroxylamines are the major products of irradiation of 2-nitrosoisobutyronitrile and 1-nitrosocyclohexanecarbonitrile with red light (570-700 nm) and appear An identical homolysis to be derived exclusively by C-N bond hom01ysis.~~~ is involved in the preparation of gem-chlorobromo-compounds by photolysis of the corresponding gem-chloronitroso-derivatives in the presence of excess bromine,166and contrary to earlier reports the photo-oxidation of nitrosoalkanes is now claimed to involve the same primary homolytic process.166 The NO group also appears to be removed photochemically from N-methyl-N'-nitro-N-nitrosoguanidine without oxygen parti~ipati0n.l~~ Nitrogen-nitrogen bond homolysis has been proposed to account for the formation of piperidine and N-formylpiperidine from N-nitropiperidine on irradiation in methanol.ls8 The details of this process are by no means clear, and indeed other workers have failed to observe homolysis of this type. In the presence of HCl and cyclohexene, however, addition of N-nitropiperidine occurs via the piperidinylium radical to give 2-nitro- and 2-methoxy-1-piperidinocyclohexanes. A complex series of reactions has been described for the irradiation of nitroethane in c y c l o h e ~ a n e and , ~ ~ ~attempts to rationalize the sequence have been complicated by further reaction of the photoproducts. trans-Azocyclohexane di-N-oxide (210) is the major product of irradiation (254 nm) of nitroethane in cyclohexane; it is further converted into hydroxyiminocyclohexane (21 1) and N-cyclohexylcaprolactam (212) by irradiation ( A > 290nm and X > 250nm respectively). The formation of nitrosocyclohexane appears to preclude the possibility of a direct photochemically induced deoxygenation of the nitroalkane and requires that, even in solution, all major products of the irradiation of nitroalkanes arise by an initial C-N homolytic bond cleavage. The primary processes 160 lS1

lS2

lK4

lK6 lSB 16'

lKB lKB

G. Lucas, U.S.P. 2 853 729 (Chem. Abs., 1975, 83, 18 926). Y. A. Gromoglasov, A. V. Iogansen, L. A. Levashova, V. V. Karchikhina, M. N. Enikeeva, G. A. Kurkchi, 0. V. Levina, V. A. Valovoi, V. P. Baeva, and A. A. Samoilenko, Neftekhimiya, 1974, 14, 770. M. P. Lazareva, V. V. Karchikhina, I. A. Levashova, 0. V. Levina, L. G. Zelenskaya, Y. A. Gromoglasov, and M. N. Enikeeva, Chem. Abs., 1976, 84,4125. K. E. Kuznetsova, L. E. Levashova, A. A. Streltsova, A. D. Proshenkova, and Y . A. Gromoglasov, Chem. Abs., 1975, 83, 95 979. B. G. Gowenlock and J. Pfab, Annalen, 1975, 1903. J. Pfab, Tetrahedron Letters, 1976, 943. J. Pfab, J.C.S. Chem. Comm., 1976, 297. Y . Ioki, A. Imamura, C. Nagata, and M. Nakadate, Photochem. andPhotobiol., 1975,21,387. R. W. Lockhart, R. W. Snyder, and Y . L. Chow, J.C.S. Chem. Comm., 1976, 52. S. T.Reid and E. J. Wilcox, J.C.S. Chem. Comm., 1975, 647.

490

Photochemistry

IIV, 254 nm

EtNoz

cyclohexane

’

hv, X > 290 nm

observed in the gas-phase photolysis of 1-nitropropane are analogous to those previously described for nitroethane and 2 - n i t r 0 p r o p a n e ~whereas ~ ~ ~ the photochemical decomposition of tetranitromethane in a variety of solvents is thought to take place predominantly by an ionic mechanisrn.ls1 Attempts to prepare imidazoles by photocyclization of 3-amino-Znitrobenzo[blthiophen derivatives (213) were unsuccessful; a product mixture was obtained from which the nitro-compound (214) and the oxime (215) were separated.lsa

Unsaturated oximes are themselves photochemically reactive. 3-0xo-17/3acetoxyandrosta-ly5-dieneoxime (216), on irradiation in methanol, is converted into the lactam (217) together with the parent ketone and four photoproducts derived The triplet states of 0-acyl aromatic ketoximes have excitation energies close to those of the parent ketones and have considerable A. R. Khan, Chem. Letters, 1975, 879. V. I. Slovetskii and V. P. Balykin, Izvest. Akad. Nauk. S.S.S.R.,Ser. khim., 1975,2186. le2 P. N. Preston and S . K. Sood, J.C.S. Perkin I, 1976, 80. 162 P. N. Preston and S. K. Sood, J.C.S. Perkin I, 1976, 80. le3 J. Repoil6s, F. Servera, and J.-J. Bonet, Helv. Chim. Acta, 1974, 57, 2454. l80

lE1

Photoreactions of Compounds containing Heteroatoms other than Oxygen

491

T,T*character.le4

They readily undergo homolytic cleavage of the N-0 bond to iminyl radicals and acyloxyl radicals. The diphenylmethaniminyl radical (218), generated in this way from benzoate (219), undergoes aromatic substitution in benzene as well as dimerization to the azine (220).lS6 The mechanisms of such 0

II ,0-C-Ph

Ph

0 Irv

Ph

)=No

Ph

Ph)=N

+

II

*O-C--Ph

Ph Ph

Ph

Ph

substitutions in benzene and in toluene have been studied.lsa Radical phthalimidation of aromatic compounds can also be effected photochemically by decomposition of N-tosyloxyphthalimide in the presence of electron-rich aromatic substrates.lS7 The recently reported photocyclization of 3-dialkylaminoacrylophenoneshas now been extended to 3,3-bis(dialkylamino)acrylophenones (221) which, on

(221) R

=

H, Et, or Prn

irradiation in benzene, are converted into the pyrroles (222).lSS The mechanism of these transformations is still not clear. The benzophenone-sensitized photoreactions of 2-quinolinecarbonitrile derivatives in ethanol are dependent on the nature of the 4 - s ~ b s t i t u e n t . ~Nucleophilic ~~ photosubstitutions of halogen in aminochloropyridines and in aminohalogenopyrimides have been described.170 Articles reviewing the photochemistry of pyrazolone derivatives used as the photosensitized reactions of amino-acids and and the M. Yoshida, H. Sakuragi, T. Nishimura, S. Ishikawa, and K. Tokumaru, Chem. Letters, 1975, 1125. las H. Ohta and K. Tokumaru, Bull. Chem. SOC.Japan, 1975, 48, 2393. lSa S. Ishikawa, H. Sakuragi, M. Yoshida, N . Inamoto, and K. Tokumaru, Chem. Letters, 1975, 8 19. la7 J. I. G. Cadogan and A. G. Rowley, J.C.S. Perkin I, 1975, 1069. la8 H. Aoyama, T. Hasegawa, T. Nishio, and Y. Omote, Bull. Chem. SOC.Japan, 1975,48, 1671. laS N. Hata and R. Ohtsuka, Chem. Letters, 1975, 1107. 170 A. N. Frolow, A. V. El'tsov, and 0. V. Kul'bitskaya, Khim. geterotsikl. Soedinenii, 1974,12, 1645. 171 J. Reisch, Gyogyszereszet, 1975, 19, 81. G. Jori, Photochem. and Photobiol., 1975, 21, 463. lS4

492 Photochemistry photochemistry of the hydrazo-, azo-, and azoxy-groups 173 have appeared during the course of the year.

2 Sulphur-containing Compounds Certain aspects of the photochemistry of organic sulphur compounds have been reviewed.174 Interest in the study of photorearrangement reactions in sulphurcontaining compounds has been maintained. The stable Z-form of a-(thiopyran2-ylidene) ketone (223) is transformed into the E-isomer (224) on irradiation;175

the photoproduct reverts to starting material by a dark process which obeys first-order kinetics. Z-Methyl thiobenzohydroximates are photochemically converted into their thermally stable E - i ~ o m e r s and , ~ ~ ~an equilibrium mixture of stereoisomers (225) and (226) is obtained on irradiation of the sulphone (225).17' Irradiation of 3-phenyl-2H-thiopyran 1, l-dioxide (227) in methanol yields a mixture of adducts (228) and (229).17* Isolation of the latter provides convincing evidence that the cyclic sulphone does not require the incorporation of an

atom bearing a free electron pair for ring-opening to occur, and therefore argues in favour of a mechanism involving cycloreversion to the sulphene (230). Full details of the 'Dewar' thiophen structure (231) of the photoproduct of 2,3,4,537s

174 176

17' 178

R. J. Drewer, in 'Chemistry of the Hydrazo, Azo, and Azoxy Groups', Vol. 2, ed. S. Patai, Wiley, 1975, p. 935. J. D. Coyle, Chem. SOC.Rev., 1975, 4, 523. C. T. Pedersen, C. Lohse, N. Lozach, and J.-P. Sauv6, J.C.S. Perkin I, 1976, 166. W. Walter, C. 0. Meese, and B. Schroder, Annalen, 1975, 1455. H. A. Selling, Tetrahedron, 1975, 31, 2387. J. F. King, E. G. Lewars, D. R. K. Harding, and R. M. Enanoza, Canad. J. Chern., 1975,53, 3657.

Photoreactions of Compounds containing Heteroatorns other than Oxygen

493

tetrakis(trifluoromethy1)thiophen (232) have been p~b1ished.l~~ The role, if any, of this species in the photorearrangement of substituted thiophens remains uncertain. The incorporation of deuterium, observed on irradiation of 5-phenylisothiazole in D,O-diethyl ether solution, supported the formation of a tricyclic sulphonium cation intermediate.lsO Various intermediates have been proposed to account for the conversion of 1,2,3-thiadiazole 2-oxides (233) into the isomeric

F3kfF3 hv

F3C

CF3

~

’ A

0-

0-

(235)

3-oxides (234);ls1the isolation of a low yield of the 1,2,5-thiadiazole (235) from the diphenyl derivative (233; R1 = R2 = Ph) is somewhat surprisingly taken as evidence for an intramolecular cycloaddition pathway. As a continuation of the study of the photochemistry of mesoionic systems, the photoreactions of the 1,3,4-thiadiazoles (236) have been examined.la2 Spectral evidence for initial rearrangement to the acyclic species (237) has been presented,

(236) R1 Ph Me Ph Ph

R2 Me Ph Ph p-MeOC,H,

Y . Kobayashi, I. Kumadaki, A. Ohsawa, Y. Sekine, and H. Mochizuki, Chem. and Pharm. Bull. (Japan), 1975, 23, 2773. lBo M. Maeda, A. Kawahara, M. Kai, and M. Kojima, Heterocycles, 1975, 3, 389. lS1 H. P. Braun, K.-P. Zeller, and H. Meier,AnnaZen, 1975, 1257. R. Mukherjee and R. M. Moriarty, Tetrahedron, 1976, 32, 661.

17

494

Photochemistry

and the products are presumed to be derived by a further photochemical N-N bond homolysis. A re-examination of this decomposition provided further evidence for a ring-opening process and also led to the detection of COS as a A revised and simplified mechanism has been proposed for these transformations. Competition between reversible ring-opening and elimination of C02 in the mesoionic 4-phenyl-l,3,2-oxathiazolylio-5-oxide(Scheme 9) has 0

Scheme 9

been shown to be dependent on the molecular environment of the reactant.ls4 1,2-Dithiolyl radicals and dithioketonate anions are the initial products of irradiation of a series of 3,5-disubstituted 1,2-dithiolylium salts in ethanol,lss and irradiation of diary1 sulphides in cyclohexane in the presence of iodine affords dibenzothiophens.166 The synthesis of medium- to large-ring azathiocyclols has been achieved by an unusually regioselective remote photocyclization of sulphide-containing phthalimides.ls7 For example, the phthalimides (238) are converted on irradiation in acetone into a mixture of nine-membered (239) and seven-membered (240) ring compounds. This reaction can surprisingly be extended to even larger rings, and a special mechanism must therefore be implicated in which the sulphur atom facilitates the formation of the macrocyclic transition state; a tentative explanation involving enhanced proton transfer from the methylmercapto-group in a chargetransfer complex has been advanced. The corresponding O-methyl derivatives fail to undergo the same cyclization. One direct application of this cyclization has been the construction of a cyclic peptide Polycyclic aromatic thiones (241) having a freeperi-position cyclize, on excitation in the n -+ T band, to give thiophen derivatives (242).189 At least in one case, the formal 1,3-hydrogen lE3 18(

lS6

A. Holm, N. H. Toubro, and N. Harrit, Tetrahedron Letters, 1976, 1909. I. R. Dunkin, M. Poliakoff, J. J. Turner, N. Harrit, and A. Holm, Tetrahedron Letters, 1976, 873. C. T. Pedersen and C. Lohse, Acta Chem. Scand., 1975, 29B, 831. K. P. Zeller and H. Peterson, Synthesis, 1975, 532.

lR6

Y.Sato, H. Nakai, T. Mizoguchi, Y . Hatanaka, and Y. Kanaoka, J. Amer. Chem. SOC.,1976, 98, 2349.

lE8 189

Y . Sato, H. Nakai, T. Mizoguchi, and Y . Kanaoka, Tetrahedron Letters, 1976, 1889. A. Cox,D. R. Kemp, R. Lapouyade, P. de Mayo, J. Joussot-Dubien, and R. Bonneau, Canad. J. Chem., 1975, 53,2386.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

495

0

0 (238) R1 = Me, Et, .But, or PhCH,

P R 2

(239) R2 = H, Me, or Ph Ph

a H

YS

\

/

p h i r 2.L (243) Ar = Ph or /3-naphthyl

migration has been shown to be intermolecular. In contrast, arylalkyl thiones (243) having an activated p-position undergo cyclization to cyclopropanethiols (244) on n + ~ T T * excitation.lao The reason for the difference in behaviour between thiones and the corresponding ketones which do not undergo cyclization is not clear; a simple rationalization is that the greater size of the n 3p orbital makes hydrogen abstraction easier. p-Hydrogen abstraction is also observed in substituted thiochromanone sulphoxides, but the major reaction pathway involves rearrangement to the cyclic sulphenate.lQ1 This rearrangement is formally analogous to the formation of oxacarbenes from ketones, but the photochemistry is made more complex by further cleavage of the weak 0 - S bond. The disulphide (245) is the only product of irradiation of the sulphoxide (246) and is believed to be formed via the sulphenate (247) as shown in Scheme 10. Homolytic S-S bond cleavage followed by radical recombination rather than a concerted 1,3-sigmatropic rearrangement is proposed to account for the novel A. Couture, M. Hoshino, and P. de Mayo, J.C.S. Chem. Comm., 1976, 131. 1. W.J. Still, P. C. Arora, M. S. Chauhan, M. H. Kwan, and M. T. Thomas, Canad.J. Chem., 1976,54,455.

lBo 191

Photochemistry

496

(245) Scheme 10

as& hNHc NHCOMe

hv, MeCN Pyrex

R

’

R

2

(248) R = H or OMe

R yH2Ph

COCH,Ph

/

HN +S

M

O

Me 0,C

v

11

EtOH

’ N

X

C0,Me

E;IH PhCH,CON H - p - - > N / 0

CO, Me

(253)

-

I1v 0 MeOH

NH

Photoreactions of Compounds containing Heteroatoms other than Oxygen

497

rearrangement of bis-(o-acetylaminophenyl) disulphides (248) to 2-methylbenzothiazoles (249).ID2 The rearrangement of the isothiazolone (250) to the thiazole (251) is assumed to involve initial homolytic S-N bond cleavage, but details of the formation of this and other products are not completely clear.le3 A thiazole (252) is also obtained as the major product of irradiation of the cephalosporin (253) in methan01.l~~ The photoreactions of simple aromatic and aliphatic thiones have been reviewed.lB6 The most distinctive characteristic is the frequent, if not general, ability of the excited thione function to give products derived from a higher singlet state. Adamantanethione, however, on irradiation (500 nm) gives the n,n* triplet efficiently; this species undergoes addition to alkenes to give thietans in a regiospecific manner and to adamantanethione to give a dimer, the 1,3-dithietan.lDs The regiospecificity in thietan formation is that expected from the most stable biradical; prior formation of a triplet exciplex may be involved. Photoaddition of aryl thiones (254) to bis(methy1thio)ethyne (255) has been

Rf R3

1q-S

I

*C=C -SMe

MeS'

MeS

SMe

reported to give the @-unsaturated dithioesters (256);lS7the unstable dithiet (257) is presumably an intermediate in this addition. The photochemically induced addition of a primary amine, l-aminobutane, to lY3-dimethyl-4-thiouracil (258) leads to the formation of two diastereoisomeric adducts (259).lD8 In the presence of a tertiary amine, however, 1,3-dirnethyl-4-thiouracilis converted into a mixture of tetrahydrodipyrimidine~.~~~ Photochemically generated thiyl radical additions have also been widely reported. 3-Phenylthiacyclohexane(260) and 2-methyl-4-phenylthiacyclopentane (261) have been synthesized by competing intramolecular thiyl addition in the unsaturated thiol (262).200The dithiole (263) and a 1,4-dithian were obtained by lea lea

lS6 lge 19'

lg0

aoo

Y. Maki and M. Sako, Tetrahedron Letters, 1976, 851. Y. Maki and M. Sako, Tetrahedron Letters, 1976, 375. Y. Maki and M. Sako, J, Amer. Chem. SOC.,1975, 97, 7168. P. de Mayo, Accounts Chem. Res., 1976, 9, 52. A. H. Lawrence, C. C. Liao, P. de Mayo, and V. Ramamurthy, J. Amer. Chem. SOC.,1976, 98, 2219. A. C. Rrouwer and H. J. T. Bos, Tetrahedron Letters, 1976, 209. J.-L. Fourrey, Tetrahedron Letters, 1976, 297. J.-L. Fourrey and J. Moron, Tetrahedron Letters, 1976, 301. V. P. Krivonogov, V. 1. Dronov, and N. K. Pokoneschikova, Khim. geterotsikl. Soedinenii, 1975, 9, 1204.

498

Photochemistry

S

S

irradiation of the thiol (264) and its S-substituted derivatives;201the formation of the dithiole via the isomeric disulphide (265) may involve a 1,3-sigmatropic process.

0

0

Irradiation of cyclic disulphides in the presence of aldehydes results in S-S bond cleavage and the formation of mono S-acylated dithiols.202The addition of photochemically generated thiyl radicals to diphenylvinylphosphine has been described; the formation of different products can be accounted for in terms of competing thiyl radical attack on the alkene or at Sulphur dioxide has again found use in the trapping of photochemically generated 1,4-biradi~als,~~* and O-ethyl-l-thionaphthoateis recommended for use as a photosensitizer for excitation by visible light of up to 500 nm wavelength because of its high triplet yield and low photochemical reactivity.206 aol SOa

$0‘ SO6

L. Dalgaard and S. 0. Lawesson, Acra Chem. Scand., 1974,28B, 1077. M.Takagi, S. Goto, and T. Matsuda, J.C.S. Chem. Comm., 1976,92. D. H. Brown, R. J. Cross, and D. Millington, J.C.S. Dalton, 1976,334. N.K. Hamer, J.C.S. Chem. Comm., 1975, 551. M. Gisin and J. Wirz, Helv. Chim. Acta, 1975,58,1768.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

499

3 Compounds containing other Heteroatoms The photochemistry of silicon-containing compounds continues to attract increased attention and a wide variety of reactions have been reported. Renewal of interest in silacyclobutenes prompted a study of the photochemistry of l,l-dimethyl-2-phenyI-l-silacyclobut-2-ene (266).206Irradiation in acetone gave the adduct (267) in 83% yield. The reaction is viewed as arising via cycloreversion to the siladiene (268) followed by 1,4-addition to acetone. The diene

Ei-Me Ph A e

hv.

ph

d

(266)

bk,CO

Si-Me I Me

n : e Ph ,sic Me Me

(268)

(267)

(27 1)

(268) would be expected to have some 1,2- and 1,4-zwitterionic character by analogy with the known chemistry of the C=Si double bond. Irradiation of the novel 9-silabicyclo[4,2,1]nona-2,4,7-triene(269), with or without sensitizers, led to the formation of the silabarbaralane (270) and the product (271) of intra,2] c y c l i ~ a t i o n . ~ ~ ~ molecular [,2 The first unsymmetrical photodimerization of the anthracene ring system has been claimed.208 Irradiation of bis-(9-anthryl)-l,l,3,3-tetramethyldisiloxane (272) in diethyl ether affords the [,4 ,4] intramolecular adduct (273). Steric

+

+

Me - Si-Me

I

0

d Av

I

Me-Si-Me

206

ao7 208

P. B. Valkovich and W. P. Weber, Tetrahedron Letters, 1975, 2153. T. J. Barton and M. Juvet, Tetrahedron Letters, 1975, 2561. A. G. Schultz, J. Org. Chem., 1975, 40, 3467.

500 Photochemistry effects are said to be responsible for the unsymmetrical dimerization of the anthracene nucleus, but no previous examples of this type of behaviour have been published. Sensitized photocycloaddition is observed between the silyl enol ethers of a-tetralone and a-indanone and electron-deficient a l k e n e ~ . ~ * ~ Two types of reaction are known for acylsilanes, namely reversible siloxycarbene formation and a less common radical cleavage. Irradiation of 1,l-diphenyl-l-silacyclohexan-2-one(274) in cyclohexane affords dimers (275) and Ph P h

(274)

Ph Ph

P h Ph

\/

\/

+ Ph’

I

Ph

Ph’

Ph

(275)

(276), arising presumably via the siloxycarbene (277) which is trapped in methanol as the cyclic acetal (278).210 1,l-Diphenyl-l-silacyclopentanewas also obtained. The photochemical reactions of benzoyltrimethylsilane with substituted phenylsilanes can readily be accounted for in terms of the insertion of siloxycarbene into the Si-H bond of the phenylsilane.211 A novel addition is observed between phenylpentamethyldisilane (279) and certain alkenes (280) on irradiation in benzene.212 The major products are the o-disubstituted benzene derivatives (281), which are believed to arise by addition

OSi

Me, Si Me

(279)

209

*lo

212

(281)

R1 = H, R2 = SiMe,

R1 = RZ = Me R1 = Me, R2 = CMe=CH,

G. Felix, R. Lapouyade, H. Bouas-Laurent, and B. Clin, TetrahedronLetters, 1976, 2277. K. Mizuno, H. Okamato, C. Pac, H. Sakurai, S. Murai, and N. Sonoda, Chem. Letters, 1975, 237.

zll

hv +

A. G. Brook, J. B. Pierce, and J. M. Duff, Canad. J. Chem., 1975,53,2874. H. Watanabe, N . Ohsawa, M. Sawai, Y. Fukasawa, H. Matsumoto, and Y. Nagai, J. Organometallic Chem., 1975, 93, 173.

Photoreactions of Compounds containing Heteroatoms other than Oxygen

501

of alkene to the unstable intermediate (282). Ph2Si=CH2 or its biradical equivalent has previously been described in the photolysis of pentaphenylmethyldisilane. When vinylsilanes such as dimethylvinyl-, trimethylvinyl-, and ethyldimethylvinyl-silane are used as the alkene in this reaction, silepin derivatives are always formed in low yield in addition to the normal adducts.21s The mercury-sensitized photodecompositions of trichlorosilane 214 and of halogenomethyldimethylsilanes 21s have been described. The aryl selenide (283) undergoes photocyclization in benzene in the presence of toluenesulphonic acid to give the benzoselenophen (284) in 60% yield.21s The selenocarbonyl ylide (285) is presumably involved. OH

Irradiation ( A > 280 nm) of benzyl diselenide (286) in acetonitrile results in extrusion of selenium and the formation of dibenzyl selenide (287).217Homolytic Se-Se bond cleavage is not involved, and a radical Se-C bond cleavage is proposed to account for product formation and kinetic observations. The preparation of carbazoles by the photochemical extrusion of dimethyl phenylphosphonate from oxazaphosphoranes has been reported.21s The isolation of isomerically pure 3-methoxy-7-methylcarbazole(288 ; R1 = MeO, PhCH,SeSeCH,Ph (286)

818 214

als 216

217

hv

PhCH,SeCH,Ph

+

Se

(287)

M. Ishikawa, T. Fuchikami, T. Sugaya, and M. Kumada, J. Amer. Chem. SOC.,1975, 97, 5923. M. Ishikawa, T. Fuchikami, and M. Kumada, Tetrahedron Letters, 1976, 1299. K. G. Sharp, P. A. Sutor, T. C. Farrar, and K. Ishibitsu, J. Amer. Chem. Soc., 1975,97,5612. I. N. Jung and W. P. Weber, J. Org. Chem., 1976, 41, 946. J. Y. C. Chu, D. G. Marsh, and W. H. H. Gunther, J. Amer. Chem. SOC.,1975,97,4905. J. I. G. Cadogan, B. S. Tait, and N. J. Tweddle, J.C.S. Chem. Comm., 1975, 847.

Photochemistry R2 = Me) from the phospholine (289; R1 = MeO, R2 = Me) can be satisfactorily accounted for by participation of a species such as (290). A preliminary account of the successful preparation of the phosphindole system by photocyclization of an o,o’-bis(phenylethyny1)triphenylphosphine has been published.21e Details of the photoaddition of tetrafluorodiphosphine to fluorinated ethylenes have been described,220and a new preparation of monoalkyl phosphates by irradiation of simple dianisyl alkyl phosphates has been reported.221 Mercurysensitized irradiation ( A = 254 nm) of boranes or carbaboranes has been found to be a convenient method for synthesizing the corresponding boron-boroncoupled boranes or carbaboranes.222 502

220 a21 24a

N. Winter, Tetrahedron Letters, 1975, 3913. W. K. Glanville, K. W. Morse, and J. G. Morse, J. Fluorine Chem., 1976, 7 , 153. R. A. Finnigan and J. A. Matson, J.C.S. Chem. Comm., 1975, 928. J. S. Plotkin and L. G. Sneddon, J.C.S. Chem. Comm., 1976, 95.

7 P hotoel imination BY S. T. REID

This chapter is principally concerned with the photochemically induced fragmentation of organic molecules accompanied by the formation of small molecules such as nitrogen, carbon dioxide, and sulphur dioxide. Photodecompositions resulting in the formation of two or more sizeable fragments are reviewed in the final section. Fragmentations arising by Norrish Type I and Type I1 reactions of carbonyl-containing compounds are considered in Part 111, Chapter 1. 1 Photodecomposition of Azo-compounds The photolysis of azoalkanes provides a convenient route for the generation and subsequent study of alkyl radicals; the most important competing process is trans-cis-isomerization. A low-field CIDNP study of ethane formed by photolysis of azomethane in carbon tetrachloride solution indicates that the photodecomposition occurs predominantly from the singlet state.l Examination of the Stern-Volmer plot of the nitrogen quantum yield for the photolysis (366 nm) of azoisopropane in the gas phase over an extended pressure and temperature range led to the conclusion that decomposition occurred via vibrationaIly excited upper singlet and triplet states with the onset of dissociation of the vibrationally equilibrated triplet state as the temperature is increased.a The phenyldiazenyl radical PhN2*has been established as an intermediate in the decomposition of arylazoalkanes. A recent study of the photolysis of the labelled azo-compound (1) clearly demonstrates that phenyl migration in the /

Me

N=N15

\ /

C

Ph’

‘Me

Ph Me

I 2 Ph-C* I hv

*N=Nl5-Ph

Me

diazenyl radical (2) does not readily O C C U ~ . Various ~ a,a’-dichloro-, a,a’-dialkoyloxy-, and a&-dibenzoxy-azoalkanes are reported to undergo mesu-dl photointerconversion on direct irradiation through pyrex,* and the photoelimination a

J. A. den Hollander, J.C.S. Chem. Comm., 1976, 403. G. 0. Pritchard and F. M. Servedio, Internat. J. Chem. Kinetics, 1975, 7 , 99. N. A. Porter and J. G. Green, Tetrahedron Letters, 1975, 2667. N. Levi and D. S . Malament, Israel J. Chem., 1975, 12, 925.

503

504 Photochemistry of nitrogen from methyldi-imide, MeN=NH, proceeds via a radical chain mechani~m.~ The study of the photoelimination of nitrogen from 1-pyrazolines continues to offer a unique opportunity to examine the mechanism of these photodecompositions, and also provides a useful synthetic route to cyclopropanes. Irradiation of the 1-pyrazoline(3) in diethyl ether affords ( - )-cyclocopacamphane (4) in 92% yield.s The pyrazoline diester ( 5 ) is similarly converted into the

Y

Y

,CO, Me

/

Me0,C

C0,Me (5)

Me0,C

C0,Me

C0,Me (7)

(6)

cyclopropane (6) which on further irradiation is reversibly transformed into the isomer (7).' 1,3-Biradicals are usually proposed as intermediates in photodecompositions of this type. In support of this, e.s.r. spectral evidence for the formation of the cyclopenta-l,3-diyl radical on irradiation of 2,3-diazabicyclo[2,2,l]hept-2-ene in a cyclohexane matrix at 5.5 K has now been described.8 Over 95% retention of configuration is observed in the photoelimination of nitrogen from the chiral 1-pyrazoline (8).g This is in agreement with the intervention of a short-lived singlet biradical with little zwitterionic character.

6

-0-menthyl

p h ~ ~ ~ - O - m e n t h y l hv -N,

Ph

+

Ph Ph (8)

Stereospecificity is not observed, however, in the photodecomposition of cis- and trans-3,5-diphenyl-l-pyrazoline, where rotational isomerization appears to compete with cyclization in the biradical.1° The cis-diphenylpyrazolineyields the cisand trans-diphenylcyclopropanesin a ratio of 51.5 : 48.5, whereas the trans-isomer

lo

S. K. Vidyarthi, C. Willis, and R. A. Back, J . Phys. Chem., 1976, 80, 559. E. Piers, M. B. Geraghty, R. D. Smillie, and M. Soucy, Canad. J. Chem., 1975, 53, 2849. T. Toda, K. Nakano, A. Yamae, and T. Mukai, Tetrahedron, 1975,31, 1597. S . L. Buchwalter and G. L. Closs, J. Amer. Chem. SOC., 1975,97, 3857. R. L. Dreibelbis, H. N. Khatri, and H. M. Walborsky, J. Org. Chem., 1975, 40, 2075. M. Schneider and H. Strohacker, Tetrahedron, 1976, 32, 619.

505 affords the same products in the ratio 14.5 : 85.5. A minor competing pathway in the photodecomposition of 3,5-diphenyl-l-pyrazolineleading to the formation of phenylcarbene has been detected by other workers;ll two possibilities for the generation of this carbene exist and are shown in Scheme 1, namely (a) a retro1,3-dipolar addition followed by photolysis of phenyldiazomethane or (b) a

Photoelimination

Ph

Pv Ph-

+

N2

+

PheH Scheme 1

PhcH

+

N,

direct fragmentation with elimination of nitrogen. At present, it is not possible to distinguish between these pathways. Both cis- and trans-3,5-divinyl-l-pyrazolines(9) are known to undergo photoelimination of nitrogen to give trans-l,2-divinylcyclopropane(10) and cyclohepta1,4-diene (11) via the same allylic biradical. A study at - 50 "C now confirms

(1 1)

beyond reasonable doubt that the cycloheptadiene is formed by thermal Cope rearrangement of the unstable cis-l,2-divinylcyclopropane(12).12 The view that the di-n-methane rearrangement of barrelene proceeds by way of a triplet cyciopropyldicarbinyl biradical is widely supported. The cyclopropyldicarbinyl radical (13) has now been generated independently by irradiation

(14)

(13)

(1 5 )

n S. L. Buchwalter and G . L. Closs, J. Org. Chem., 1975, 40, 2549. l2

M. Schneider, Angew. Chem. Internat. Edn., 1975, 14, 707.

(16)

506 Photochemistry of the azo-compound (14), leading to the formation of barrelene (15) and semibullvalene (16).13 Sensitized irradiation affords predominantly the semibullvalene, thus supporting the proposed intermediacy of triplet biradicals in the di-n-methane rearrangement. The greater tendency for barrelene formation in the direct irradiation indicates that intersystem crossing in the azo-compound is inefficient.

(19) R = Ph, 2,5-(MeO)2CaH3,Prn or Pri

(20)

A series of 4-substituted 3-nitro-3-methyl-1-pyrazolines (17) have been converted by irradiation in benzene into the corresponding nitrocyclopropanes (18).14 The reaction is complicated by competing but poorly understood photoreactions of the nitro-group giving the pyrazoles (19) and (20). Singlet biradical intermediates are proposed to account for the retention of configuration observed in

hv benzene

(21)

'

R1 = Ra = Me R1 = Me, R2 = But 0

R2

Ph

0

-

Ph

(23) l8

l4

H. E. Zimmerman, R. J. Boettcher, N. E. Buchler, and G. E. Keck, J. Amer. Chem. SOC., 1975, W,5635. L. Valades, M. Jimenez, and L. Rodriguez-Hahn, Rev. Latinoamer. Quim., 1975, 6, 152.

Photoelimination

507

the direct photodecomposition of four new em-double pyrazolines prepared from norbornadiene and dimethyl phenyldiazornethylphosph~nate.~~ Benzophenonesensitized photodecomposition leads to an intractable mixture of isomeric biscyclopropanes. The photodecomposition of diazobicyclohexane (21), however, takes a different course and results in the formation of the diazo-compound (22), characterized spectroscopically.l6 Further irradiation of this intermediate affords the bicyclo[l,l,O]butane (23), presumably via the carbene. Pyrazoles readily eliminate nitrogen on irradiation to yield the corresponding cyclopropenes. In this way, the first example of a [ 2 ~ , 6 nspirene ] (24)has been

prepared by irradiation of the pyrazole (25).17 The first spirocyclopropabenzenes (26) have also been prepared by photodecomposition of the pyrazoles (27) at - 20 OC.18 These products can be expected to be even less stable than previously reported cyclopropabenzenes and do in fact readily rearrange on heating to the cycloheptenes (28). Photodecomposition of pyrazole (29) yields small amounts of the indenes (30; R1 = Me, R2 = C0,Me; R1 = CO,Me, R2 = Me) together with the cyclopropene (31).le Photoelimination of nitrogen from triazolines has again been used in the synthesis of aziridines. Thus, bicyclic aziridines (32) have been prepared in good yield from the corresponding triazolines (3 3),20 and aziridines have similarly A similar approach to the been obtained from 4-cyan0-5-aminotriazolines.~~ l5

l6 l7

2o

21

H. Cohen and C. Benezra, Canad. J. Chem., 1976, 54,44. W. Welter and M. Regitz, Tetrahedron Letters, 1976, 1473. H. Durr and B. Weiss, Angew. Chem. Internat. Edn., 1975, 14, 646. H. Durr and H. Schmitz, Angew. Chem. Internat. Edn., 1975,14,647. V. V. Razin, Zhur. org. Khim., 1975, 11, 1457. M. H. Akhtar, A. Begleiter, D. Johnson, J. W. Lown, L. McLaughlin, and S.-K. Sim, Canad. J. Chem., 1975, 53, 2891. F. Texier and J. Bourgois, J. Heterocyclic Chem., 1975, 12, 505.

508

Photochemistry

H

0

hv __f

- Nz

I

R

(32)

N-N

/

Me

\

Me

synthesis of several heteromethylenecyclopropanes has been employed. The first synthesis of 1,2-dimethyldiaziridinone (34) has been accomplished by irradiation of the 2-tetrazoline ( 3 3 , the product being characterized spectroscopically.22 Analogous ring contractions are reported in imino- and methylenetetrazolines, leading to diaziridinimines and cyclic carbodi-imides respectively. The photoelimination of nitrogen from cyclic azo-compounds is not, of course, are obtained on limited to five-membered rings. Three products, (36)-(38),

(36)

(37)

(38)

direct irradiation of the azo-compound (39) and their formation is taken as evidence for the intermediacy of the 2,2’-bis-(l, l-dimethylallyl) biradical (40).23 Although full details of the nitrogen elimination process have not been established, aa

H. Quast and L. Bieber, Angew. Chem. Internat. Edn., 1975, 14, 428. T. J. Levek and E. F. Kiefer, J. Amer. Chem. SOC.,1976, 98, 1875.

509

Photoelimination

the formation of the cyclobutane (37), involving as it does rotation around the 2,2'-bond, precludes a concerted ring closure. Thermally unstable 2-arylbenzazetes (41) are produced by photolysis of 4-arylbenzotriazines (42) at - 80 "C and can be trapped as cycloadducts with suitable dimersn2*At room temperature, the benzazetes readily give dimers (43).

Rey

hv, 300nm

N+N

\

(42)

R

=

-80 O C / - N

H, Me, or C1

(41)

Ar (43)

R

Photoelimination of nitrogen is also reported in 3-chloro- and 3-bromo3-methyldia~irines,~~ whereas photochemically induced cis-trans-isomerization is observed in preference to elimination in a series of configurationally isomeric 3 ,S-diphenyl-1,2-diazacyclo-oct-1-enes .26

2 Elimination of Nitrogen from Diazo-compounds Photoelimination of nitrogen from diazo-compounds provides a simple and versatile route for the generation of carbenes. The formation of triplet carbenes by photolysis and thermolysis of diazo-compounds has been ~eviewed.~' The gas-phase photolysis of diazo-n-butane has been studied at various pressures and with added gases.28 Vibrationally excited but-1-ene and methylcyclopropane are formed via singlet carbene. The photodecomposition of diazoanthrone leads to two species of anthranylidene having different spin states;29these two states are directly interconvertible. Irradiations carried out in benzene, toluene, cyclohexene, cyclohexane, and hexafluorobenzene in the presence and absence of triphenylphosphine demonstrate that the triplet state has the ability to abstract hydrogen atoms selectively whereas the singlet state interacts with nucleophilic centres such as the terminal nitrogen atom of the diazo-group. Triplet vinylmethylene, generated by triplet-photosensitized decomposition of diazopropene, also readily participates in hydrogen abstraction reactions.30 Irradiation in cyclohexane, for example, yields allylcyclohexane by a radical pair mechanism. Studies with cyclohexene, however, have cast doubt on the view that addition is the preferred mode of reaction of triplet carbenes with alkenes. The major product of reaction of triplet vinylmethylene with cyclohexene is 3-allylcyclohex-l-ene, formed presumably by hydrogen abstraction and coupling I4 26

I6 27

I9

C. W. Rees, R. C. Storr, and P. J. Whittle, J.C.S. Chem. Comm., 1976, 411. P. Cadman, W. J. Engelbrecht, S. Lotz, and S. W. J. Van der Merwe, J. S. African Chem. Inst., 1974, 27, 149. G. Vitt, E. Hadicke, and G. Quinkert, Chem. Ber., 1976, 109, 518. H. Diirr, Topics Current Chem., 1975, 55, 87. J. M. Figuera, J. M. PerCz, and A. P. Wolf, J.C.S. Faraduy I, 1975, 71, 1905. G. Cauguis and G. Reverdy, Bull. SOC.chim. France, 1975, 1841. M. L. Manion and H. D. Roth, J. Amer. Chem. SOC.,1975, 97, 6919.

510

Photochemistry of the resulting radicals. Singlet excited carbene, generated by photodecomposition of l-phenyldiazoethane (44), undergoes addition to cis-but-2-ene to give the cis-cyclopropanes (45) and (46).31 Styrene (47) is also formed and arises by an hv

Ph-C-Me II

-N*

Ph-c-Me

N2

(44)

uncommon 1,Zhydrogen migration to the carbene centre. As with diphenylcarbene, there is apparently an equilibrium between singlet and triplet carbene, the triplet-derived products being the corresponding trans-cyclopropane, ethylbenzene, and, in the presence of oxygen, acetophenone. The effect of p-substitution on the addition of diarylcarbenes to alkenes has been studied with a view to clarifying the electronic effect of the substituent on the stereochemistry of the addition.32 Photodecomposition of diaryldiazomethanes (48) in cyclopentadiene gave adducts (49) and (50). In all cases, the major product has the

C=N2

Ph’

hv

-N2 +

Ar\C: Ph’

>

3’:

electron-rich aryl group endo to the five-membered ring. The stereoselectivities of these carbenes and corresponding carbenoids, generated by zinc chloridecatalysed decomposition of the diazomethanes, are similar. Attempts to obtain evidence for the existence of singlet bis(methoxycarbony1)carbene as a discrete intermediate in the photoreaction of dimethyl diazomalonate with cyclohexene by analysis of the activation parameters has been U ~ S U C C ~ S S ~ U ~ . ~ The role of the carbene in the direct photodecomposition is still in some doubt, and an explanation involving the formation of a complex between the excited diazo-compound and the alkene is preferred. The product of direct photolysis of 2-diazomethyl-l,3,5-triazine( 5 1) in cyclohexane is the cyclohexylmethyltriazine (52).34 Photodecomposition of diphenyldiazomethane (53) in hexane in the presence of 2,6-dimethylphenyl isocyanide (54) affords the ketenimine (55), presumably by electrophilic attack of carbene on the isocyanide, the amide (39, and a small amount of the indene (57);35the formation of the indene is the result 81 s2 88

8p 85

Y. Yamamoto, S . 4 . Murahashi, and I. Moritani, Tetrahedron, 1975, 31, 2663. D. S. Crumrine and H.-H. B. Yen, J. Amer. Chem. Soc., 1976, 98, 297. D. S. Wulfman, B. Poling, and R. S. McDaniel, Tetrahedron Letters, 1975, 4519. A. Kumagai, S. Sekiguchi, and K. Matsui, Bull. Chem. SOC.Jupan, 1975, 48, 3409. N. Obata, H. Mizuno, T. Koitabashi, and T. Takizawa, Bull. Chem. SOC.Japan, 1975, 48, 2287.

51 1

Photoelimination

hv, -Nz

0

II

NH-C-

CHPh,

of a thermal addition of the ketenimine to the isocyanide. Aryl and hydrogen migration compete in the carbenes generated by photodecomposition of a-diazo/3-hydroxyphosphine oxides (58) to give products (59) and (60) respectively, the latter pred~minating.~~ The OH insertion product (61) and dimethyl malonate (62) are the major products of direct irradiation of dimethyl diazomalonate (63)

(58) R = Ph,p-MeC,H,, p-CNC,H,, 2-naphthyl, 2-thienyi, or CH=CHPh C0,Me

/

N2C\ C02Me (63) s6

hv, ROI-I -N, +

C0,Me I R-0-CH I CO, Me (61)

W. Disteldorfand M. Regitz, Chem. Ber., 1976, 109, 546.

+

/CO, Me C\H2 C0,Me (62)

512

Photochemistry

in The yield of dimethyl malonate increases with the hydrogendonating ability of the alcohol, and it is the only major product in the corresponding benzophenone-photosensitized decomposition. These results give support for a singlet excited state in the insertion reaction and a triplet excited state in the hydrogen abstraction process. The photoelimination of nitrogen from diazo-ketones and the fate of the resulting a-oxocarbene is still an area of major interest. The Wolff rearrangement A Wolff of the diazo-ketone (64) could only be accomplished photo~hemically.~~

C~CHN, (64)

CI-I,CO,Me

hv

&cH

-N2’

Ph

rearrangement to keten (65) is also thought to be responsible for the conversion of the a,/l-epoxydiazo-ketone (66) into the ester (67) on irradiation in methan01.~~ Nucleophilic addition of methanol to the keten is presumably accompanied by ring cleavage of the epoxide. Analogous transformations in cyclic diazoketones result in ring contraction. Thus, irradiation of 2-diazo[1-13C]naphthalenl(2H)-one (68) in dioxan-water leads to the formation of the labelled carboxylic acid (69).40 The absence of any isotope scrambling in this and closely related transformations excludes the possibility of an oxiren intermediate, a species frequently proposed in the decomposition of other diazo-ketones. Ring con,02J’]traction has also been reported in 8-diazo-endo-benzo[c]tricyclo[4y2,1 non-3-en-7-0ne.~~ 87 s8

40

41

W. Ando, T. Hagiwara, and T. Migita, Bull. Chem. SOC.Japan, 1975, 48, 1951. A. J. H. Klunder and B. Zwanenburg, Tetrahedron, 1975,31, 1419. N. F. Woolsey and M. H. Khalil, J. Org. Chem., 1975, 40, 3521. K.-P. Zeller, Chem. Ber., 1975, 108, 3566. L. Enescu, F. Chiraleu, and M. Avram, Rev. Roumaine Chem., 1975,20, 957.

513

Photoelimination

(69)

Irradiation of 3-diazobenzofuranone (70) at low temperature yields two primary products, the keten (71) and the ring-opened product (72).42 These products can be photochemically interconverted, and the keten undergoes further photodecomposition with short-wavelength light to give benzyne (73), presumably

J via the carbene (74). 3-Diazobenzofuranone is therefore an ideal precursor for the preparation of matrix-isolated benzyne. The formation of the keten is easily viewed as the result of a Wolff rearrangement, whereas the formation of (72) probably involves a concerted ring cleavage in the intermediate carbene. Identical interconversions have been reported in 2-diazoindan-1-one, and the first synthesis of the 2I~-l-thiacyclobutabenzenesystem has been accomplished by irradiation of the thia-analogue (75).43 The aza-analogues (76) behave quite differently and provide the first example of reversible photochromic valence isomerization between diazo-compounds and diazirine~.~* Although a-oxocarbenes are known to undergo addition reactions with alkenes, the addition of such a species to an aromatic system is so far unrecorded. 42

0. L. Chapman, C.-C. Chang, J. Kolc, N. R. Rosequist, and H. Tomioka, J. Amer. Chem. SOC.,1975, 97, 6586.

O3 O4

E. Voigt and H. Meier, Angew. Chem. Internat. Edn., 1976, 15, 117. E. Voigt and H. Meier, Chem. Ber., 1975, 108, 3326.

514

Photochemistry

a‘’ 0

hv,

___, MeOH

> 290 nm

- Na

\

\

R

R

(76)

R

=

H or Me

Irradiation of 2-diazoacenaphthen-l-one(77) in benzene, however, affords the spiro-compound (78) in 84% yield.46 Similar reactions were effected with toluene and p-xylene. A carbene insertion reaction has again been used in the synthesis

hv, benzene

- Na (77)

Et02C H hv

0

of a penam analogue; photodecomposition of the diazo-ketone (79)in carbon tetrachloride gave the 7-oxa-l-azabicyclo[3,2,0]heptane(80) in an estimated yield of 55%.4s The key step in a new synthesis of (+)-glaziovine (81) is provided by the

(82)

(8 1)

C. G. F. Bannerman, J. I. G. Cadogan, I. Gosney, and N. H. Wilson, J.C.S. Chem. Cumm., 1975, 618. 46

B. T. Golding and D. R. Hall, J.C.S. Perkin 1, 1975, 1517.

Pho toelim ination

515

photolysis of the o-diazo-oxide (82).47 This reaction is a significant improvement over the previously reported synthesis in which the final cyclization is accomplished by photoelimination of HBr from ( k )-8-bromo-N-methylcoclaurine. 3 Elimination of Nitrogen from Azides The photoreactions of azides can almost without exception be rationalized in terms of an intermediate nitrene, and the decomposition provides an easy and efficient method for the generation of these species. The reactions of photochemically generated nitrenes have been reviewed.48 Photolysis of aromatic azides is claimed to produce nitrenes primarily in the singlet state; the lifetimes formed by photoof these nitrenes have been d e t e ~ m i n e d . 1-Pyrenylnitrene, ~~ decomposition of 1-azidopyrene,reacts to give 1-aminopyrene and 1,l ’-azopyrene in degassed methanol, whereas only 1,l’-azopyrene is formed in benzene.50 The quantity of 1-aminopyrene formed was measured and was used to calculate rate constants for hydrogen abstraction by 1-pyrenylnitrene from a wide variety of solvents. Hydrogen abstraction does not occur from benzene. When phenol was present in the degassed benzene solution, N-( 1-pyreny1)-p-benzoquinonemonoimine was formed in addition to the other products. Photoelimination of nitrogen from azidoacetonitrile (83) at - 196 “C gave formimidoyl cyanide (84), undoubtedly via the nitrene, and on further irradiation this was converted into

formimidoyl isocyanide (85).51 The mechanism for this cyanide to isocyanide rearrangement has been the subject of much discussion, and a three-membered ‘zwitterionic’ azirinimine intermediate is now suggested. 9-Azidotriptycenes (86), on irradiation in methanol, are converted into the azahomotriptycenes (87) which can be regarded as solvent adducts of the unstable imines (88).52 Irradiation in cyclohexane affords imine dimers. The formation of imine is presumably the result of a 1,2-aryl migration in the nitrene. The parallel photochemical behaviour of such diverse bridgehead azides as 1-azidoadamantane and 9-azidotriptycene indicates that solvation effects do not play a decisive role in this reaction and that bridgehead azides can in general be regarded as simple precursors of highly strained bridgehead imines. Products arising by 1,2-hydrogen transfer and 1 ,2-alkyl migration in nitrene intermediates were formed by photolysis of a variety of 3-, 6-, 17-, and 20-steroidal a z i d e ~ In . ~ only ~ one case, that of 6P-azido-5a-pregnane (89), was pyrrolidine formation observed and then in only 6% yield. The major products of this photodecomposition are the imine (go), the azepines (91) and (92), and pyrrolidine (93). p7 48

48 6o 61 62

63

C. Casagrande and L. Canonica, J.C.S. Perkin I, 1975, 1647. 1. F. Goryainova and Y. A. Ershov, Khim. uysok. Energii, 1975, 9, 99. A. V. Oleinik, V. M. Treushnikov, and N. N. Gessen, 2hur.fiz. Khim., 1976, 50, 202. T. Tsunoda, T. Yamaoka, and M. Takayama, Nippon Kagaku Kaishi, 1975,12,2074. J. H. Boyer, J. Dunn, and J. Kooi, J.C.S. Perkin I, 1975, 1743. H. Quast and P. Eckert, Angew. Chem. Internat. Edn., 1976, 15, 168. A. Pancrazi and Q. Khuong-Huu, Tetrahedron, 1975, 31, 2041.

516

Photochemistry

hv __3

(86) R

=

H or Me

1

MeOH

The principal products of photodecomposition of azidothiopyran (94) are the pyridines (95) and (96) and the thiophen (97).64 Details of the mechanism of this transformation are not clear, although thiazepine intermediates have been proposed.

The photoreactions of vinyl azides have been reviewed.65 Decomposition leads at least initially to 1-azirines; on the basis of kinetic results, it is clear that a nitrene is not involved in the formation of this azirine. A concerted process 64

b5

J. P. LeRoux, J. C. Cherton, and P. L. Desbene, Compt. rend., 1975, 280, C, 37. G. Labbe, Angew. Chem. Internat. Edn., 1975, 14, 775.

517

Photoelimination

involving cyclization with simultaneous loss of nitrogen is the most likely pathway for this reaction, but the possibility of an unstable intermediate 1,2,3-4Htriazole must still be considered. l-Azirine intermediates have been proposed to account for a variety of photoreactions of unsaturated and aryl azides. The formation of the nitrile (98) by photodecomposition of 2,5-diazido-3,6-di-t-butyl1,Cbenzoquinone (99) in benzene is believed to occur via the azirine

+ hv But

N3

But&N

d

- N2

But

N3

N3

0

0

0

(99)

( 100)

(98)

( 102)

In the photodecomposition of 6-, 7-, and 8-azidoquinolines and in 2-azidonaphthalene, the azirine intermediates can be trapped as 1,Zdiamines with secondary amine~.~'7-Azidoquinoline (101), for example, is converted into the diamine (102) in 80% yield on irradiation in the presence of diethylamine. Ring expansion of the azirine to the corresponding 1H-azepine competes with diamine

aN3 -N2' hv

(103)

68

57

W. Weyler, W. G. Duncan, and H. W. Moore, J. Amer. Chem. SOC., 1975, 97, 6187. S. E. Carroll, B. Nay, E. F. V. Scriven, and H. Suschitzky, Synthesis, 1975, 11, 710.

518 Photochemistry formation in phenyl azide. Following many failures, this ring-expansion process has now been extended to azido-naphthalenes and -anthracenes, but only in the presence of concentrated potassium methoxide in Thus, irradiation followed by gentle heating under reflux of 2-azidoanthracene (103) affords a nearly quantitative yield of 3-methoxy- 1H-napht ho [2,3-c]azepine (104), whereas irradiation followed by immediate neutralization affords 1-amino-2-methoxyanthracene (105). These results are easily rationalized in terms of an unstable methoxyaziridine (106). Ring expansion to 8H-thieno[2,3-c]azepines has been observed in 6-azidobenzo[b]thiophensbut not in the corresponding 4- or S-azidoderivatives.69 Substituted carbomoyl azides have been found to undergo a photo-Curtius rearrangement to give aminoisocyanates;60these can be trapped by nucleophiles. Thus, phenylcarbamoyl azide (107) on irradiation in methanol is readily converted into methyl 2-phenylhydrazinecarboxylate(108). Attempts to detect intermediate 0

II PhNH-C-NB

hv -Nzh

PhNH-NHC02 Me

PhNH-N=C=O

Me

Me (1 10)

nitrenes on photolysis in benzene, cyclohexene, and cyclohexane were unsuccessful. The reasons for the difference in behaviour between these carbamoyl azides and diarylcarbamoyl azides (which afford relatively stable nitrenes in aprotic solvents) are not clear. NN-Dimethylaminoisocyanate (109) was detected spectroscopically on neon-matrix-isolated photolysis of dimethylcarbamoyl azide (1 10). Doubts have been raised concerning the proposal that triplet biphenylnitrene is the sole carbazole precursor in the photocyclization of 2-a~idobiphenyl.~~ A difference in rate constants for the formation of carbazole and for the disappearance of a short-lived intermediate absorbing at X 360nm was observed, suggesting that two separate processes are involved in the formation of carbazole. Photolysis of a series of pyrazole azides (1 11; R = 5-C1,4-CF8, or 5-NMe2)in the presence of acetophenone gave products (112) and (113) derived solely from triplet nitrene.62 These were also formed on direct irradiation, but the ylides (114) were also obtained by what appears to be a singlet-derived process. The electrophilic character of singlet nitrene is further demonstrated by its facility I*

6o

J. Rigandy, C. Igier, and J. Barcelo, Tetrahedron Letters, 1975, 3845. B. Iddon, M. W. Pickering, H. Suschitzky, and D. S. Taylor, J.C.S. Perkin I, 1975, 1686. W. Lwowski, R. A. deMauriac, M. Thompson, R. E. Wilde, and S.-Y. Chen, J. Org. Chem., 1975,40,2608.

R. J. Sundberg, D. W. Gillespie, and B. A. DeGraff, J. Amer. Chem. SOC.,1975, 97, 6193. I. M. McRobbie, 0. Meth-Cohn, and H. Suschitzky, Tetrahedron Letters, 1976, 925.

519

Photoelimination

I

to attack nitrogen nucleophiles, a reaction which is preferred to attack on sulphur nucleophiles as shown for the azidobenzothiazole (115).63 Competitive singletand triplet-derived cyclizationswere also observed in 2-azidophenylbenzimidazole.

Qc)-p

-=? v I1

N3

(115)

Pentafluorophenyl nitrene, generated photochemically from the corresponding azide, adds stereospecifically to cis- and trans-1,Zdichloroethylene to afford a ~ i r i d i n e s . The ~ ~ aziridine (116) is presumably an intermediate in the addition of pentafluorophenyl nitrene to thiophen to give the 2-aminothiophen

(117). Pivaloyl nitrene, generated by photodecomposition of pivaloyl azide, adds to alkenes stereospecifically in its singlet state and stereoselectively in its triplet state.6S The addition of triplet nitrene to 4-methylpent-2-ene, for example, is highly stereoselective, producing the cis-aziridine from both cis- and transalkenes. Addition and C-H bond insertion are observed on reaction of the I. M. McRobbie, 0. Meth-Cohn, and H. Suschitzky, Tetrahedron Letters, 1976, 929. R. A. Abramovitch, S. R. Challand, and Y. Yamada, J. Org. Chem., 1975,40, 1541. G. R. Felt and W. Lwowski,J. Org. Chem., 1976,41, 96.

520 Photochemistry nitrene generated by the photolysis of 2-azido-4,6-dimethoxy-l,3,5-triazine with cyclohexene and cyclohexane re~pectively.~~ Reaction with ketones, however, leads to the formation of 5,7-dimethoxy-3H-1,2,4-oxadiazolo[4,3-a]-s-triazines by a pathway which is thought to involve electrophilic attack of singlet nitrene on the carbonyl oxygen atom as outlined in Scheme 2.

Me0

)(

Me0

Me0

R1R2 Scheme 2

The first convincing evidence for the formation of silaimines in the condensed phase has been de~cribed.~'The two silaimines (118) and (119), arising respectively by methyl and t-butyl migration in the nitrene derived from the silyl azide (120), have been trapped as the adducts (121) and (122) with t-butyl ButMe,SiN,

I

kv, 254 nm -N,

Me, Si =NBut (1 19)

i

hv, 254 nm

------+ - N,

ButMeSi=NMe

1

ButOH

ButMcSi -NHMe I OBut (121)

B ~ O H

Me, Si -N H But

I

OBut (122)

alcohol. Silylation of the alcohol by the azide, presumably accompanied by the formation of hydrazoic acid, is a competing reaction. Sulphamoyl nitrenes do not appear to be obtained on photodecomposition of sulphamoyl azides; complex reactions are reported to occur involving mainly S-N bond cleavage 66

6'

R. Kayama, H. Shizuka, S. Sekiguchi, and K. Matsui, Bull. Chem. SOC.Japan, 1975,48, 3309. D. R. Parker and L. H. Sommer, J . Amer. Chern. Soc., 1976,98, 618.

Photoeliminat ion

52 1

but with some C-N cleavage.68 Certain aryl azides have been used as photoaffinity labels because of the ease with which reactive nitrenes can be generated.6g-71 4 Photodecomposition of other Compounds having N-N Bonds Photodecomposition of sodium salts of toluene-p-sulphonylhydrazonesis a well established and mild route for the generation of carbenes. In many instances, intermediate diazo-compounds can be detected.72 2,3-Homocycloheptatri-

1 (126)

enylidene (123), formed by photodecomposition of the sodium salt of 2,3-homotropone toluene-p-sulphonylhydrazone(124), undergoes rapid ring-opening to afford the reactive highly strained cyclic allene cyclo-octa-l,2,4,6-tetraene(125).73 Na'

68

(128) R. A. Abramovitch and K. Miyashita, J.C.S. Perkin I, 1975, 2413. D. F. Wilson, Y. Miyata, M. Erecinska, and J. M. Vanderkooi, Arch. Biochem. Biophys., 1975,171, 104.

70

71 72

73

S. H. Hixson and S. S. Hixson, Biochemistry, 1975, 14, 4251. F. Seela and F. Cramer, Z. Physiol. Chem., 1975, 356, 1185. R. Siegfried, Tetrahedron Letters, 1975, 4669. M. Oda, Y. Ito, and Y . Kitahara, Tetrahedron Letters, 1975, 2587.

522

4

Photochemistry

N-NHTs

MeOH-MeONa

&C€-IN2 CHO

I

J-f

HO

hv, MeOH

(130)

(1 32)

Ph o toe lim inat ion

523

The tetraene, which is rapidly converted into the dimer (126), can be trapped as an adduct with cyclopentadiene. A carbene-carbene rearrangement is observed on low-temperature photodecomposition of the tosylhydrazone salt (127) to give as final product the fulvalene (128).74 This constitutes the first reported example of a low-temperature rearrangement of an arylcarbene to an aromatic carbene, and, by analogy with other carbene-carbene rearrangements in solution, an intermediate cyclopropene (129) is proposed. Unexpectedly, the tosylhydrazone of 4-hydroxy-endo-tricyclo[4,2,102~6]non-7-en-3-one (130) underwent photodecomposition involving retroaldol cleavage exclusively on irradiation in methanol containing sodium methoxide to yield the ethers (131) and (132).75 The corresponding 4-methoxy-derivatives gave the ring-contracted product expected from the carbene. Evidence for the formation of a dibenzobicyclo[4,1,O]heptatriene (133) has been r e p ~ r t e d Irradiation .~~ of the tosylhydrazone salt (134) at - 110 "C affords the diazoalkane (135) which in turn partitions into the 3H-pyrazole (136) and the carbene (137). The latter is converted into the bicycloheptatriene (133) by intramolecular addition of the carbene to the triple bond; a stable adduct (138) is formed with butadiene. Intramolecular electrophilic addition of carbenes to sulphur has been observed in photochemically generated j?-arylthioalkyl carbenes, leading to the formation of products derived from novel thietanonium ylides.?' Further evidence for the intervention of thietanonium ylides comes from a study of the photodecomposition of tosyl salts (139) as illustrated in Scheme 3.

W

R

[2,31

% & P h

I

YYPh R Scheme 3 74

7s 70

77

U. H. Brinker and W. M. Jones, Tetrahedron Letters, 1976, 577. W. Kirmse and T. Olbricht, Chem. Ber., 1975, 108, 2629. J. P. Mykytka and W. M. Jones, J. Amer. Chem. SOC.,1975,97, 5933. K. Kondo and I. Ojima, Bull. Chem. SOC.Japan, 1975,48, 1490.

524 Photochemistry The synthesis of a series of fluoropyrazoles and 3-fluoro-l,2,4-triazole has been accomplished by irradiation of the corresponding diazonium salts in HBF4.78 5 Photoelimination of Carbon Dioxide Elimination of carbon dioxide is one of a variety of reaction pathways observed on irradiation (A = 254 nm) of monochloroacetic acid in aqueous ~ o l u t i o n . ~ ~ Semidione radicals have been detected in the photofragmentation of a-oxocarboxylic acids in aqueous solution, and they appear to arise by decarboxylative substitution of a-oxo-carboxylic acids by acyl radicals.80* Decarboxylation is also observed on irradiation of nalidixic acid (140) in alkaline oxygen-free solution;82the mechanism proposed to account for the decarboxylation and for the formation of the lactam (141) is outlined in Scheme 4. 7,8-Dimethylisoalloxazine-10-acetic acid yields 7,8-dimethylalloxazine (lumichrome), carbon 0

Naf

co,

fJ?J

+ *co,-

hv ___,

-

coz

Et

Et

Scheme 4

dioxide, and formaldehyde by an intermolecular triplet process.83 3-Methyland 3,/3-dimethyl-isoalloxazine-1O-propanoicacids (142), on the other hand, undergo intramolecular photodecarboxylation and cyclization to give the tetracyclic products (143), again confirming the photoreactivity of the N-1 position in this system. Photoexcited l-cyanonaphthalene reacts with substituted phenylacetic acids to give excited complexes which deactivate preferentially via exciplex emission in benzene, but via electron transfer followed by chemical reaction in acetonitrile ;84 photoreduction and reductive alkylation of l-cyanonaphthalene 78

79 8a

82

83 84

J. Vilarrasa, C. Galvez, and M. Calafell, Anales de Quim., 1975, 71, 631. M. Neumann-Spallart and N. Getoff, Monatsh., 1975, 106, 1359. S. Steenken, E. D. Sprague, and D. Schulte-Frohlinde, Photochem. and Photobiol., 1975, 22, 19. S. Steenken, Photochem. and Photobiol., 1975,22, 157. N. Detzer and B. Huber, Tetrahedron, 1975, 31, 1937. W.-R. Knappe, Chem. Ber., 1975,108, 2422. J. Libman, J. Amer. Chem. SOC.,1975, 97, 4139.

Photoelimination

525

accompanied by the elimination of carbon dioxide is observed. Analogous reactions have been reported for l-methoxynaphthalene.8s Examples of the photoelimination of carbon dioxide from esters and lactones have again been widely reported. Photodecomposition of 4-acetoxysantonene (144) in benzene proceeds by way of a triplet excited state and affords 4-methylsantonene (145), the 1l-methyl isomer (146), and 4-methylphotosantonene

q

0

*+ o

0

0

(147)

(146)

(147).86 Santonenyl and acetoxyl radicals are formed initially, the latter undergoing loss of carbon dioxide followed by radical recombination to give (145) and (146). In arylmethyl esters, a novel oxygen scrambling reaction has been shown to compete with photodecarb~xylation.~~ This process probably arises by recombination of initially formed radical pairs, although other explanations are possible; in arylmethyl phenylacetates, it is a major pathway. A new synthesis of unsymmetrical biphenyls has been accomplished by irradiation of l-aroyloxy3,5-dinitro-2(1H)-pyridones in benzene.88 These pyridones are a useful source of aryl radicals which arise by homolytic N-0 bond cleavage followed by loss of carbon dioxide from the aroyloxy-radical. The photolyses of perfluoroacetic anhydride and perfluoropropionic anhydride in the gas phase are quantitatively described by the equation:8g (RC0),0 86

*’

88

hv

R,

+ C 0 2 + CO

J. Libman, Tetrahedron Letters, 1975, 2507. T. B. H. McMurray and R. R. Talekar, J.C.S. Perkin I, 1976, 442. R. S. Givens and B. Matuszewski, J. Amer. Chem. SOC.,1975, 97, 5617. E. C. Taylor, H. W. Altland, F. Kienzle, and A. McKillop, J. Org. Chem., 1976, 41, 24. G . A. Chamberlain and E. Whittle, J.C.S. Faruduy I, 1975, 71, 1978. 18

526 Photochemistry Evidence for the intermediacy of unstable tetrafluorocyclobutadiene (148) in the photodecomposition of the anhydride (149) has been reported.90 Irradiation in the presence of furan leads to the formation of the cyclobutadiene adduct (150), whereas irradiation alone yields the cyclobutadiene dimer (1 51).

11v - co,, - co+

J

Fw F F

Cyclic carbonates have been found to be useful precursors for arylcarbenes (Scheme 5).01 A photoelimination process is involved with elimination of carbon dioxide; the properties of phenylcarbene obtained from meso- and dl-hydrobenzoin carbonates are virtually identical with those of phenylcarbene obtained 0

from conventional precursors such as trans-2,3-diphenyloxiransand phenyldiazomethane. Photodecomposition of silver trifluoroacetate in solution yields silver, carbon dioxide, and trifluoromethyl radicals.92 This reaction, therefore, can be used as a convenient source of trifluoromethyl radicals and on irradiation in benzene a 57% yield of benzotrifluoride was obtained. 6 Fragmentation of Organosulphur Compounds The photolyses of methanethiol and ethanethiol at 185 nm have been studied;93 the quantum yields for the formation of hydrogen and methane from methanethiol are 0.70 and 0.26 respectively. Ethyl and ethylthiyl radicals are the principal products of triplet-mercury-photosensitized decomposition of diethyl sulphide O0 g1

ga

93

M. J. Gerace, D. M. Lemal, and H. Ertl, J. Amer. Chem. Soc., 1975, 97, 5584. G. W. Griffen, R. L. Smith, and A. Marmade, J . Org. Chem., 1976,41, 338. E. K. Fields and S. Meyerson, J. Org. Chem., 1976, 41, 916. D. Kamra and J. M. White, J. Photochem., 1975, 4, 361.

Photoelimination

527

in the vapour phase at 25 O C . 0 4 The products of irradiation of thietan, 3-ethyl2-propylthietanYand 3-methylthietan in the vapour phase, in solution, and in glassy matrices at low temperature can be explained in terms of an initial C-S cleavage to give 1,4-biradical~.~~ Ring contraction is observed on irradiation of the 1-thiacycloheptan-4-one derivatives (152) with the formation of 3,3-dimethyly-butyrothiolactones (153) as the major ~ T O ~ U C ~These S . ~ ~results can best be accounted for by postulating a one-electron-transfer quenching process as outlined in Scheme 6 in preference to C-S bond cleavage. Other products obtained in lower yield appeared to be the result of Type I processes.

(152) R

= H, AcO, or CI,CCH,OCO,

Scheme 6

The first preparation of a stable, crystalline dithiet tautomer of a dithioo-quinone has been reported.g7 Photodecomposition of the 2,3-dihydro-lY4benzodithiin (154) in n-heptane gave the dithiet (155) and ethylene in virtually quantitative yield. The exceptional stability of this dithiet must, at least in part, be attributed to the steric protection afforded it by the host lanostane molecule. The synthesis of fluoranthene (156) has been accomplished in low yield by photoelimination of sulphur from the spirodihydrothiopyran (157).08 Further examples of the extrusion of sulphur from cyclic sulphides in the presence of trimethyl or triethyl phosphite have been described. This approach is of particular value in the synthesis of cyclophanes,gB-loland the extrusion of sulphur from the sulphide (158) is a key step in a new coronene synthesis.102 Fragmentation is the only pathway observed on irradiation of bis(dipheny1methyl) sulphide (159) ;lo3 diphenylmethane (160), 1,lY2,2-tetraphenylethane(161), and bis(diphenylmethy1) disulphide (162) were obtained in yields of 28, 44, and 14% respectively. The major sulphur-containing products of irradiation of C. S. Smith and A. R. Knight, Canad. J. Chem., 1976, 54, 1290. D. R. Dice and R. P. Steer, Canad. J. Chem., 1975,53, 1744. P. Y. Johnson and M. Berman, J. Org. Chem., 1975,40, 3046. R. B. Boar, D. W. Hawkins, J. F. McGhie, S. C. Misra, D. H. R. Barton, M. F. C. Ladd, and D. C. Povey, J.C.S. Chem. Comm., 1975,756. K. Praefcke and Ch. Weichsel, Tetrahedron Letters, 1976, 1787. H. Tatemitsu, T. Otsubo, Y. Sakata, and S. Misumi, Tetrahedron Letters, 1975, 3059. l o o T. Umemoto, S. Satani, Y . Sakata, and S. Misumi, Tetrahedron Letters, 1975, 3159. lol K. Galuszko, Roczniki Chem., 1975, 49, 1597. loa J. T. Craig, B. Halton, and S.-F. Lo, Austral. J. Chem., 1975, 28, 913. Io3 R. W. Binkley, S.-C. Chen, and D. G. Hehemann, J. Org. Chem., 1975,40,2406. g4

M

528

Photochemistry

AcO

(155)

(154)

v + I1

\

& /

\

/

(156)

(157)

-@ I1v

\

(MeO),P

I’ll

Ph

PI:

PI1

S-(cis-l-propeny1)-L-cysteine are prop-l-ene-l-thiol, 2,4-dimethylthiophen, 3,4dimethylthiophen, and 3-methy1thi0phen.l~~All of these appear to arise directly from photochemically generated l-propenylthiyl radicals. Evidence for methyleneoxaziridine radical intermediates in the photolysis of sulphur-containing nitrones has been reported.loS Irradiation of the nitrone (163), for example, gave three products, benzophenone (164), the oxaziridine (165), and the 1,Zthiazine (166) in approximately equal amounts. These are viewed as arising via the methyleneoxaziridine radical (167) as shown in Scheme 7. The photodecomposition of 5-nitrosoimino-4-phenyl-3-phenylimino1,2,4-thiadiazolidine is also thought to be the result of an initial C-S homolytic bond cleavage.lo6 The photochemically induced conversion of the thioesters (168) into the same 1-p-tolylmercapto-7-methylthioxanthone(169) appears to involve a photoH. Nishimura and J. Mizutani, J. Org. Chem., 1975, 40, 1567. W. M. Leyshon and D. A. Wilson, J.C.S. Perkins I, 1975, 1925. lo6 K. Akiba, T. Tsuchiya, I. Fukawa, and N. Inamoto, Bull. G e m . SOC.Japan, 1976, 49, 550. lo4

lo6

529

Pho toelimination

substitution of the intermediate 1 -halogenothioxanthone (1 7O).lo7 The products of the photodecomposition of dithiocarbamic anhydrides and of acyl xanthanes can also be rationalized in terms of an initial C-S bond cleavage.lo8 S-S and S-N bond cleavage compete on irradiation of bis-(2,2,6,6-tetramethylpiperidl-yl) disulphide in a solid matrix at low temperature.log

(168) R = F or C1

( 1 70)

( 169) G. Buchholz, J. Martens, and K. Praefcke, Tetrahedron Letters, 1975, 3213. lo* S. N. Singh and M. V. George, Tetrahedron, 1975, 31, 2029. lo# B. Maillard and K. U. Ingold, J. Amer. Chem. SOC.,1976, 98, 520. lo7

530 Photochemistry Direct but not triplet-sensitized photolysis of 5-phenyl-1,2,3,4-thiatriazole (171) leads to the formation of phenyl isothiocyanate (172), benzonitrile (173), sulphur, and nitrogen.l1° Phenyl isothiocyanate is apparently formed directly from the thiatriazole, whereas benzonitrile sulphide (174) was identified as an intermediate in the formation of benzonitrile and may itself arise via phenylthiazirene (175). The related 5-phenyl-l,2,3,4-thiatriazole 3-oxide affords

I

R v -N2

Ph

\pS

--+

(175)

R3

(176)

+

PhC-N-S-

---+

PhC=N

+

S

(173)

(174)

R3

(177)

benzonitrile and phenyl isothiocyanate on irradiation in ethanol.lll Photolysis of a series of N-(N-arylimidoy1)sulphimides (176) results in cleavage of the S-N bond and formation of 2-substituted benzimidazoles (177) by cyclization of the resulting imidoyl nitrene.l12 The 3aH-benzimidazoles formed by photodecomposition of analogous ortho-blocked sulphimides undergo further rearrangement to derivatives of 5H-~yclopenta[d]pyrirnidine.~l~Benzimidazole and benzimidazole-2-carboxamideare photodegradation products of the fungicide thiabendaz01e.l~~ The sulphoxide (178) is converted into the isoquinolone (179) on irradiation in benzene.l16 In contrast to this, photolysis of the sulphoxide (180) gave the ketone (181) with elimination of sulphur.ll6 Photoelimination of sulphur from certain cyclohexa-l,4-diene-3-thioneS-oxides to give the corresponding 1,4-dien3-ones has also been re~0rted.l~' A. Holm, N. Harrit, and N . H. Toubro, J. Amer. Chem. SOC.,1975,97, 6197. A. Holm, L. Carlsen, S.-0. Lawesson, and H. Kolind-Anderson, Tetrahedron, 1975,31, 1783. 118 T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Perkin Z, 1975, 1964. T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Chem. Comm., 1976, 44. 114 T.A. Jacob, J. R. Carlin, R. W. Walker, F. J. Wolf, and W. J. A. Vanden Heuvel, J. Agric. Food Chem., 1975,23, 704. 116 H. Kato, S. Nakazawa, T. Kiyosawa, and K. Hirakawa, J.C.S. Perkin I, 1976, 672. K. Praefcke and Ch. Weichsel, Tetrahedron Letters, 1976, 2229. 117 D. H. R. Barton, L. S. L. Choi, R. H. Hesse, M. M. Pechet, and C. Wilshire, J.C.S. Chem. Comm., 1975, 557. ll1

531

Photoelimination Ph

Ph

(179)

fi

,s=o

The photolysis of methyl benzenesulphonate in methanol has been described and yields benzene, biphenyl, and aniso1e.ll8 The photodecomposition of toluene-p-sulphinamides has also been studied ; in methanol, photoalcoholysis took place yielding methyl sulphinates, whereas in aprotic solvents such as benzene or acetonitrile products arising by S-N bond homolysis were obtained.lle The photodecomposition of N-arylsulphonyl-SS-dimethylsulphoximides (182) appears to proceed by a radical mechanism with no evidence for the

04-

I

Me,S=NH +

+

Me,SO

+

Me,SO,

intermediacy of singlet sulphonylnitrene.120 S-N Bond homolysis is also implicated in the dye-sensitized photolysis of an aryldiazo-sulphone.121 The formation of nitriles (183) in addition to hydroxamic acids (184) and acetylenes (185) on irradiation of the 1,2,3-thiadiazole 1,l,Ztrioxides (186) suggests that rearrangement occurs in the heterocyclic nucleus before fragmentation.lzZ The primary step in the photodecomposition of the hitherto unknown ethoxy(diphenylmethy1ene)sulphonium tetrafluoroborate to benzophenone is probably dealkylation with the formation of ethyl fluoride and thiobenzophenone A Type I1 photoelimination has been described in thiobenzoic acid 1-0xide.l~~ O-esters.lZ4 11* 119 120

lZ1 lZ2 12*

lZ4

Y.Izawa and N. Kuromiya, Bull. Chem. SOC.Japan, 1975,48, 3197. H. Tsuda, H. Minato, and M. Kobayashi, Chem. Letters, 1976, 149. R. A. Abramovitch and T. Takaya, J.C.S. Perkin I , 1975, 1806. T. Yamase, H. Hisada, S. Suzuki, and T. Ikawa, Bull. Chem. SOC.Japan, 1976,49, 351. H. Meier, G . Trickes, and H. P. Braun, Tetrahedron Letters, 1976, 171. L. Carlsen and A. Holm, Acra Chem. Scand., 1976,30B, 277. Y. Ogata, K. Takagi, and S. Ihda, J.C.S. Perkin I, 1975, 1725.

Photochemistry

532

7 Miscellaneous Decomposition and Elimination Reactions Fragmentation and elimination reactions which cannot be included in any of the above categories are briefly reviewed in this section. It has not proved possible to classify these processes, although analogous reactions are grouped together. A common intermediate is proposed in the sensitized photodecomposition of benzylamine with a variety of sensitizers.126 A novel photochemical N-demethylation has been observed in cocaine and related bicyclic amines;126 the reaction appears to be intermolecular in character. Photochemical N-aryl bond cleavage is less common than N-alkyl bond cleavage. The photolysis of alkylated 0- and p-phenylenediamine derivatives in acidic methanol, however, has been reported to lead via homolytic bond cleavage to the corresponding aniline.12’ Photochemically induced C-0 bond homolysis in alkyl ethers has

R3 R 2 0 0 R 1

hv Ph,CO

(187) R1 = Me or MeCHCOMe R2, R3 = H, Me, or M e 0

lz6

’

lRH

Z. A. Sinitsyna, Y.I. Kiryukhin, and K. S. Bagdasaryan, Doklady Akad. Nauk S.S.S.R., 1975,225, 361.

120

lZ7

V. I. Stenberg, S. P. Singh,N. K. Narain, and S . S . Parmar, J.C.S. Chem. Comm., 1976, 262. D. P. Specht, J. L. R. Williams, T.-H. Chen, and S . Farid, J.C.S. Chem. Comm., 1975, 705.

Photoelimination

533 been observed in ( + )-O-methyl mandelate,128 in 2,4-diphenyl-S-ethoxyvinyl5,6,7,8-tetrahydrobenzopyrylium p e r ~ h l o r a t e ,and ~ ~ ~in 3,3-dietho~ypropene.~~O Aryl glycosides with an a-linkage undergo photochemical cleavage more rapidly than those with a P-linkage.l3l The benzophenone-sensitizedphotodecomposition of 2-alkoxytetrahydropyrans (187) is initiated by hydrogen abstraction and not C-0 cleavage; the ratio of products (188) and (189) is dependent on the nature of the s ~ b s t i t u e n t s . ~ ~ ~ New examples of the well known [02-+ ,2] photocleavage of cyclobutane derivatives have been reported. Bond cleavages which release the highest amount

of steric interaction are preferred. Thus, the cyclobutane (190) affords only the biphenyl (191), whereas the all-cis-isomer undergoes the alternative cleavage resulting in fragmentation to phenanthrene and t r a n ~ t i 1 b e n e . lThe ~ ~ formation

Ph

lz8 lZ9 130

131 132

13a

4

+ (-jJ;+co

M. Yoshida and R. G. Weiss, Tetrahedron, 1975, 31, 1801. V. P. Karmazin, E. P. Olekhnovich, M. I. Knyazhanskii, and G. N. Dorofeenko, Zhur. org. Khim., 1975, 11, 1137. R. Sastre, M. V. Dabrio, and Y. J. L. Mateo, Anales de Quim., 1974, 70, 905. T. Yamada, M. Sawada, and M. Taki, Agric. and Biol. Chem. (Japan), 1975, 39, 909. C. Barnasconi and G. Descotes, Compt. rend., 1975, 280, C , 469. G . Kaupp and W. H. Laarhoven, Tetrahedron Letters, 1976, 941.

Photochemistry

534

of phenanthrene from a tetrabenzo[a,c,g,i]cyclododecene seems to involve an A new and efficient synthesis of barrelene (192) analogous photo~1eavage.l~~ has been accomplished in this way by irradiation of the cyclobutane (193) in t e t r a h y d r ~ f u r a n ,and ~ ~ ~ conclusive evidence for the intermediacy of cyclobutadiene in the photodecomposition of cyclobutene (194) has now been A number of substituted c&dimethoxycarbonylstilbene oxides p~b1ished.l~~ 11 photocycloelimination reaction to give aryl methoxyundergo a 13 -+2 carbonylcarbene~.~~~ In alkyl amides, a Type I1 elimination process is inefficient in comparison with Type I ~1eavage.l~~ Triarylmethane leuconitriles undergo heterolytic cleavage on irradiation in ethanol to form a dye cation and cyanide The cyano-radical, rarely encountered in a liquid-phase organic system, has been generated by photolysis of benzoyl cyanide in cyclohexane or benzene Photodecomposition of the a-peracetoxynitrile (195) in benzene or t-butyl alcohol yields the 8-ketonitrile (196) regioselectively in 52% ~ i e 1 d . l A ~ ~mechanism consistent with this

+

1

(195)

- EO II

- H.

(196) Scheme 8

observation is outlined in Scheme 8. Irradiation of the isomeric triphenylmethylA%oxazolines (1 97) is accompanied by elimination of the triphenylmethyl radical followed by fragmentation of the isoxazoline radical to give the ketone (198) and the nitriles (199).142 Photochemical intra- and inter-molecular elimination of HCI, HBr, and HI, arising in many cases by initial carbon-halogen bond cleavage, has again been widely described. In this way, the photodecomposition of a variety of substituted 2-iodobenzylamine hydrochlorides (200) in aqueous solution provides a convenient route to the corresponding 6,7-dihydro-5H-dibenz[c,e]azepines la4 135

G. Wittig and G . Skipka, Annalen, 1975, 1157. W. G. Dauben, G . T. Rivers, R. J. Twieg, and W. T. Zimmerman, J. Org. Chem., 1976,41, 887.

lS6 lS7 lS8

lSo 140 142

R. D. Miller, D. L. Dolce, and V. Y. Merritt, Tetrahedron Letters, 1976, 1845. G. W. Griffin, D. M. Gibson, and K. Ishikawa, J.C.S. Chem. Comm., 1975, 595. P. H. Mazzocchi and M. Bowen, J. Org. Chem., 1976,41, 1279. M. L. Herz, J. Amer. Chem. SOC.,1975,97, 6777. J. Kooi and J. H. Boyer, J.C.S. Perkin I, 1975, 2374. D. S. Watt, J. Amer. Chem. SOC.,1976, 98, 271. H. Kaufmann and J. Kalvoda, J.C.S. Chem. Comm., 1976,210.

Photoelimination

535 OAc

Ph3C

H

"0

(197)

I

536

Photochemistry

(201).143An analogous approach has been successfully employed in the synthesis of many alkaloids including ( k )-n~rpredicentrine,l~~ ( k )-actin~daphnine,l~~ a t h e r ~ l i n eprotoberberine,14' ,~~~ and ( k )-bo1dine.l4* Similarly, irradiation of the metacyclophane (202) affords the tricycle (203).149 Numerous examples of the photocyclization of chloroacetamide derivatives have again been described. Thus, for example, the indole (204) is converted into the azepinone (2O5).l5O The cyclizations of N-chloroacetyl-3-methoxyphenethylarnine,l5l N-~hloroacetyl-2,5-dimethoxyphenethylamine,~~~ and N-chloroacetyl derivatives of indolylethylamines153 have also been reported, and the cyclization has been used in the synthesis of the quebrachamine-dihydrocleavamine skeleton 154 and of benzazocine derivatives.155 (206) Elimination of HBr from 3,5-dibromo-2,6-dimethylhepta-2,5-dien-4-one by irradiation (300nm) in hexane yields the cyclopentenone (207) as the major

product together with a cyclobutane dimer.166 The photoelimination of HBr from trans,trans-2,4-dibromo-l,5-diphenylpenta-l,4-dien-3-one has also been examined.15' On irradiation of dibromomaleic anhydride (208) with N-phenylpyrrole (209), photosubstitution and photocyclization took place in successive

143 144

146

P. W. Jeffs, J. F. Hansen, and G. A. Brine, J. Org. Chem., 1975, 40, 2883. M. S. Premila and B. R. Pai, Indian J. Chem., 1975, 13, 13. M. S. Premila, B. R. Pai, and P. C. Parthasarathy, Indian J. Chem., 1975, 13, 945. T. Kametani, R. Nitadori, H. Terasawa, K. Takahashi, and M. Ihara, Heterocycles, 1975, 3, 821.

T. Kametani, K. Fukurnoto, M. Ihara, M. Takernura, H. Matsumoto, B. R. Pai, K. Nagarajan, M. S. Premila, and H. Suguna, Heterocycles, 1975, 3, 811. S. M. Kupchan, C.-K. Kim, and K. Miyano, J.C.S. Chem. Comm., 1976,91. 149 S . Hirano, H. Hara, T. Hiyama, S. Fujita, and H. Nozaki, Tetrahedron, 1975, 31, 2219. lri0 R. J. Sundberg and F. X. Smith, J. Org. Chem., 1975, 40, 2613. 151 Y. Okuno and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 1039. lS2 Y. Okuno, M. Kawamori, K. Hirao, and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 2584. I b 9 S. Naruto and 0 . Yonernitsu, Tetrahedron Letters, 1975, 3399. 164 R. J. Sundberg and R. L. Paton, Tetrahedron Letters, 1976, 1163. lS6 Y. Sawa, T. Kato, A. Morimoto, M. Toru, M. Hori, and H. Fujimura, Yakugaku Zasshi, 1975, 95, 261 (Chem. A h . , 1975, 83,28 074). lri6C. W. Shoppee and Y . Wang, J.C.S. Perkin I, 1975, 1595. lS7 C. W. Shoppee and Y . Wang, J.C.S. Perkin I, 1976, 695. 14?

Photoelimination

537

steps to yield pyrrolo[l,2-a]quinoline-4,5-dicarboxylic anhydride (210) as the final product.15* The photoinduced benzoylation of anthracene with benzoyl chloride has been described.159 Many other decomposition reactions arising by carbon-halogen homolytic bond cleavage have been described, but these are essentially radical processes having no special photochemical significance, and so are not included in this Report. lS8 lsS

T. Matsuo and S . Mihara, Bull. Chem. SOC.Japan, 1975, 48, 3660. T. Tamaki, J.C.S. Chem. Comm., 1976, 3 3 5 .

Part IV POLYMER PHOTOCHEMISTRY By D. PHILLIPS

1 Introduction The severe limitations of available space in this volume have necessitated drastic curtailment of this section this year: thus radiation effects are now excluded.

2 Photopolymerization Two useful reviews of photopolymerization have appeared,lV pertinent to the coatings, printing ink, and lithography industries.

the former

Photoinitiation of Addition Polymerization.-A thorough review of the chemistry involved in the widely used aromatic carbonyl-type photoinitiators has been presented by an authority in this field.3 A review of novel photoinitiators of the metal carbonyl type, such as [Mn,(CO),,], [Re,(CO),,], etc., has appeared:, the rhenium compound is the best for polymerization of fluoro-olefins, facilitating the formation of block copolymers.6 The Lewis acids VCI,, TiCl,, TiBr,, SnCl,, and AIBr, photoinitiate radical-cation polymerization of isobutylene with excitation in the 400-480 nm region.s Charge-transfer complex formation between monomer and metal compound prior to excitation is implicated. Vinyl polymerization by iron(~~~)-salt-saccharide,~ iron(r~~)-amine-CCI,,~and poly(viny1amine)-copper(r1) O a systems has been reported. Vanadium(v) and platinum(r1) chelates have also been used to initiate addition polymerization.9b Complexes of molecular chlorine with vinyl monomers such as methyl methacrylate (MMA), ethyl methacrylate, vinyl acetate, styrene, and methyl acrylate upon photoexcitation exhibit initiation of polymerization with efficiencies of monomers increasing in the order shown.lo Benzoin-pyridine and CCl, l1 and 1,1,l-trichloro-3-phenylpropanel2 have been used to initiate 1

4 6

9

7 8

0)

10 l1

12

R. B. Cundall, J. Oil Colour Chemists' Assoc., 1976, 59, 95. S. S. Labana, J . Macromol. Sci. (Chem.), 1974, C11, 299. A. Ledwith, J . Oil Colour Chemists' ASSOC.,1976, 59, 157. S. M. Aliwi, C. H. Bamford, and S. U. Mullik, J. Polymer Sci.,Polymer Symposia, 1975,50,33. C. H. Bamford, Polymer, 1976,17, 321; C . H. Bamford and S. U. Mullik, ibid., p. 225; J.C.S. Farady I, 1976, 72, 368. M. Marek, L. Toman, and J. Pilar, J. Polymer Sci., Polymer Chem., 1975, 13, 1565. T. Okimoto and Y . Inaki, Angew. Makromol. Chem., 1974, 36, 27. Y . Inaki, M. Takahashi, and K. Takemoto, J . Macromol. Sci. (Chem.), 1975, A9, 1133. K. Kimura, Y . Inaki, and K. Takemoto, J. Macromol. Sci.(Chem.), 1975, A9, 1399. S. M. Aliwi and C. H. Bamford, J.C.S. Faraday I, 1975, 71, 1733; C. H. Bamford, S. U. Mullik, and R. J. Puddephatt, ibid., p. 2213. P. Ghosh and S. Chakraborty, J. Polymer Sci., Polymer Chem., 1975, 13, 1531. K. Inoue, N. Nakagawa, and T. Tanigaki, Polymer J., 1976, 8, 254. C.A. Barson, R. A. Batten, and J. C. Robb, European Polymer J., 1975, 11, 381.

541

542 Photochemistry polymerization of methyl methacrylate and styrene respectively. Br, (for MMA),13 the pyridine-Br, charge-transfer (CT) complex (for MMA and other vinyl monom e r ~ )the , ~ ~quinoline-Br, CT complex (for MMA),15 N-bromosuccinimide (for MMA and other vinyl monomers),lS 2,4,6-tribromophenol (for MMA),17 and 1,2-dibromotetrafluoroethane (for tetrafluoroethylene, TFE) have been used successfully as photoinitiators. Photopolymerization of the following monomers has been reported : ethylene,lg ethylene with formamide,20vinyl fluoride,21viny1 acetate (under high pressure),22 3-0xaperfluorobutene,~~ MMA with an acriflavine dye,24MMA in the presence of saccharides,2Sacrylonitrile with aromatic hydrocarbons and benzophenone 28 (summarized in Scheme l), acrylonitrile with substituted triphenyl ph~sphites,~' (u)

N

+ hv@>

310 nm)

AN + lN* + *(N

(b) B

+ hu(A>310nm)

__+

'B

4 3B

R* + BH. B

intermediates giving freeradical polymerization

N

B

N

+ Iwl,

3

N

N

AN)'$

AN

> 3(N.. . - . AN)

+ hVl,

AN = acrylonitrile, N = naphthalene, B = benzophenone; (a) case where naphthalene alone is excited ; (b) benzophenone-naphthalene mixtures with B preferentially excited

Scheme 1

methacrylonitrile,28acrylamide, 29 NN-bis-(2-cyanoethyla~rylarnide),~~ and N-cycloalkylacrylamides of types (1) and (2).31 l3

l4 l6

l6 17 la

20

21 22

ad 25 26

27 28

z9 30

31

P. Ghosh, J. Polymer Sci., Polymer Letters, 1975, 13, 439. P. Ghosh and P. S. Mitra, J . Polymer Sci., Polymer Chem., 1976. 14, 981. P. Ghosh and P. S. Mitra, J . Polymer Sci., Polymer Chem., 1975, 13, 921. P. Ghosh and P. S . Mitra, J. Polymer Sci., Polymer Chem., 1976, 14, 993. T. Tanigaki and S . Asami, Nippon Kagaku Kaishi, 1975, 1076. W. S. Mungall, C. L. Martin, and G. C. Borgeson, Macromolecules, 1975,8, 934. T. J. Pullukat, Mukromol. Chem., 1975, 176, 2479. H. P. Rath, A. Saus, and B. Dederichs, 2.Nuturforsch., 1975, 30b, 740. D. Raucher and M.Levy, J. Polymer Sci., Polymer Chem., 1975, 13, 1339. M. Yokawa and Y. Ogo, Makromol. Chem., 1976, 177,429. V. A. Novikov, E. A. Manuilova, L. F. Sokolov, and S. V. Sokolov, Vysokomol. Soedineniya Ser. (B), 1975, 17, 82. K. P. Chakrabarti, J. Polymer. Sci., Polymer Chem., 1975, 13, 2051. H. Kubota, Y. Ogiwara, and K. Matsuzaki, J. Appl. Polymer Sci., 1976, 20, 1405. J. Barton, I. Capek, and P. Hrdlovic, J. Polymer Sci., Polymer Chem., 1975,13,2671; J. Capek and J. Barton, ibid., p. 2691. T. Taninaka, T. Ogawa, and Y. Minoura, J. Polymer. Sci., Polymer Chem., 1975, 13, 2353. P. Smith, R. D. Stevens, and L. B. Gilman, J. Phys. Chem., 1975, 79, 2688. T. Yamase and T. Ikawa, Bull. Chem. SOC.Japan, 1975, 48, 3738. C. Azuma and N. Ogata, J. Polymer Sci., Polymer Chem., 1975, 13, 741. N. Ogata, C. Azuma, and H. Itsubo, J. Polymer Sci., Polymer Chem., 1975, 13, 1959.

543

Polymer Photochemistry

H,C =CHCO

la 0

5

H,C=CHCON 0

(1) N-acrylylpyrrolidone (2) N-acrylylsuccinimide

A laser flash photolysis study of the interaction of triplet benzophenone with the monomers styrene ( S ) , methyl methacrylate (MMA), acrylonitrile (AN), vinyl acetate (VA), and the solvent THF gave measured second-order rate constants of 3.3 x lo8, 6.9 x lo7, 3.4 x lo7, 5.4 x lo6, and 3 x lo61 mol-1 s-l re~pectively.~~ The rate constant for styrene is so high (presumably because of efficient exothermic energy transfer) that benzophenone is an inefficient photoinitiator for this monomer in THF. The reaction of ketyl radicals with VA, AN, and MMA had rate constants of 5.5 x lo3, 3.8 x lo3, and 9.0 x lo3 I mol-1 s-l respectively, meaning that pinacol is not a major reaction product when monomer concentrations are less than 1 mol I-1 and incident light intensities are low. The benzophenone-amine initiator 33 and alkyl aryl benzoin ether,36 disulphide~,~~ and peroxidic initiators 37 have been discussed. In the last study, luminescence in MMA polymerization was observed. Photopolymerization in the presence of pigments 38 has been discussed, and many useful references on technological applications will be found in ref. 39. The patent literature is summarized in the Appendix. Ionic polymerization has some advantages over free-radical polymerization, 41 N-Vinylcarbazole is easily and the field has been reviewed briefly polymerized by a radical-cation mechanism, and recent studies on this monomer , ~ ~ halogens and halogenusing Rhodamine 6G,42bromanil and ~ h l o r a n i l other ated and metal salts45 have been reported. The production of photosensitive polymers by the cationic polymerization of vinyl ethoxyacrylate has been described.46a The benzaldehyde-sensitized photopolymerization of penta-1,3-dienes from the gas phase on to surfaces has been reported.46b R. KuhImann and W. Schnabel, Polymer, 1976, 17,419. J. F. Kinstle and S. L. Watson, jun., J. Radiation Curing, 1976, 3, 2. a4 M. Hamity and J. C. Scaiano, J. Photochem., 1975, 4, 229. 36 S. P. Pappas and A. K. Chattopadhyay, J . Polymer Sci., Polymer Letters, 1975, 13, 483. 38 G. V. Leplyanin, S. R. Rafikov, E. G. Varisova, 0. I. Korchev, and F. Z. Galin, Vysokomol. Soedineniya Ser. A , 1976, 18, 597. 37 L. Matisovarychla, J. Rychly, and M. Lazar, Makromol. Chem., 1975, 176, 2701. 38 P. S. Pappas and W. Kuhhirt, J. Paint Technol., 1975, 47, 42; Z . W. Wicks, jun, and W. Kuhhirt, ibid., p. 49. 39 ‘Non-silver Photographic Processes’, ed. R. J. Cox, Academic Press, London, 1975. 40 M. Irie, Y. Yamamoto, and K. Hayashi, J. Macromol. Sci. (Chem.), 1975, A9, 817. 4 1 D. Phillips, J. Oil Colour Chemists’ ASSOC., 1976, 59, 202. p 2 R. A. Crellin and A. Ledwith, Macromolecules, 1975, 8, 93. 45 M. Shimizu, K. Tada, Y. Shirota, S. Kusabayashi, and H. Mikawa, Makromol. Chem., 1975, 176, 1953. 44 M. Biswas, J. Macromol. Sci. (Chem.), 1976, C14, 1. 46 M. Asai, Y. Takeda, S. Tazuke, and S. Okamura, Polymer. J., 1975, 7, 359; Y. Takeda, M. Asai, and S. Tazuke, ibid., p. 366. 4e0 T. Nishikubo and T. Ichijyo, J. Appl. Polymer Sci., 1976, 20, 1133. 466 G. R. de Mare, J. R. Fox, M. Termonia, and B. Tshibangila, European Polymer J., 1976, 12, 119. sa

33

544

Photochemistry

Photocondensation Polymerization and Photochemical Cross-linking.-The Paterno-Biichi reaction of aromatic diketones with tetramethylallene forms a photopolymer (Scheme 2) through oxetan formation.47 Attempts to form 1 : 1

J-? c-c, ,c-c'y ?-7 X Scheme 2

&

adducts of aromatic diketones with furan were unsuccessful, but 2 : 1 furan : diketone adducts were formed more easily. Photosensitive poly(amin0-acids) with acrolyl, methacrolyl, and cinnamoyl side-chains have been 49 Four-centre photopolymerization in the solid state of m-phenylene diacrylic acid dimethyl ester proceeds as in Scheme 3.60 The formation of amorphous oligomer is in contrast with the related case of distyrylpyridazine where a crystalline polymer results. The reaction in

,(MeO,CHC=HC O

C

H =CHC0,Me 1

C0,Me z ! : H = C C0,Me H C O , M e ) MeO,CHC=CH Scheme 3

Scheme 3 has been shown to proceed in two stages, i.e. the formation of a topochemical dimer in a regular lattice followed by subsequent random cycloaddition in a disordered lattice. Solid-state polymerization has been reviewed.61

49

D. J. Andrews and W. J. Feast, J. Polymer Sci., Polymer Chem., 1976, 14, 319, 331. M. Nanasawa and H. Kamogawa, Bull. Chem. SOC.Japan, 1975,48,2588. Y. Kadoma, A. Ueno, K. Takeda, K. Uno, and Y. Iwakura, J. Polymer Sci.,Polymer Chem.,

so

F. Nakanishi, H. Nakanishi, M. Hasegawa, and Y. Yamada, J . Polymer Sci., Polymer Chem.,

s1

M. Nishii and K. Hayashi, Ann. Rev. Materials Sci., 1975, 5 , 135.

0

1975, 13, 1545. 1975,13,2499.

Polymer Photochemistry

545

Photocross-linkable resins based upon the benzylideneacetophenone (chalcone) 6 2 and expoxide 63 moieties have been discussed, and cross-linking of poly-(Zphenylbutadiene) through charge-transfer interactions with tetracyanobenzene 54 and poly(isopropeny1 styryl ketone) 66 has been investigated. The surface photopolymerization of TFE 6g and u.v.-induced self-adhesions of poly(ethy1ene terephthalate) (PET) 67 have been reported. Photografting.-The photografting of MMA on to poly(viny1 alcohol) fibres,6a acrylic monomers on to fibrous substrates using b i a ~ e t y l ,amine~~ and ketosubstituted polystyrenes,60AN,gf and other polymers g2 to silica, and grafting of poly(styrene-ah-acrylonitrile) g3 have been discussed. 3 Optical Properties and Luminescence of Polymers An excellent review of photoluminescence in synthetic polymers and its many uses has appeared.g4 Excitons and polaritons in polymers have been discussed,g5 as has the influence of molecular conformation upon intramolecular energy transfer in macromolecules reviewed by leading authorities in the field.sg Observation of depolarization of luminescence from polymeric species (often with luminescent probe molecules attached) yields important information about rotational relaxation in solution. A new technique to improve such measurements, in which simultaneous measurements of fluorescence polarization and quenching are made, has been reportedg7 and a simplified theory describing orientational relaxation by fluorescence correlation described.gs The use of such techniques on PMMA,69 PS,'O and other systems 7 2 including copolymers of 4-vinylpyridine and anthrylmethyl methacrylate 73 has been reported. The use of fluorescence depolarization in this manner limits the range of environments for study owing to the short duration of this luminescence. The time-scale for rotational relaxation can be extended, permitting the study of more viscous 71t

63

63 64 66 66 I'

eo 62

63 64 66

66

67

'O

71 72

73

S. P. Panda, J. Polymer Sci., Polymer Chem., 1975, 13, 1757. S. P. Panda and D. S. Sadafule, J. Polymer Sci., Polymer Chem., 1975, 13, 2415. K. Kato, S. Okamura, and H. Yamaoka, J. Polymer Sci., Polymer Letters, 1976, 14, 211. I. Naito and A. Kinoshita, Kobunshi Ronbunshu, 1975, 32, 321. D. H. Maylotte and A. N. Wright, Faraday Discuss. Chem. SOC.,1974, No. 58, p. 292. D. K. Owens, J. Appl. Polymer Sci., 1975, 19, 3315. Y. Ogiwara, T. Yasunaga, and H. Kubota, J . Appl. Polymer Sci., 1976,20, 1413; Y. Ogiwara and T. Yasunaga, ibid., p. 1119. R. P. Seiber and H. L. Needles, J, Appl. Polymer Sci., 1975, 19, 2185. J. F. Kinstle and S. L. Watson, jun., J . Radiation Curing, 1975, 2, 7. N. I. Litsov and A. A. Kachan, Vysokomol. Soedineniya Ser. B, 1976, 18, 182. E. Papirer, V. T. Nguyen, J.-C. Morawski, and J. B. Donnet, European Polymer J., 1975, 11, 597. N . G. Gaylord and T. Tomono, J. Polymer Sci., Polymer Letters, 1975, 13, 697. A. C. Somersall and J. E. Guillet, J. Macromol. Sci. (Chem.), 1975, C13, 135. M. R. Philpott, J. Chem. Phys., 1975, 63, 485. R. E. Dale and J. Eisinger, Proc. Nut. Acad. Sci. U S A . , 1976, 73, 271. J. P. Bentz, J. P. Beyl, G. Beinert, and G. Weill, European Polymer J., 1975, 11, 711. J. T. Yardley and L. T. Specht, Chem. Phys. Letters, 1976, 537, 43. E. V. Anufrieva, Yu. Ya. Gotlib, and I. A. Torchinskii, Vysokomol. Soedineniya Ser. A , 1975, 17, 1169; M. G . Krakovyak and E. V. Anufrieva, Izuest. Akad. Nauk S.S.S.R., ser.$z.,l 975, 39, 2354. B. Valeur and L. Monnerie, J . Polymer Sci., Polymer Pliys., 1976, 14, 11, 29. I. A. Torchinskii and A. A. Darinskii, Vysokomol. Soedineniya Ser. A , 1976, 18, 413. G. Beck, J. Kiwi, D . Lindenau, W. Schnabel, Colloid and Polymer Sci., 1976, 254, 162. Yu. E. Kirsh, N. R. Pavlova, and V. A. Kabanov, European Polymer J., 1975, 11, 495.

Photochemistry media if the longer-lived phosphorescence is used as the probe, and such studies on the MS time-scale have recently been reported on PVA, PEMA, PS, poly(buty1 methacrylate), and PMMA,74using benzophenone and anthrone as probes. Energy migration may also contribute to fluorescence depolarization, and phosphorescence studies on copolymers of vinylbenzophenone and styrene and fluorescence studies on S-MMA, S-MA, and 1-vinylnaphthalene-MMA copolymers show that this does not occur in copolymers containing less than a few mole per cent of the luminescent comonomer, but increases (i.e. p-l, where p is the polarization, increases) linearly with the percentage of fluorophore up to 50-65%.76 At very high levels of fluorescent comonomer, fluorescence depolarization is complete. A comparison has been made of fluorescence, birefringence, and X-ray methods to study molecular orientation in poly(ethy1ene terephthalate) (PET) drawn fibres, and it was shown that use of 4,4’-dibenzoxazolylstilbene as a fluorescent probe provides a method capable of characterizing quantitatively the distribution of chain orientations only in the non-crystalline regions of semi-crystalline polymers, since fluorescent molecules are excluded from crystalline regions.76 It was further shown that PET behaves as a rubber with 5.6 freely jointed links between cross-link points. Polymer conformation through monitoring of magnetic field modulation of delayed fluorescence 7 7 and the use of oriented polymer films to determine optical transition moment directions in solute molecules 78 have been discussed. Relaxation processes near the glass transition temperature (T,)in polymers by means of excimer fluorescence 79 and the effects of tacticity on excimer formation in poly-(p-methylstyrene) 8o have been reported. Exciplex formation in poly(vinylnaphthalene) and poly(acenaphtha1ene) has been studied and compared with the model compounds ethylnaphthalene and acenaphthalene.sl In one series of experiments, the aromatic models and polymers were electron acceptors, with diethylaniline as donor, and in another the same aromatic molecules were used as electron donors with dicyanoanthracene as acceptor. In these latter experiments no exciplex emission from the aromatic polymers was observed. There have been several other reports of luminescence in polymers. In one, the distribution of localized electronic states in atactic poly(styrene) was discussed,82and in another the fluorescence of styrene as a model for lignin compounds was c ~ n s i d e r e d . The ~ ~ luminescence of the amide chromophore in poly(amides) such as Nylon 66 is still a matter of controversy. Strong emission in commercial samples of the polyamide has been attributed to ap-unsaturated carbonyls arising from aldol condensation reactions, and upon prolonged irradiation this type of emission disappears, implicating such species in photo-oxidation mechanism^.^^ One recent report, however, suggests that the amide chromophore 546

74 76 76

77 78 70 80 81

82

83 84

L. J. Miller and A. M. North, J.C.S. Faraday IZ, 1975, 71, 1233. C. David, D. Baeyens-Volant, and G. Gueskens, European Polymer J., 1976, 12, 71. J. H. Nobbs, D. 1. Bower, and I. M. Ward, Polymer, 1976, 17,25. P. Avakian, R. P. Groff, A. Suna, and H. N. Cripps, Chem. Phys. Letters, 1975, 32, 466. C. C. Bott and T. Kurucsev, J.C.S. Faraday ZI, 1975, 71,749. C.W. Frank, Macromolecules, 1975, 8, 305. T. Ishii and S. Matsunaga, Makromol. Chem., 1976, 177,283. C. David, N. Putman de Lavareille, and G. Gueskens, European Polymer J., 1976, 12, 365. T.J. Fabish, H. M. Satsburg, and M. L. Hair, J. Appl. Phys., 1976, 47, 940. H. Konschin, F. Sundholm, and G. Sundholm, Actu Chem. Scand., 1976, B30, 262. N. S. Allen, J. F. McKellar, and G. 0. Phillips, J. Polymer Sci., Polymer Chem., 1975,13,2857.

547

Polymer Photochemistry

itself is phosphorescent, albeit weakly compared with impurities and oxidation products, in the 443-470 nm region with a decay time of 0.2-0.6 s in ethanol at 77 K.ss These authors suggest that such phosphorescence can be excited via direct So+TI absorption, although such a process is too improbable to be of importance in photodegradation. Dye-polymer interactions in Nylon 66 have been discussed.86

1

Exclhtion

Wavelength n m

Figure Fluorescence excitation and emission spectra of polymer films (Reproduced by permission from Chem. and Ind., 1976, 692)

a/3-Unsaturated carbonyl groups, diagnosed earlier as of importance in the are believed also to photo-oxidation of poly(butadiene) and related be of importance in poly(propylene), as a new study reveals.88 Thus luminescence attributed wrongly earlier 89 to naphthalene contamination from exposure to urban atmospheres has been shown not to be due to this source, as the excitation spectra in the Figure clearly demonstrate, and is probably due to unsaturated c a r b o n y l ~which , ~ ~ may well render the polymer light-sensitive. A thorough study has shown that triplet energy migration in copolymers of styrene and vinylbenzophenone is efficient in both films and glassy solutions at 77 K.gl The results show that exchange interactions are not sufficient to account for observed efficiencies, and that more efficient transfer in ordered regions of the polymer must be of importance. The effect of PS on the quenching rate of benzil phosphores~ence,~~ fluorescence quenching of PS by OZyg3 and, more 85 86

n2

n3

J. A . Dellinger and C. W. Roberts, J. Polymer Sci. Polymer Letters, 1976, 14, 167. A. M. Athale and M. R. Padhye, J. Appl. Polymer Sci., 1976, 20, 403. S. W. Beaven and D. Phillips, European Polymer J., 1974, 10, 593; Rubber Chem. Technol., 1975, 48, 692; J. Photochem., 1975, 3, 349; S. W. Beavan, P. A. Hackett, and D. Phillips, European Polymer J., 1974, 10, 925. N . S. Allen, R. B. Cundall, M. W. Jones, and J. F. McKellar, Chem. and Ind., 1976, 110. D. J. Carlsson and D. M. Wiles, J. Polymer Sci.,Polymer Letters, 1973, 11, 759. N. S. Allen, J. Homer, and J. F. McKellar, Chem. and Znd., 1976, 692. C. David, V. Naegelen, W. Piret, and G. Gueskens, European Polymer J., 1975,11, 569. K. Horie and 1. Mita, Polymer J., 1976, 8, 227. M. Nowakowska, J. Najbar, and B. Waligora, European Polymer J., 1976, 12, 387.

Photochemistry

548

interestingly, fluorescence enhancement in organic polymers by o2(lAg)through reaction (1),94 where A is an aromatic chromophore, have been discussed, O,(lA,)

+ 3A

-

lA

+ 02(3Eg-)

(1)

E-Type delayed fluorescence of 1,l’-diacronyl in a plastic matrix g5 and the behaviour of fluorescent molecules in a polymerizing medium 96 have been investigated. There have been many studies on poly-(N-vinylcarbazole) (PVCZ) and related photoconducting polymers. MO calculations on the electronic structure of PVCZ 9 7 and energy migration in the solid state in this polymer 98-100 have been reported. In PVCZ, poly-(N-ethyl-2-vinyIcarbazole),and poly-(N-ethyl-3-vinyIcarbazole),lol an intrachain excimer fluorescence some 5400 cm-l to the red of the monomer 0-0 band is seen, but PVCZ is unique in exhibiting a second, higher-energy excimer band in addition. Results show that the geometrical arrangement required for this higher-energy band exists in the gound state of the polymer. Excimer formation in PVCZ in solution has also been studied,lo2and triplet energy migration and delayed luminescence in the species have been investigated.lo3 Excimer formation appears to require pendant carbazole groups on adjacent chromophores separated by three carbon atoms.lo2 Charge-transfer complexes of PVCZ and related polymers of the types (3) and (4) have been widely studied because of their p h o t o c ~ n d ~ ~ t i ~ i tTyYPe . ~ ~(3)~ - ~ ~ ~ was found to be a very inefficient photoconductor, probably owing to the poor electron-donor character and short lifetime of singlet excitons in the presence of R. D. Kenner and A. U. Khan, Chem. Phys. Letters, 1975,36,643; J. Chem. Phys., 1976,64, 1877. 96 M. Zander, 2.Naturforsch., 1975, 30a, 1097. 9B R. D. M. Neilson, I. Soutar, and W. Steedman, J. Polymer Sci., Polymer Chem., 1976, 14, 1005. 97 K. Hattori and Y. Wada, J. Polymer Sci., Polymer Phys., 1975, 13, 1863. gs B. Jezek, J. Pospisil, and I. Chudacek, Czech. J. Phys., 1975, 25, 1176. O0 R. M. Siegoczynski, J. Jedrzejewski, and A. Kawski, Acta Phys. Polon., 1975, A47, 707. l o o G. E. Venikouas and R. C. Powell, Chem. Phys. Letters, 1975, 34, 601. lol G. E. Johnson, J. Chem. Phys., 1975, 62,4697. lo2 M. Yokoyama, T. Tamamura, M. Atsumi, M. Yoshimura, Y. Shirota, and H. Mikawa, Macromolecules, 1975, 8 , 101. l o 3 R. D. Burkhart, Macromolecules, 1976, 9, 234. lo4 K. Okamoto, M. Ozeki, A. Itaya, S. Kusabayashi, and H. Mikawa, Bull. Chem. SOC.Japan, 1975,48, 1362. lo5 K. Okamoto, A. Itaya, and S. Kusabayashi, Polymer J., 1975, 7 , 622. log K. Okamoto, A. Itaya, and S. Kusabayashi, J. Polymer Sci., Polymer Phys., 1976, 14, 869. lo’ S. Moriwaki, K. Okamoto, S. Kusabayashi, and H. Mikawa, Bull. Chem. SOC.Japan, 1975, 48, 2623. l o 8 H. Ito, S. Tazake, and M. Okawara, Makromol. Chem., 1976, 177, 621. loo P. K. C. Pillai and R. C. Ahaja, Polymer, 1976, 17, 192. S. Tazuke and Y . Matsuyama, Macromolecules, 1975, 8, 280. M. Yokoyama, Y. Endo, and H. Mikawa, J. Luminescence, 1976, 12, 865. P. J. Reucroft, S. K. Ghosh, and K. Takahashi, J. Polymer Sci., Polymer Phys., 1975, 13, 1275. 113 M. F. Froix, D. J. Williams, and A. 0. Goedde, Macromolecules, 1976, 9, 81. 114 Y. A. Cherkasov, A. D. Lopatko, M. S. Borodkina, and T. V. Cheltsova, Zhur. nauch. priklad. Fotograf. Kinemat., 1975, 20, 370. 115 V. Gaidyalis, I. Vapshinskaite, A. Undzenas, and A. Lyudkyavichyus, Zhur. nauch. priklad. Fotograf. Kinemat., 1976, 21, 57. 116 K. Okamoto, N. Oda, A. Itaya, and S. Kusabayashi, Chem. Phys. Letters, 1975, 35, 483; K. Okamoto and A. Itaya, Chem. Letters, 1976, 99. 94

Polymer Photochemistry

549

the carbonyl group.1os The PVCZ-trifluoroenone CT complex was found to be a better photoconductor than PVCZ.log Electric ll1 and magnetic fields were found to increase the photocurrent by assisting the dissociation into free carriers from the non-relaxed exciplex state. EDA properties of thin polymer films of PTFE on silicon,l17 and photoconduction in poly(olefins), poly(ethylene-CO),l18 e p o x y - r e s i n ~ ,and ~ ~ ~ poly(diacetylene) single crystals 120 ( 5 ) have been investigated. There have been reports of photoelasticity in poly-(n-alkyl acrylates), poly(methacrylamide), and poly(methacry1amide-co-2-hydroxyethyl methacrylate) gels,121poly(viny1idene fluoride),12, and styrene-2-ethylhexyl acrylate copolymer fiims.123 Phot ochromic polymers have been further discussed.124-126

4 Photochemical Reactions in Polymers reactions in Photochemical Reactions in the Absence of O,.-Photochemical polymeric systems have been 128 Specific subjects covered in recent papers include photolysis of poly(forma1dehyde)12@ and glutaric anhydride-type 11'

H. R. Anderson, jun., F. M. Fowkes, and F. H. Hielscher, J. Polymer Sci., Polymer Phys.,

118

1976, 14, 879. G. Y. C. Chan and H. J. Wintle, J . Polymer Sci., Polymer Phys., 1975, 13, 1187.

G. E. Golubkov, V. I. Krainyukov, and B. N. Satyukov, Vysokomol. Soedineniya Ser. B, 1975, 17, 133. R. R. Chance and R. H. Baughman, J . Chem. Phys., 1976, 64, 3889. lal M. Ilavsky, J. Hasa, and K. Dusek, J . Polymer Sci., Polymer Symposia, 1975, 53, 239; M. Ilavsky and K. Dusek, ibid., p. 257. laa H. Ohigahi, J. Appl. Phys., 1976, 47, 949. l Z 3 A. E. Grishchenko, E. P. Vorob'eva, and V. T. Surkov, Vysokomol. Soedineniya Ser. B, 1975, 17, 820s. la' G. Smets, J . Polymer Sci., Polymer Chem., 1975, 13, 2223. J. Verborgt and G . Smets, J. Polymer Sci., Polymer Chem., 1975, 13, 2415. 126 M. Kryszewski, B. Nadolski, and A. Fabrycy, Rocznilci Chem., 1975, 49, 2077. la' G. Smets, Pure Appl. Chem., 1975, 42, 509. 12* G. Smets, J. Thoen, and A. Aerts, J . Polymer Sci., Polymer Symposia, 1975, 51, 119. la* L. L. Yasina and V. S. Pudov, Vysokomol. Soedineniya Ser. B, 1975, 17, 153. lrO

Photochemistry and photochemical transformations of methylox in p r ~ p y l e n e , ~ ~ ~ poly(viny1-p-azidobenzoate)in the presence of ole fin^,^^^ naphthalene in cellulose t r i a ~ e t a t e ,134 ~ ~photointeractions ~' of SO, with poly(viny1 photoisomerization of stilbene by a phenyl vinyl ketone-2-vinylnaphthalene copolymer,136photoreactions of fumaric and maleic derivatives in m ~ l t i l a y e r s and ,~~~ ferocene containing polymers.138 The photolysis of poly(propy1ene) (PP) under vacuum at 253.7 nm results in the formation of methane and ethane as additional products compared with thermal d e c o m p o ~ i t i o n . ~The ~ ~ photoreactions are caused by absorption of 550

Ph CONHPh

p-MeC,H,NHCOPh

(6)

(7)

P h C O N H o N HCOPh

e H C O P h PhCONH

253.7 nm radiation by Ti02 pigment residues. It was found that blending PMMA with PP stabilizes the former. Thermal and photoinduced phenomena in PMMA have been further 141 The quenching of chain-scission processes in copolymers of biphenyl methacrylate and 2-naphthyl methacrylate PhCONHPh

'"

[PhkO

+ fiHPhlGege-

PhkO

+ rjHPh

Scheme 4 lao

H.Hiraoka, Macromolecules, 1976, 9, 359.

E. M. Slobodetskaya, M. G. Vorob'ev, and 0. N. Karpukhin, Vysokomol. Soedineniya Ser. A , 1975, 17, 2533. lsa A. G. Filimoshkin, R. N. Nevedomskaya, I. P. Zherebtsov, and R. M. Livshits, Vysokomol. Soedineniya Ser. A, 1975, 17, 2260. lS3 L. N.Guseva, Yu. A. Mikheev, and D. A. Toptygin, Bull. Acad. Sci. U.S.S.R., 1974,23, 1910. n4 A.A. Degtyareva, A. A. Kachan, L. N. Sharovol'skaya, and V. A. Shrubovich, Vysokomol. Soedineniya Ser. A , 1975, 17, 2144. A. V. Oleinik, V. M. Treushnikov, and N. V. Frolova, Vysokomol. Soedineniya Ser. A , 1975, 17, 1989; 1975,17, 361. la6 S. Irie, M. Irie, Y. Yamamoto, and K. Hayashi, Macromolecules, 1975, 8, 424. la' R. Ackerman and D. Naegele, Makromol. Chem., 1974,175, 699. lS8 K. Kojima, S. Iwabuchi, T. Nakahira, T. Uchiyama, and Y. Koshiyama, J. Polymer Sci., Polymer Letters, 1976, 14, 143. lS0 N. Grassie and W. B. H. Leeming, European Polymer J., 1975, 11, 809, 819; N. Grassie, A. Scotney, and T. I. Davis, Makromol. Chem., 1975, 176, 963. 140 A. Torikai, T.Asai, and T. Suzuki, J. Polymer Sci., Polymer Chem., 1975, 13, 797. 141 E. Ya. Davydov, G. B. Pariiskii, and D. Ya. Toptygin, Vysokomol. Soedineniya Ser. A, 1975, 17, 1504. lal

55 1

Polymer Photochemistry

-

has been ~ e p 0 r t e d . l Using ~~ the Perrin model, effective radii of -16 A were found for these copolymers, compared with 13 A for quenching in copolymers of 1- and 2-vinylnaphthalene and acrylophenone and 10 A for related small model compounds. The photolysis of the fully aromatic amides (6)-(9) is believed to occur through the free-radical mechanism exemplified in Scheme 4, rather than the a1ternat ive molecular concerted react ion.143 Other photoreactions in poly(amides) have been d i s c ~ s s e d , l ~and ~ - ~photo~~ processes in aromatic poly(su1phones) 14' and n-alkylpyrrolidones 148 [as models for photoreactions of poly(vinylpyrrolidone)] investigated. N

Photo-oxidation and Weathering.-There have been several useful reviews of photodegradation (and stabilization) of p ~ l y m e r s , ~ specifically ~ ~ - ~ ~ ~ poly( o l e f i n ~ )lSo . ~ ~Reports ~~ on photodegradation in particular polymers are collated below. PoZy(oZefins) (PE, PP). The photo-oxidation of PE sensitized by ferric acetylacetonate-tristearyl phosphate has been described.153 Photoinitiation processes in PE 164 and PP 164s lS6 have been investigated. Titania pigments were found to be important initiators, and quenching of the anatase form by Nil1 chelates was found to be effective as a stabilizing method. The kinetics of photo-oxidation of isotactic PP 166 and the influence of light intensity 16' and 1-benzoyl-2-naphthol and 6-hydroxybenzanthrone lS8on this process have been investigated. PoZy(styrene) (PS). A review of photodegradation in styrene polymers has appeared lSQ and the effects of this upon permeability in PS and poly-(p-methylstyrene) have been reported.lso In copolymers of PS with poly(viny1 acetate) (PVA) it was found that the presence of PS does not affect the photodegradation of PVA, whereas PS is stabilized by the presence of PVA, probably because the PVA prevents extensive energy migration in PS.lS1 14%

I. Lukac and P. Hrdlovic, European Polymer J., 1975, 11, 767.

llS

D. J. Carlsson, L. H. Gan, and D. M. Wiles, Canad. J. Chem., 1975, 53,2337.

144

H.S. Koenig and C. W. Roberts, J . Appl. Polymer Sci., 1975, 19, 1847.

G. S. Zhdanov and U. K. Milinchuk, Vysokomol. Soedineniya Ser. A , 1976, 18, 3. S. Caccamese, P. Maravigna, G. Montaudo, A. Recca, and E. Scamporrino, J. Polymer. Sci., Polymer Letters, 1975, 13, 51. 147 N. V. Eliseeva, L. T. Danilina, and A. N. Pravednikov, Vysokomol. Soedineniya Ser. B, 1976, 18, 189. P. H. Mazzochi, F. Danisi, and J. J. Thomas, J. Polymer Sci., Polymer Letters, 1975, 13, 737. 149 N. S. Allen and J. F. McKellar, Chem. SOC.Rev., 1975, 4, 533. lS0 E. Cernia, E. Mantovani, and W. Marconi, J. Appl. Polymer Sci., 1975, 19, 15. lS1 V. Ya. Shlyapintokh, Plast. Massy, 1976, 47. lS2 D. J. Carlsson and D. M. Wiles, J. Radiation Curing, 1975, 2, 2. lSs T.Sato, H. Tamai, H. Deura, and K. Oba, Kobunshi Ronbunshi, 1975, 32, 598. lS4 N. S. Allen, J. F. McKellar, and D. G. M. Wood, J. Polymer Sci., Polymer Chem., 1975, 13, 23 19. lS5 D.J. Carlsson and D. M. Wiles, J. Macromol. Sci. (Chem.), 1976, C14, 65. 145

146

ls6 lS7

lS8

lS9 lE0

E. M. Slobodetskaya, 0. N. Karpukhin, and V. V. Amerik, Vysokomol. Soedineniya Ser. B, 1976, 18, 184. 0.N. Karpukhin, E. M. Slobodetskaya, V. V. Amerik, T. M. Fes'kova, and M. G . Vorob'ev, Vysokomol. Soedineniya Ser. B, 1975, 17, 749. P. Bentley and J. F. McKellar, J. Appl. Polymer Sci., 1976, 20, 1145. G. E. Sheldrick and 0. Vogl, Polymer Eng. Sci., 1976, 16, 65. R. Greenwood and N. Weir, Makromol. Chem., 1975, 176,2041 ;J. Appl. Polymer Sci., 1975, 19, 1409. A. Jamieson and I. C. McNeill, J. Polymer Sci., Polymer Chem., 1976, 14, 603.

552

Photochemistry

PuZy(amides)and PoZy(uretharzes). The role of ap-unsaturated carbonyls in amide d e g r a d a t i ~ nand , ~ ~ luminescence 84-86 and direct photolysis 145, 146 of amides have been discussed earlier. The effects of heat pretreatment and orientation in the photo-oxidative degradation of poly(caproamide) have been reported.ls2 The effects of metal acetylacetonates,ls3 adamantane,ls4 ethyl phenyl carbamates,ls5 and the structure of urethane groups 166 on the photo-oxidation of poly(urethanes) have been investigated. PuZy(vinyl chloride) (PVC). 1.r. and U.V. spectroscopy and gel permeation chromatography have been used in a recent study of the photo-oxidation of unprocessed PVC.le7 It was concluded that the mechanism is an autocatalytic chain scission initiated by carbonyl species which gives carboxylic acids among the main products (Scheme 5). Additives such as calcium stearate and an

c1

Cl 0 CI Cl JIPCH-CH,-CH--C-CHJVI I I I1 I Norrish Type I1

I’ypc 1

I

c1

c1 I

mC=CH2

Noi-ri~h I1 v +

+

I

I

c1

CICH,COCHm

I

c1

I

mcH-cH2&H-eo

J

c1 c1 I

mCH-CH,CH-CO,H a-chloro-acid

+

I

*CH-

I

c1

.1

I

HOO-CHm

CI I

mc=o p-chloro-acid chloride

Scheme 5

organotin stabilizer alter the rate but not the mechanism of degradation. The light and weather resistance of PVC,lSEthe role of THF in photodegradation of PVC,le9and the interaction of thermal stabilizers and U.V. absorbers in PVC 170 have been discussed. Elastomers. Photodegradation of chlorinated rubbers 171, 172 and the effect of zinc mercaptobenzothiazolate and its derived basic zinc salt in vulcanized rubbers 162 163 164

166

168

leS 170 171 172

E. V. Vichutinskaya and L. M. Postnikov, Vysokomol. Soedineniya Ser. B, 1976,18,279. E.-L. Cheu and Z. Osawa, J. Appl. Polymer Sci., 1975, 19, 2947. S. S. Novikov, A. P. Khardin, N. G. Gureev, and S. S. Radchenko, Vysokomol. Soedineniya Ser. A , 1976, 18, 619. 2. Osawa, E.-L. Cheu, and Y. Ogiwara, J. Polymer Sci., Polymer Letters, 1975, 13, 535. 0.G. Tarakanov, M. N. Kurganova, E. K. Anisimova, and L. V. Nevskii, Vysokomol. Soedineniya Ser. B, 1975, 17, 461. G. Scott and M. Tahan, European Polymer J., 1975, 11, 535. G. Menzel, Angew. Makromol. Chem., 1975, 47, 181 ;K. V. Bassewitz and G. Menzel, ibid., p. 201. J. F. Rabek, J. Shur, and B. Ranby, J. Polymer Sci., Polymer Letters, 1975, 13, 1285. J. Wypych, J. Appl. Polymer Sci., 1976, 20, 279. C. More and H. Valot, Compt. rend., 1976, 282 C, 113. R. A. Petrosyan, K. A. Ordukhanyan, and R. V. Bagdasaryan, Vysokomol. Soedineniya Ser. A, 1975, 17, 1831.

553

Polymer Photochemistry

in preventing oxidation 173 have been investigated. Reactions of 02(lAg) with 176 unsaturated polymers have been Cellulose. There have been a series of papers reporting an extensive e.s.r. study of radicals produced in photo-irradiated c e l l u l o ~ e . ~ Other ~ ~ - ~ papers ~~ on cellulose triacetate 179s 180 and lignin have appeared. Wool. In a thorough study on the triplet state of tryptophan in the solid environments of poly(viny1 alcohol) (PVAL) film and in the wool protein keratin, it has been concluded that triplet-triplet interactions play a major role in the deactivation of this species in PVAL films, whereas in wool keratin in the presence of air the major loss mechanism appears to involve interaction of triplet tryptophan with oxygen.182 The effects of metal ions18S and of 2-pyrazoline-type fluorescent whitening agents lE4on the photo-yellowing of wool have been discussed. Photodegradable Polymers. Photodegradable vinyl lE6 poly(ethy1ene),lE7and a degradable polymer containing a pyrazine moiety 188 (10) have been reported. The patent literature is surveyed in Table A1 (Appendix).

(10)

U.V.-Stabilization.-A polymeric u . v . - a b s ~ r b e r , ~ 2-(-2’-hydroxypheny1)benzo~~ triazole absorbers,lQoand the interaction of thermal antioxidants and U.V. absorbers in PVC170 have been discussed. Other papers have reported the ~~~ testing of diffusion of 2-hydroxy-4-octoxybenzophenonein p o l y ( ~ l e f i n s ) ,the F. A. A. Ingham, G. Scott, and J. E. Stuckey, European Polymer J., 1975,11, 783. N. B. Zolotoi, M. N. Kuznetsova, V. B. Ivanov, G. V. Karpov, V. E. Skurat, and V. Ya. Shlyapintokh, Vysokomol. Soedineniya Ser. A , 1976, 18, 658. 176 A. Zweig and W. A. Henderson, jun., J. Polymer Sci., Polymer Chem., 1975, 13, 717, 993. 176 N . 4 . Hon, J. Polymer Sci., Polymer Chem., 1975, 13, 1933, 2363, 2416, 2641, 2653. N.-S. Hon, J. Polymer Sci., Polymer Letters, 1976, 14, 225. 178 N . 4 . Hon, J. Appl. Polymer Sci., 1975, 19, 2789. 17@ L. N. Guseva, L. E. Mikheeva, Yu. A. Mikheev, D. Ya. Toptygin, and V. F. Shubnyakov, Vysokomol. Soedineniya Ser. B, 1975,17, 117. l a 0 V. I. Gol’denberg, E. V. Bystritskaya, V. I. Yustl, 0. A. In, V. Ya. Shlyapintokh, and I. Ya. Kalontarov, Vysokomol. Soedineniya Ser. A , 1975, 17, 2779. lB1 G . Gellerstedt and E. L. Pettersson, Acta Chem. Scand., 1975, B29, 1005. la* K. P. Ghiggino, C. H. Nicholls, and M. T. Pailthorpe, Photochem. and Photobiol., 1975, 22. 169. lS3 G. H. Smith, Textile Res. J., 1975, 45, 483. u4 N. A. Evans, D. E. Rivett, and P. J. Waters, Textile Res. J., 1976, 46, 214. la6 B. Freedman and M. J. Diamond, J. Appl. Polymer Sci., 1976, 20, 463. la8 B. Freedman, J. Appl. Polymer Sci.,1976, 20, 911, 921. lS7 V. Pozzi, A. E. Silvers, and L. Giuffre, J. Appl. Polymer Sci., 1975, 19, 923. laBM. Sakuragi, M. Hasegawa, and M. Nishigaki, J. Polymer Sci., Polymer Chem., 1976, 14, 521. lB9 Y. Mizutani and K. Kusumoto, J. Appl. Polymer Sci., 1975, 19, 713. l B 0 M. N. Volkotrub, T. A. Rubstova, and A. F. Lukovnikov, Vysokomol. Soedineniya Ser. A , 1976, 18, 62. lg1 M. Johnson and J. F. Westlake, J. Appl. Polymer Sci.,1975, 19, 1745. 173 17’

554 Photochemistry materials as potential stabilizersyfQ2the measurement of U.V. radiation in accelerated weathering and the use of poly(pheny1ene oxide) as a dosimeter for U.V. r a d i a t i ~ n . ~ IBS ~*~ The patent literature is surveyed in Table A2 (Appendix).

5 Appendix: Review of Patent Literature Photopolymerizable Systems.-Pa tents of interest concerning pho topolymerizable and U.V. curing systems can be found in references 196-296* and under the following British patent numbers : 1400504 1404378 1 406 467 1407795 1 408 466 1 411 966 1 413 410 1 415 378 1417750 1 420 888 1 422 192

1400798 1404497 1 406 741 1407898 1 409 832 1 412 015 1 414 065 1 415 883 1418804 1 420 958 1 422 778

1400978 1404687 1 406 742 1408265 1 409 833 1 412 252 1 414 521 1 417 088 1419187 1 421 078 1 423 548

1400979 1405324 1 406 780 1408412 1 411 295 1 412 290 1 414 671 1 417 396 1420064 1 421 538 1 424 443

1400 988 1 405 865 1 407 069 1 408 413 1 411 677 1 412 754 1 414 837 1 417 404 1 420 351 1421 854

V. F. Tsepalov, Uspekhi Khim., 1975,44, 1830. E. Capron and J. R. Crowder, J. Oil Colour Chemists’ ASSOC.,1975, 58, 9. lg4 A. Davis, G. H. W. Deane, and B. L. Diffey, Nature, 1976, 261, 169. lor,A. Davis, G. H. W. Deane, D. Gordon, G. V. Howell, and K. J. Ledbury, J. Appl. Polymer lg2 lgs

Sci., 1976, 20, 1165.

Agency of Industrial Sciences and Technology, JA 75 24 393. Agency of Industrial Sciences and Technology, JA 75 45 076. lg8 Agency of Industrial Sciences and Technology, JA 75 123 138. leg Agency of Industrial Sciences and Technology, JA 74 128 992. aoo Agency of Industrial Sciences and Technology, JA 75 24 392. 201 Arakawa Forest Chemical Industries Ltd., JA 74 15 633. 2oa Dainippon Ink and Chemicals Inc., JA 74 130 983. 2os Dainippon Ink and Chemicals Inc., JA 75 72 990. 204 Dainippon Printing Co. Ltd., JA 75 105 729. 2os Fuji Systems Co. Ltd., JA 76 20 788. 206 Grace W. R. and Co., JA 75 103 536. 807 Hokuetsu Paper M.F.G. Co. Ltd., JA 75 112 506. 208 Japan Oil Seal Co. Ltd., JA 75 114 489. Japan Oil Seal Co. Ltd., JA 75 124 983. 210 Kansai Paint Co. Ltd., JA 75 02 189. 211 Leben Utility Co. Ltd., JA 75 154 378. 212 Leben Utility Co. Ltd., JA 75 154 390. *lS Leben Utility Co. Ltd., JA 75 154 391. 214 Leben Utility Co. Ltd., JA 75 154 333. 216 Matsuda and Haruo, JA 75 67 885. 216 Matsushita Electric Works Ltd., JA 75 123 150. *I7 Mitsubishi Rayon Co. Ltd., JA 75 17 435. 218 Mitsubishi Rayon Co. Ltd., JA 75 92 341. 219 Mitsui Toatsu Chemicals Inc., JA 74 26 061. e20 Mitsui Toatsu Chemicals Inc., JA 75 56 425. 221 Nippon Oil Seal Industry Co. Ltd., JA 74 115 134. 222 Nippon Oil Seal Industry Co. Ltd., JA 75 63 087. 22s Nippon Oil Seal Industry Co. Ltd., JA 75 70 101. * Patents references: CA Canada, CZ Czechoslovakia, DT West Germany, GB Great Britain, IS Israel, JA Japan, NL Netherlands, SU Soviet Union, S W Switzerland, US United States. lg6

lg7

Polymer Photochemistry Nippon Oil Seal Industry Co. Ltd., JA 75 67 886. 225 Nippon Oil Seal Industry Co. Ltd., JA 75 29 600. 226 Nippon Paint Co. Ltd., JA 75 112 435. 227 Nippon Paint Co. Ltd., JA 74 115 128. 228 Nippon Paint Co. Ltd., JA 75 126 072. 229 Nippon Paint Co. Ltd., JA 75 129 650. 230 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 151 981. 2s1 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 149 73 1. Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 133 238. 233 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 04 152. 2s4 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 66 596. 2ss Nippon Telegraph and Telephone Public Corp., JA 75 98 832. 236 Research Institute for Production Development, JA 74 15 633. Sakuranomiya Kaguku K.K., JA 75 137 206. Shiotsu et al., JA 75 514 392. Shiotsu et Tutsumi, JA 75 116 539. 240 Teijin Ltd., JA 74 33 995. 241 Teijin Ltd., JA 74 44 935. 24a Toa Paint Co. Ltd., JA 74 47 889. 243 Toa Gosei Chemical Industry Co. Ltd., JA 75 56 423. a44 Toray Industries Inc., JA 74 23 304. 245 Toyo Ink M.G.F. Co. Ltd., JA 75 50 440. a46 Toyo Ink M.F.G. Co. Ltd., JA 75 50 441. Toyo Ink M.F.G. Co. Ltd., JA 75 59 487. 248 Toyo Ink M.F.G. Co. Ltd., JA 75 59 497. 24g Toyota Auto Body Co. Ltd., JA 75 67 871. 2 6 0 Wako Pure Chemical Industries Ltd., JA 74 85 174. 2s1 Bayer A.G., DT 2 430 081. 26a Bayer A.G., DT 2 349 979. Bayer A.G., DT Prog. Org. Coal. 32-115-39 1975. 264 Cellophane S.A. (France) DT 2 510 873. 2s6 Ciba-Geigy A.G., DT 2 507 008. 266 Ciba-Geigy A.G., DT 2 528 358. Continental Can Co., DT 2 505 448. Felten und Guilleaume Kabelwerke A.G., DT 2 459 320. 26,B Finna Michael Huber Miinchen, DT 2 438 724. 2Eo W. R. Grace and Co., DT 2 402 390. General Electric Co., DT 2 518 639. a6a I.C.I. Ltd., DT 2 457 575. 268 I.C.I. Ltd., DT 2 454 800. 264 I.C.I. Ltd., DT 2 522 756. 266 Knonos Titan G.m.b.H., DT 2 350 468. 266 Matsumoto et al., DT 2 420 409. 267 Mobil Oil Co., DT 2 521 986. 268 National Starch and Chemical Corp., DT 2 512 642. 26g Nippon Paint Co. Ltd., DT 2 442 879. 2 7 0 Nippon Paint Co. Ltd., DT 2 514 249. 271 Oce-Van der Grinten N.V., DT 2 503 526. 272 Reliance Universal Inc., DT 2 437 885. 273 Unisearch Ltd., DT 2 458 959. 274 Agency of Industrial Sciences and Technology, US 3 882 084. 276 Bridgestone Tyre Co. Ltd., US 3 870 620. 276 E. I. Du Pont de Nemours and Co., US 3 926 643. 277 W. R. Grace and Co., US 3 900 594. 278 W. R. Grace and Co., US 3 877 971. 279 W. R. Grace and Co., US 3 908 039. 280 Keuffel and Esser Co., US 3 909 273. 281 P.P.G. Industries Inc., US 3 861 945. 282 S.C.M. Corp., US 3 876 519. 283 S.C.M. Corp., US 3 878 075. 284 Sun Chemical Corp., US 3 926 641. 286 Sun Chemical Corp., US 3 926 638. 288 Sun Chemical Corp.,US 3 926 640. 287 University of California, US 3 933 607. 224

555

556 288

2sn 2n0

281 292

2n4 296

286

Photochemistry

Western Ltiho Plate and Supply Co., US 3 852 256. Western Litho Plate and Supply Co., US 3 923 761. Hoechst A.G., GB 1 377 526. W. R. Grace and Co., Fr. Demande 2 258 436. J. Parrein and E. Marechel (Fr.), Chim. Peint 22 238, 2-77-83. Muanyagipan Kuto. Intez., Budapest, Hungary, Magyar Kdm.Lapja, 1975,30, 241. Polychrome Corp., NL (Appl.) 73 17 187. All-Union Scientific Research Institute of the Chemical Industry (U.S.S.R.), SU 465 384, 488 113. Akhonedor and Tulyuganov, Russ. Khim. Drev., 1975, H3643.

Prodegradants and u.v.-sensitizers are given in Table A1 and references 297385. Sekisui Kagaku Kogyo K.K., GB 1 409 439. Hoechst A.G., GB 1 411 539. 299 Badische Anilin- and Soda-Fabrik A.G., GB 1 412 335. Badische Anilin- and Soda-Fabrik A.G., GB 1 424 620. *01 I.C.I. Ltd., GB 1 400 570. Mitsui Toatsu Chemicals Inc., GB 1 408 307. Sekisui Kagaku Kogyo K.K., GB 1 409 439. Hoechst A.G., GB 1 410 641. Mitsubishi Chemical Ind., GB 1 404 927. ao6 Hoeschts A.G., GB 1 411 538. 807 Union Carbide Corp., GB 1 412 021. Union Carbide Corp., GB 1 412 396. Badische Anilin- and Soda-Fabrik A.G., GB 1 412 861. *lo Badische Anilin- and Soda-Fabrik A.G., GB 1 412 877. Hoechst A.G., GB 1 414 693. s12 Hoechst A.G., GB 1 416 604. Konishiroku Photo. Ind. Co. Ltd., GB 1 418 216. 814 Daicel Ltd., GB 1 420 008. I.C.I. Ltd., GB 1421 913. s16 I.C.I. Ltd., GB 1423 655. s17 I.C.I. Ltd., GB 1 423 657. sls Sahi Chem. Ind. Co. Ltd., JA 75 100 141. sln Canon, K. K., JA 74 33 659. s20 Dain ichiseika Color and Chemicals M.F.G. Co. Ltd., JA 75 60 523. 821 Fr. SociCte Anono Jet, JA 74 71 030. a22 Ibonai and Masaru, JA 75 65 592. s2s Kayiya et al., JA 75 10 376. 324 Kureha Chemical Ind. Co. Ltd., JA 75 24 340. s25 Kureha Chemical Ind. Co. Ltd., JA 74 133 438. s26 Kureha Chemical Ind. Co. Ltd., JA 75 09 643. s27 Kureha Chemical Ind. Co. Ltd., JA 75 24 338. s28 Japan Oil Seal Ind. Co. Ltd., JA 75 70 485. s29 Mitsubishi Monsanto Chemical Co.. JA 74 61 234. 330 Mitsubishi Monsanto Chemical Co., JA 75 37 882. s31 Mitsubishi Monsanto Chemical Co., JA 75 38 741. s32 Mitsubishi Petrochemical Co. Ltd., JA 75 67 889. s3s Mitsubishi Plastics Industries Ltd., JA 74 52 243. ss4 Mitsui Toatsa Chemicals Inc., JA 75 61 444. sss Mitsui Toatsa Chemicals Inc., JA 75 16 741. 3s6 Mitsubishi Rayon Co. Ltd., JA 74 114 660. Nippon Soda Co. Ltd., JA 75 18 596. ss8 Nissek Plastic Chem. Co. Ltd., JA 74 78 740. s8n Nippon Zeon Co. Ltd., JA 76 06 242. s40 Sagami Chemical Research Center, JA 74 117 600. s41 Shiseido Co. Ltd., JA 75 158 630. s42 Shiseido Co. Ltd., JA 75 67 861. s43 Shiseido Co. Ltd., JA 75 82 152. s44 Shiseido Co. Ltd., JA 75 67 346. 345 Shiseido Co. Ltd., JA 75 52 153. 346 Shiseido Co. Ltd., JA 75 113 550. 2Q7

28s

557

PoZymer Photochemistry 347 348

34Q 350

351 35a

353 354 365

356 367

368 350

361 362

363 364 365

366 367

360 370

371 s72

373 374

876 876

377 378 37g 880

381 883

88s

Sumitomo Chemical Co. Ltd., JA 75 34 087. Sumitomo Chemical Co. Ltd., JA 75 34 044. Sumitomo Chemical Co. Ltd., JA 75 34 047. Toyobo Co. Ltd., JA 75 15 827. Bayer A.G., DT 2 554 534. Bayer A.G., DT 2 436 260. Bayer A.G., DT 3 915 823. Ciba-Geigy A.G., DT 2 516 168. Ethylene-Plastique S.A., DT 2 432 689. Fuji Photo Film Co. Ltd., DT 2 445 038. Hoechst A.G., DT 2 400 418. Hoechst A.G., DT 2 418 834. Kureha Chemical Industry Co. Ltd., DT 2 364 875. I.C.I. America Inc., DT 2 410 219. Montedison S.P.A. (Italy), DT 2 529 617. Ruhrchemie A.G., DT 2 357 035. Snama Progetti S.P.A. (Italy), DT 2 450 359. Snama Progetti S.P.A. (Italy), DT 2 450 367. Snama Progetti S.P.A. (Italy), DT 2 450 398. Solray et Cie. (Belg.), DT 2 513 200. Arco Polymers Inc., US 3 917 545. Arc0 Polymers Inc., US 3 903 024. Arc0 Polymers Inc., US 3 929 690. Continental Oil Co., US 3 808 272. Eastman Kodak Co., US 3 871 887. Eastman Kodak Co., US 3 912 697. I.C.I. Ltd., U.S. Patent Office T 921 026. Owens-Illinois Inc., US 3 941 759. Polaroid Corp., US 3 929 829. U.O.P. Inc., US Publ. Pat. (appl.) B 596 692. United States Dept. of Agriculture, US 3 932 352. United States Dept. of Agriculture, US 3 932 338. J. E. Guillet, CA 983 200. Bio-degradable Plastics Inc., Fr. Demande 2 234 337. Lion Fat and Oil Co. Ltd., Fr. Demande 2 249 903. Shin-Etsa Chemical Ind. Co. Ltd., Fr. Demande 2 235 353. Lion Fat and Oil Co. Ltd., NL (appl.) 73 14 895. Dainippon Ink and Chemicals Inc., JA 75 59 431. Koga et al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 482.

U.V.stabilizers are collated in references 386-452 386 387

388 38B 3B0

3Q1 3Q2 3Q3 394

3Q5 3Q6 3g7 3g8

30* 400 401

*02 403

Oo5 406 407

and Table A2.

Koga er al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 483. Koga et al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 494. Kyodo Chem. Co. Ltd. and Sumitomo Chem. Co. Ltd. JA 75 136 291. Mitsubishi Paper Mills Co. Ltd., JA 74 121 893. Nippon Shokubai Kagaku Kogyu Co. Ltd., JA 75 35 287. Nippon Shokubai Kagaku Kogyu Co. Ltd., JA 75 41 988. Sumitomo Chemical Co. Ltd., JA 75 121 178. Sumitomo Chemical Co. Ltd., JA 75 120 486. Sumitomo Chemical Co. Ltd., JA 74 61 070. Teijin Ltd., JA 75 54 670. Yoshitomi Pharmaceutical Industries Ltd., JA 75 125 978. Sandoz Ltd., DT 2 432 098. Cincinnati Milacron Inc., US 3 888 823. Eastman Kodak Co., US 3 936 419. Eastman Kodak Co., US 3 939 115. Eastman Kodak Co., US 3 900 442. Martin Processing Co. Inc., US 3 943 105. Monsanto Co., US 3 928 264. Phillips Petroleum Co., US 3 867 342. R. F. Reinisch and G. R. Hermilo, US (appl.) 414 043. Weston Chemical Corp., US Publ. Pat. (appl.) B 54 859. Canadian Titanium Pigments Ltd., CA 962 142.

19

General formula

U.V.

sensitizers Specification R is a G-C, alkyl which may be substituted by halogen C1--6alkoxy, or nitro or may be bonded to the C atom at the ortho position of the benzene nucleus directly or in a CO group or a group of formula

R2

R3

II

provided that the C atom adjacent to the CO group is bonded to the cation at the ortho position of the benzene nucleus directly or via the CO group and R1--Bis either H, halogen, OH, C,, alkyl, C14 alkoxy, or nitro, provided that when at least one of the said group is not bonded to the C atom at the ortho position of the benzene nucleus

R1ac-R

0

Table A1 Prodegradants and

Application Used in conjunction with at least one aliphatic carboxylic acid or a Zn, Mg, Al, Ca, or Ba salt thereof for the photodegradation of polyolefins

297

Ref.

V

Oxymethane polymers

0.1/5% used by weight as a light sensitizer for polystyrene, polyolefin, poly(viny1 chloride) or polylactam, polyurethane, polyester, or polyamide systems Used as a sensitizer in the above system, especially for polyolefins such as polyethylene, polypropylene, and polybut-1-ene

A light sensitizer which is acenaphthene quinone and/or aceanthrene quinone or a mixture thereof with anthrone and/or naphthaquinone X is H, CH20R3[R3 = H, COCR4=CH,, or CO(CH,),,Me]; R, R1,R2,and R4 are H or C14 alkyl

R is H, ClW3alkyl or alkoxy, sulpho, nitro, or halogen; A = ammonium or alkali-metal ion; n = 2 or 3, a = 2or3,andn - a = Oorl

300

299

298

4

2> 2F

0

ca

3

3

&*cr

560 408

409

410

411 412

413 414

416 416 417 418

'19 OZ0

421 422

423 424 425 426

427 428

430

431 432 433

434 435 436

4s7 438

43s 440 441

442 443 444 445 446

447 448 44g

450

451 452

Photochemistry

Luston et al., CZ 159 526. Manasek et al., CZ 159 525. Kvuzat Poalim Lehityashret Shitufit B.M. (Israel), IS 39 037. U.S.S.R. Institute of Chemistry, Academy of Sciences, Tadzhik, U.S.S.R., SU 480 780. Institut FranGais du Petrole, GB 1 401 234. Unilika Ltd., GB 1401 895. Sanyko Co. Ltd., GB 1 401 924. Ciba-Geigy A.G., GB 1402 888. Ciba-Giegy A.G., GB 1 402 889. Ciba-Giegy A.G., GB 1 403 942. Ciba-Giegy A.G., GB 1401 163. Ciba-Geigy A.G., GB 1 411 301. I.C.I. Ltd., GB 1 411 436. Ciba-Giegy A.G., GB 1 411 515. Ciba-Giegy A.G., GB 1 411 656. Ciba-Geigy A.G., GB 1 411 657. Ciba-Geigy A.G., GB 1 415 266. Sanyko Co. Ltd., GB 1415 741. Bayer A.G., GB 1 416 415. Sanyko Co. Ltd., GB 1 417 835. Ciba-Geigy A.G., GB 1 418 701. Ciba-Geigy A.G., GB 1 418 783. Ciba-Geigy A.G., GB 1420 882. Daihto Chemical Industry Co. Ltd., JA 74 61 482. Chisso Corp., JA 76 11 839. Kanebo Ltd., JA 75 22 155. Mitsui Toatsu Chemicals Inc., JA 76 07 026. Nippon Kayaku Co. Ltd., JA 75 25 877. Nippon Kayaku Co. Ltd., JA 75 25 876. Showa Chemicals Industries Ltd., JA 75 200 79. Sumitomo Chemical Co. Ltd., JA 74 59 844. Toray Indust. Inc., JA 75 03 813. Uehara et al., JA 74 53 219. Bayer A.G., DT 2 419 766. Bayer A.G., DT 2 412 785. Ciba-Geigy A.G., DT 2 441 102. Ciba-Geigy A.G., DT 2 529 564. Ciba-Geigy A.G., DT 2 529 568. Ciba-Geigy A.G., DT 2 538 816. Ciba-Geigy A.G., DT 2 538 815. Ciba-Geigy A.G., DT 2 454 946. Eastman Kodak Co., DT 2 427 404. Henckel and Cie. G.m.b.H., DT 2263 940. Hercules Inc., DT 2 452 870. Hoechst A.G., DT 2 442 514.

Optical brighteners are collated in Table A 3 and references 453-475. 453

454

455 466 467 458 459

460 461 462 463

464 466

466 467

468

Sterling Drug Inc., US 3 935 195. Ciba-Geigy A.G., SW 560 277. Ciba-Geigy A.G., SW 567 607. Ciba-Geigy A.G., SW 560 236. Ciba-Geigy A.G., SW 557 917. Ciba-Geigy A.G., SW 561 746. Ciba-Geigy A.G., SW 566 420. Ciba-Geigy A.G., SW 561 327. Ciba-Geigy A.G., GB 1 400 963. Hoechst A.G., GB 1 402 326. Bayer A.G., GB 1 402 371. Ciba-Geigy A.G., GB 1 402 803. Sandoz Ltd., GB 1 403 564. Bayer A.G., GB 1 410 31 1. Sandoz Ltd., GB 1 411 989. Ciba-Geigy A.G., GB 1 412 049.

Polymer Photochemistry 468 470 471

47a 473

m 476

Sandoz Ltd., GB 1 414 155. Hoechst A.G., GB 1 414 669. Ciba-Geigy A.G., GB 1 416 116. Sandoz Ltd., GB 1 417 019. Ciba-Geigy A.G., GB 1 418 572. Ciba-Geigy A.G., GB 1 422 530. Bayer A.G., GB 1422 621.

561

R 2

R'

Y

I

6;:

R3- CH-CH,OH

Me

-

Me

CIass and general formula

Table A2 U.U.absorbers and stabilizers

R1 is q4alkyl; R2 is H or G4 alkyl; R3 is Cl-20 alkyl, alkenyl, phenylbenzyl, alkyl-phenyl, or alkylbenzyl, there being no more than two alkyl substituents, each having 1-8 C atoms. Others are based similarly on the same formula

R1 and R2 are the same or different and each is Me or Et or R1 and R2 having 1 to 12 C atoms, together with the C atom to which they are bound, form a Cs-12 cycloalkyl residue. Y is zero, H is a straight- or short branched-chain C1-,, alkyl residue, a C3-12 alkenyl or alkynyl residue, a C,-12 aralkyl residue, or a group having the general formula CH,CH(R)OH, where R = H, Me, or Ph R3 is H or a straight- or branchedchain C1-,, alkyl residue

Specification R1 and R2 are straight- or branchedchain alkyl residues. A full description can be found in the relevant patent

U.V.

by weight, used in the stabilization of polyolefins 0.01-5%

for example

Stabilization of polypropylene

AppIication Used as a stabilizer in many polymer systems

418

416

415

Ref.

4

5

g

2 2 Q

/N- .A-!0{R3

HN\

11

R2 0

R1 I

,c-c

F-F

II

R
R2 0

C-0

.OH

R4

\

R3

R5

Or C 4 4 cycloalkyl; R5 iS C2-18 alkylene or a group fCHR6CH,0f,CHR6CH2 where R6 = H or Me a n d n = 1-5; m = 2 4

C4-8 cycloalkyl; R4 is H, C14 alkyl,

R1,R2, and R3 are c1-8 alkyl or

R1 and R2 are the same or different alkyl groups or R1 and R2 together with a common C atom form a monocyclic C5-12 ring; n = 1, 2, 3, or 4 and each of R3 and R4,which are the same or different, is a alkyl group, at least one of RS and R4being an a-branched alkyl group

Polyolefins can be light-stabilized by the incorporation of an NG-,, alkyl or Neb-, cycloalkylphthalimide, the alkyl substituent being optionally further substituted by aryl or hydroxyl

Novel piperazine diones where R1 and R2 are the same or different alkyl groups or R1 and R2together with a common C atom form a monocyclic C5-1, ring; n = 1 or 2; when n = 1, R3 is a alkyl and when n = 2 R3 is a C2-8 alkylene group and A is a cl-6 straight- or branched-chained alkylene group in which the terminal C atoms each bear only H atoms on one alkyl group

Used in the photostabilization of,

As above

Photostabilization of poly(olefins)

polystyrene amongst a wide range of others

e.g., polyethylene, polypropylene,

424

421

420

419

3

4

(D

; s s2.

2F

ca

3 2

Q

Me

H

I

R

X N

~

Class and general formula

Table A2 (cont.)

R1-R5 are the same or different and each represents H or G4 alkyl, provided that 2-4 of R1-R5 are C14 alkyl; Rsis H, alkyl alkenyl, alkynyl, aralkyl, hydroxyalkyl, alkoxyalkyl, aliphatic or aromatic acyloxyalkyl, cyanoalkyl, halogenoalkyl, epoxyalkyl, alkoxycarbonyalkyl, aliphatic acyl, alkoxy CO group, or an aralkoxy CO group

Stabilization against light of polyethylene, polypropylene, polystyrene, etc.

U.V. stabilizer of polyacetates, polyesters, polyamides, polyuret henes, acrylonitrile-st yrenebutadiene copolymers, olefin, diene, or styrene polymers, or PVC

R is alkyl, alkenyl, alkenoyl (which may be aryl substituted), hydroxyalkyl, alkoxyalkyl, alkoxycarbyalkyl, acyloxyalkyl, cyanoalkyl, or nitroso ; X and Y are both S or 0

R1 is CN, CONR,, or C02R; R2 is CONR, or C0,R; R is H, straightor branched-chain C1-12 alkyl, or cyclohexyl

Application

Specification

427

426

425

Ref.

Further

U.V.

R2

stabilizers can be found in the following British patents: 1 403 210 1 403 324 1404925 1406872 1 417 437 1 417 502 1 421 937 1 422 853 1 424 119 1 403 916 1408 165 1 418 389 1 423 117

1 404 340 1415492 1 420 008 1 423 559

R1and R2 are c1-8 alkyl or C4-8 cycloalkyl; R3,R4,and R6are H, c1-8 alkyl, or c4-8 cycloalkyl or R5is a/s p-tolyl; R6is C144 alkyl when n = 1 or an n-valent C2--10 alkane residue when n = 2-6, whereby each C in the alkane residue may not be bonded to more than one N atom and n is an integer from 1 to 6 U.V. stabilization of polypropylene, Nylon 66, polystyrene, styrene-butadiene rubber, and polyacetal

0

N=Y-X

Class and general formula

Table A3 Optical brightening agents Specification R is an optionally non-chromophorically substituted phenyl, naphthyl, or biphenylyl group; X = H, halogen, an alkyl group, or an optically nonchromophorically substit u ted phenyl, naphthyl, or biphenylyl group. A fuller description can be found in the relevant patent

Application

466

463

Ref. 461

m

VI Q\

I

I\3

H

XP-n

R l

I.

Y

R4

I

H

k3.

XZ-n

R3

Optical brightening of Nylon 6 or 66, polyethylene, terrephthalate, PVC, and polystyrene

Each R or R represents H, C1-12 aralkyl with 1 4 C atoms in the alkyl part, G-18alkoxy, a phenylalkoxygroup with 1 4 C atoms in the alkoxy part C, or C, alkenyloxy, sulpho, carboxy or carboxyalkoxy or an ester or amide derivative of such a group, CN, or a sulphone group

474

473 Optical brightening of polyesters, PVC, polyacrylnitrile, polyurethanes, polyamide, polystyrene, and acetylcellulose

E.g. optical brightening 471 of PVC, polyacryonitrile, acetylcellulose, polyamide, polyurethane, and terephthalic acid-ethylene glycol polyester

R is ClW8alkyl optionally substituted by a non-chromophore radical, phenyl optionally substituted by a nonchromophore radical, C2+ alkenyl, or C, or C, cycloalkyl; X = H, C1, Me, MeO, or EtO; n = 1 or 2

R is C1-,,alkyl, C,, alkenyl, cycloalkyl, aryl, or aralkyl, optionally bearing one or more non-chromophore substituents; X is H, halogen, or C,, alkyl; A is H, sulphamyl, or halogen; B is H, halogen, or G4 alkyl or alkoxy

Optical brightening of polyesters, polypropylene, polyethylene, terephthaltate, and PVC

469

A is a residue of naphthalene, methoxynaphthalene, or acenaphthalene or benzene optionally substituted by Me and/or Me0 provided that if R1is CN and R4is H then either both of R2and R3 are other than H or one of them is H and the other CN, CO,Et, or CONH,, and if R1 = H at least one of R2and R3is CN or CONH,

R1is H or CN; R2 = H, CN, C02Et, CONH,, or Me; R3is H, CN, C02Et, or CONH,; R4 = H or Me;

2

4

3 5

%

2

z2

P 3 3

Class and general formula

Table A3 (cont.) Specification

biphenyl, or naphthyl radical; R2 = H, halogen, alkyl, or Ph; R3is CN, C02H, or carboxylic acid ester

R1is an optionally substituted styryl,

Application

Part V PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By

M. D. ARCHER

1 General Reviews Of the many general articles and reviews that have appeared this year on solar energy utilization, the following may be of particular interest to chemists. Two books,l$ both of which include sections on photobiological and photochemical solar energy conversion, have appeared. A series of seven review articles, covering all aspects of solar energy, has been published in the Bulletin of the Atomic S ~ i e n t i s t s : ~the - ~ articles on photochemistry,6 photovoltaics,s and photosynthesis are useful general introductions. Brinkworth lo has also provided a general discussion. An interesting review of the application of thin films and coatings to solar energy utilization l1 covers reflector films, antireflection coatings, and selectively absorbing coatings, as well as thin films in photovoltaic devices. Stein l2has reviewed the chemical storage of solar energy and photochemical fuel formation, concentrating particufarly on hydrogen formation by irradiation of aqueous solutions of Fe2+ and Eu3+. Simple thermal decomposition reactions for the storage of solar energy have also been discussed,13and ten criteria for a suitable reaction proposed. The thermal decomposition of ammonium hydrogen sulphate at ca. 500 “C seems to be the most promising: NH4HS04

NH3(I)

+ H20(Z) + S03(I);

AHo

-N

250kJmol-I

Finally, direct conversion methods have been generally reviewed at some length by Porter and Archer,14with emphasis on the similarity of the general principles that underlie superficially diverse devices.

a

@

lo l1 la

l3 l4

J. O’M. Bockris, ‘Solar Energy: The Hydrogen Alternative’, Wiley Interscience, New York, 1975. ‘Solar Energy’, ed. H. T. Messel and S. Butler, Pergamon Press, Oxford, 1975. C. Zener, Bull. At. Sci., 1976,32, 17. W. A. Shurcliff, Bull. At. Sci., 1976, 32, 30. E. Broda, Bull. At. Sci., 1976, 32, 49. M. Wolf, Bull. At. Sci., 1976, 32, 26. A. D. Poole and R. H. Williams, Bull. At. Sci., 1976, 32, 48. M. J. Antal, Bull. At. Sci., 1976, 32, 58. A. Makhijani, Bull. At. Sci., 1976, 32, 14. B. Brinkworth, Chem. in Britain, 1975, 11, 311. D. M. Mattox, J. Vacuum Sci. Technol., 1976,13, 127. G. Stein, Israel J. Chem., 1975, 14, 213. W. E. Wentworth and E. Chen, Solar Energy, 1976,18,205. G.Porter and M. D. Archer, Interdisc. Sci. Rev., 1976, 1, 119.

571

572

Photochemistry

2 Photochemistry Valence 1somerizations.-Jones l6 has examined the energy-storing isomerization of the dienone (1) to the cage isomer (2). The forward reaction has A H = 68.6 kJ

(1)

(2)

mol-l, and a photochemical quantum yield of 0.35-0.40 at 330-380nm; the energy storage efficiency, $AH/EA is thus ca. 0.08. The thermal process (2) --f (1) is slower than the isomerization of other cage compounds [(2) decomposes at 295 "C], but it can be catalysed by rhodium(1) complexes at 140 "C. However, the catalysts tend to decompose. Photochemical heat storage by a reaction of this type has been compared18 with a conventional hot water solar energy system. Conversion efficiency,

c

b

1

\ )s

c

3 a d

U

i?

c

W U .u.U

aJ

a

m

0.1

0.2

0.3 0.4 0.5 Chemical Efficiency

0.6

0.7

Figure 1 Plot of the specific energy capacity and chemical efficiency required of a photochemical system to match the winter operation of a typical (35% efficient) solar water heating system in the southern United States. The specific energy capacity is the specific thermal capacity of the reaction B --f A. The chemical efficiency is the fraction of the incident solar energy that is stored as chemical energy (Adapted from Solar Energy, 1975, 17, 367, with permission) l6 l8

G. Jones and B. R. Ramachandran, J. Org. Chem., 1976,41,798. S. G. Talbert, D. H. Frieling, J. A. Eibling, and R. A. Nathan, Solar Energy, 1975, 17, 367.

Photochemical Aspects of Solar Energy Conversion

573

energy storage capacity, and life-cycle costs were the primary bases of comparison. Among the potential advantages of photochemical systems that this research identified were effectiveness on cloudy days, a smaller storage tank, and uniform energy levels in the stored fluid, irrespective of season. Other conclusions are summarized in Figure 1, which is a plot of the specific energy capacity and overall energy storage efficiency required of a photochemical system to match the winter operation of a hot water thermal system. The area to the upper right of these curves represents conditions where a photochemical system will have the advantages of lower cost or better performance. The curves in Figure 1 indicate that in developing a photochemical fluid to be competitive with a hot water thermal system, chemical efficiency can be traded off against energy storage capability, to some extent. However, there are limits. Even with the highest reasonable storage capacity, a minimum energy storage efficiency of 0.2 is necessary, and even with very high efficiency, a minimum thermal capacity of about 150 J g-l is necessary. The latter condition is not unduly hard to meet, but the first certainly is. Photochemical Decomposition of Water.-The production of hydrogen by photoassisted electrolysis is described in Section 3, and production by photosynthetic organisms in Section 5. The review paper by Balzani et al. on photochemical formation of hydrogen from water by the use of transition metal complexes (see Vol. 7, p. 564) has now appeared in a more accessible journa1.l' A preliminary report by Whitten and co-workers on the photochemistry of monolayer-bound complexes of ruthenium(I1) adjacent to an aqueous phase has aroused great interest this year. Two surfactant analogues of tris-(2,2'-bipyridyl)ruthenium(II)Z+, (3) and (4), were prepared as monolayer assemblies on glass

17

18

V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta, and M. Gleria, Science, 1975, 189, 852. G. Sprintschnik, H. W. Sprintschnik, P. P. Kirsch, and D. G. Whitten, J. Amer. Chem. SOC., 1976,98,2337.

574

Photochemistry

slides (either hydrophilic or hydrophobic). The luminescence from these monolayers was observed to be almost entirely quenched on immersion in water. On irradiation through Pyrex with a 100 W medium pressure mercury lamp, a steady production of hydrogen and oxygen was observed over a period of several days with a reported quantum yield of ca. 0.1. This finding, if confirmed by subsequent work, is indeed remarkable. It may be tentatively explained in the following way : the parent compound tris-(2,2'bipyridyl)ruthenium(II)2+has a luminescent triplet charge-transfer excited state, which is quenched by oxidizing species such as iron(w) through an electrontransfer mechanism, although no permanent products result. The excited state has an energy of ca. 180 kJmol-l, thermodynamically more than enough to dissociate water to hydrogen and oxygen, which requires 118 kJ per mol electrons. The fact that the parent ion has no photochemistry in aqueous solution, whereas similar ruthenium(I1) complexes are apparently reactive in monolayer assemblies, might be due either to a lowering of the barrier for electron transfer to solvent in the assembly or to the provision in the assembly of a barrier to the geminate recombination of H* and Ru"', so that the latter is able to oxidize water in a normal thermal process. (Why the monolayer is apparently unaffected by the adsorbed He and OH* is not known.) This hypothesis gains some support from a separate study l9 of the effect of ligand and solvent deuteriation on the properties of the charge-transfer state of tris-(2,2'-bipyridyl)ruthenium(11)~+ in aqueous solution, which suggests that transient electron transfer to the solvent (rather than to the ligand) does occur even in homogeneous solution. There has been some interest20-22in hydrogen production from water by thermochemical cycles, and a hybrid thermochemical water splitting cycle, using solar energy to drive one stage of the process, reaction (l), has been examined.23 2Fe2+

+ I,-

-

2Fe3+ + 31-

(1)

Electron Transfer Reactions.-Bimolecular electron transfer reactions of electronically excited states are of potential interest in connection with photogalvanic cells. There have been three studies on the electron transfer quenching of the 3CT state of tris-(2,2'-bipyridyl)ruthenium(11)~+. The possibility of using such a 24925

[Ru(bipy),I2+

+ P2+ + NPh,

[Ru(bipy),12+ * [Ru(bipy),I3+

Overall: NPh,

+ P2+

hV

A

[Ru(bipy),12+

*

[Ru(bipy),13+

+ P+

[Ru(bipy),12+

+ NPh3+

NPh,+

+ Pt

Scheme 1 J. Van Houten and R. J Watts, J Amer. Chem. SOC.,1975,97, 3843. C. E. Bamberger and D. M. Richardson, Cryogenics, 1976, 16, 197. ar K. F. Knoche, H. Cremel, and G. Steinhorn, Hydrogen Energy, 1976, 1, 23. aa J. E. Funk, Hydrogen Energy, 1976, 1, 33. a3 T. Ohta, S. Asakura, M. Yamaguchi, N. Kamiya, N. Gotoh, and T. Otagawa, Hydrogen Energy, 1976,1, 113. F. Bolletta, M. Maestri, L. Moggi, and V. Balzani, J.C.S. Chem. Comm., 1975,901. aK R. C. Young, T. J. Meyer, and D. G. Whitten, J. Amer. Chem. Soc., 1976, 98, 286; 1975, 97, l9

ao

4781.

575

Photochemical Aspects of Solar Energy Conversion

reaction to drive a second chemical reaction in the non-spontaneous direction has been examined 26 for the sequence of Scheme 1, in which P2+is paraquat (methyl viologen). Lichtin et aZ.26have examined the feasibility of separating the energy-rich products of the photolysis of aqueous ferric bromide by the use of nitrogen to sweep bromine out of the photolysis apparatus: 2Fe3f

+ 2Br-

-

2Fe2+

+ Br,:

AGO = 61 kJ mol-l

(2)

The quantum yield of Br, at 436 nm is ca. 0.001 initially, but it falls as Fe*+ accumulates. 3 Photoelectrochemistry Photogalvanic Effects and Cells.-Clark and Eckert’s data on a thin-layer ironthionine cell, reported last year, have now appeared in a more accessible Creutz and Sutin 28 have suggested a photoelectrochemical solar energy storage system in which irradiation of an alkaline aqueous solution of tris-(2,2’-bipyridyl)ruthenium(rI)2+produces the 3CT state. This injects an electron into an n-type semiconductor anode and is regenerated in the bivalent state by the oxidation of water. At the metallic counter-electrode, hydrogen is evolved by the reduction of water. [Ru(bipy),12+in acid solution has been shown to behave rather differentl~.~~ Cathodic, rather than anodic, photocurrents are observed at a platinum or SnO, electrode on irradiation of a ca. lo-, M solution in O.5M-H,SO,. Limiting currents of the order of 1 pAcm-* were observed, corresponding to a quantum yield of current of 0.005 at 470 nm. The photocurrent was enhanced by methyl viologen or oxygen, which quench the 3CT state of [Ru(bipy),12+ oxidatively. The cathodic photocurrent is due to the reduction of [Ru(bipy),I3+ at the illuminated electrode. The cell 3.1 x 10-2M-Ru(bipy)3Cl,,

SnO, 7.8 x 10-3M methyl viologen

1.OM-NaOH, 1.OM-Na,S04 Pt

in O.OSM-H,SO4

produced a short-circuit current of 13 pA cm-a on irradiation, the anodic process being oxygen evolution. Lin and Sutin have reported substantial photogalvanic effects in a cell based on the reversible photo-oxidation of [Ru(bipy),12+ by Fe3+. The observed photopotentials AE (i.e. the change in potential of a platinum electrode on irradiation of the solution) are summarized in Table 1. The observed photopotentials are all positive, showing that the Ru couple acts more reversibly at the electrode than the Fe couple, which may be partially suppressed by adsorbed bipyridine complexes. Knowing the composition of the photostationary state 26 27 28

2s 30

S. N. Chen, N. N. Lichtin, and G. Stein, Science, 1975, 190, 879. W. D. K. Clark and J. A. Eckert, Solar Energy, 1975,17, 147. C. Creutz and N. Sutin, Proc. Nat. Acad Sci. U S A . , 1975,72,2858. S. 0. Kobayashi, N. Furata, and 0. Simamura, Chem. Letters, 1976, 503. C. T. Lin and N. Sutin, J. Phys. Chem., 1976, 80, 97; J. Amer. Chem. Soc., 1975,97, 3543.

576 Photochemistry Table 1 Photogalvanic potentials of a reversible [Ru(bipy),I2+-Fe3+ system in various acids at -22 "C" 105[Ru(bipy),12+/M 103(Fe3+)/M Medium 0.10M-CFaSO3H 0.024M-HC10, 1 .o 4.8 2.0 5.0 0.025M-HC104 O.lOM-CF3SO3H 0.125 M-HCIO, 2.0 5.0 O.lOM-H,SO, 0.025M-HC10, 2.0 5.0 O.1OM-HCI 0.025M-HCl04 2.0 5.0 2.0 5 .O 0.50M-HC104 O.5OM-H2S04 2.0 5.0 O.5OM-HCl 2.0 5.0

+ +

+

AEobsdlV

+

N

0.16b 0.17 0.17 0.14 0.10 0.18 0.14 0.03

The incident light intensity in the range 400-480 nm was -2.3 x lo-' einstein cm-2 s-l. In this range, the ED value for [Ru(bipy),la+was averaged as 1 x lo4 M-' cm-l. No photopotentials were observed for a deaerated solution containing [Ru(bipy),12+ but not Fe3+, or an air-saturated solution containing Fe3+but not [Ru(bipy),12+. In a deaerated solution. a

and assuming only Ru"/Ru"' reacts at the electrodes, calculated photopotentials that are in fair agreement with the experimental values were obtained. Photogalvanic effects observed with metal electrodes coated with chlorophylls and phthalocyanines continue to attract some interest. Anodic photopotentials were observed at platinum electrodes coated with monolayers of chlorophyll a,31 as expected for a p-type semiconducting film. The cell 5 x 10-3M quinone,

Pt Chl a O.1M-LiCIO,

5 x 10-3M hydroquinone, Pt

pH = 10.3

showed a quantum yield of current of about 0.2 in the red and 0.5 in the blue. However, very weak light was used and the monolayers absorb very little light, so that the actual currents are of the order of nanoamps only, a current which takes several hours to oxidize a monolayer of material. Enhanced photoelectrochemical effects have been observed with electrodes coated with 1 : 1 mixtures of chlorophyll (a mixture of the a and b forms) and q ~ i n o n e s .The ~ ~ cell Pt I Chl

+ naphthoquinone I NAD 11 K,Fe(CN), I Chl + anthraquinone I Pt

had an open-circuit potential on illumination at lo4 lux (full sunlight II lo5 lux) of 0.25 V and a short circuit current of 8 pA cm-2. The left-hand electrode is the cathode, the right-hand the anode. The quinones have a catalytic role, as many more coulombs may be passed than are required for their oxidation or reduction. Small photogalvanic effects have been observed in metal electrodes coated with copper(@ phthaIocyanine 33$ 34 and iron(n) or metal-free phthalocyanine;34 the electrode reaction is probably oxygen reduction in both cases. Photogalvanic effects have also been observed in solid chlorophyll and magnesium phthalocyanine films under pulse i l l ~ m i n a t i o n . ~ ~ 31 32 33 34

36

F. K . Fong and N. Winograd, J. Amer. Chem. SOC.,1976,98, 2287. F. Takahashi and R. Kikuchi, Biochirn. Biophys. Acta, 1976,430, 490. B. Schreiber and M. Savy, Cornpt. rend., 1976, 282, C, 787. G. A. Alferov and V. I. Sevast'yanov, Russ. J. Phys. Chem., 1976, 50, 118. V. B. Evstigneev, Stud. Biophys., 1975,49,27.

Photochemical Aspects of Solar Energy Conversion

577 The discharge behaviour of redox thermogalvanic cells has been investigated both theoretically and e~perimentally.~~ The analogy with photogalvanic cells is quite close, particularly in that mass transfer polarization is the major factor limiting power output. A combined photo/thermogalvanic cell : (80 "c)n-TiO, I F ~ ( C N ) G ~ Fe(CN)e4-, I Pt (30 "c) illuminated dark anode cathode

is proposed. The cell voltage in the absence of illumination is some tens of millivolts and is due to the Soret effect. Under illumination, a substantial photogalvanic effect would be superimposed. Semiconductor Electrodes.-Nozik general type

37

has considered the efficiency of a cell of the

n-semiconductor I electrolyte 1 p-semiconductor

in which both electrodes are irradiated. An upper theoretical limit of ca. 0.45 is predicted for sunlight conversion. Some data on the cell n-TiO, I O.lM-H,SO,

I g-GaP

are reported, and are illustrated in Figure 2. The current efficiency for hydrogen and oxygen evolution was 0.0025 in simulated sunlight. Anderson and Chai 38 have reviewed the characteristics of the semiconductorelectrolyte interface and of solar cells incorporating such interfaces.

a

E

OA-

-0.6

-0.4

-0.2

0 0.2 0.4 Potential, volts

0.6

0.8

0

Figure 2 Current-voltage characteristic for the n-TiO,/p-GaP cell with 0.1 M-H,S04. Current is plotted against both the terminal cell voltage and the electrode potentials (vs. SCE). The area and carrier concentration of the TiO, electrode are 0.7 cm2 and 2 x lOlS ~ m - ~ Internal . cell resistance is 700 i2 (Reproduced by permission f r o m Appl. Phys. Letters, 1976, 29, 150) s6 37 88

B. Burrows, J. Electrochem., Soc., 1976,123, 154. A. J . Nozik, Appl. Phys. Letters, 1976, 29, 150. W.W. Anderson and Y. G. Chai, Energy Conversion, 1975, 15, 85.

578

Photochemistry

Titanium Dioxide Electrodes. The photoelectrolysis of water using n-TiO, electrodes continues to attract much interest, although the process is now reasonably well understood. Nozik 39 studied single crystals (with the c axis in the wafer plane) over a wide range of light intensities and obtained the currentvoltage characteristics shown in Figure 3 for the cell n-Ti02 I 1 N phosphate buffer I platinized Pt

Electrode potential (against SCE)(V) Figure 3 Electrode potentials vs. SCE of TiO, (-) and Pt (- - -) as a function of light intensity: 1.0 N phosphate bufer (pH = 6.5). A = 0 corresponds to broad-band U.V. (300-400nm) light of intensity 26 mW crnA2. A = 1, 2, 3 results obtained with neutral density Jilters of absorbance 1, 2, 3. The arrows indicate reversible hydrogen and oxygen evolution potentials (Reproduced by permission from Nature, 1975, 257, 383)

These data corroborate what other studies have established, namely that the range of working potential of the TiO, electrode in such a cell is well below that required for the attainment of limiting currents at this electrode. Carey and Oliver 40 also examined intensity effects at TiOa electrodes. They found that the current efficiency decreased in a pH-dependent manner as the light intensity increased. This is to be expected if the hole-electron recombination rate in the crystal is the major factor limiting the photocurrents. Below 12 mW cm-2, they found the usual linear behaviour. A. J. Nozik, Nature, 1975, 257, 383. ‘O

J. H. Carey and B. G . Oliver, Nature, 1976, 259, 554.

Photochemical Aspects of Solar Energy Conversion

579

Two other papers reporting the usual effects on Ti02 single crystals have appeared 4 2 and three more on Ti02 polycrystalline or amorphous films, formed either electrochemically 4 3 ~44 or thermally.43,45, 46 Honda and coworkers 43 have reported the operation under sunlight of a modular unit containing 20 cells, which had a mean daily conversion efficiency of ca. 0.4%. However, the anolyte was 1M-NaOH and the catholyte 0.5M-H2S04,which increases the ‘efficiency’at the expense of consuming alkali and acid. The role of surface states in photoelectrochemical processes at Ti02 is of interest. Three distinct surface phases have been observed in TiO, crystals subjected to argon ion bombardment ,47 Electrochemical properties of singlecrystal Ti02 (in the dark) have been thoroughly investigated by means of impedence, current, and potential mea~urements.~~ Other Semiconducting Oxide Electrodes. Strontium titanate electrodes have been shown in three independent studies49-61to be superior to Ti02 itself in that the current quantum efficiency (electrons flowing per photon absorbed) is about an order of magnitude higher at zero bias voltage in a working photoelectrochemical cell than for Ti02. Thus, irradiation of the semiconductor electrode in the cell 41s

SrTiO, ] 9.5M-NaOH ] Pt

drove the electrolysis of water photocatalytically without any external bias at X < 390nm (the band gap of SrTiO, is 3.2eV).60 The stability of the photoelectrode was confirmed by experiments carried out on 180-labelledH,O and by the lack of weight loss in the electrode on prolonged use.6o The increased quantum efficiency occurs because the flat band potential of SrTiO, is ca. 0.3 V more cathodic for SrTiO, than for Ti02.49, 51 This produces increased band bending at the SrTiO, surface in its working potential range. Figure 4 shows the current-potential behaviour of an irradiated electrode as a function of pH. The limiting current quantum efficiency is 1 at V % 1.55 V and A < 330 nm.60 At zero bias voltage, it is ca. 0.1.499 Two other perovskites, KTaO, and KTa0.77Nb0.2303, behave similarly to SrTi03.62 The band gap in these is 3.5 eV, which is even higher than for TiOz (3.0eV). As the flat band potential of the electrode should be as cathodic as possible for the efficient photoelectrolysis of water, and as this correlates with a large band gap, it may unfortunately be the case that good performance will not be obtained with smaller band gap oxide electrodes.

4a

4a 44 45

47

6o

61

aa

V. A. Benderskii, J. M. Zolotovitskii, J. L. Kogan, M. L. Khidekel, and D. M. Shub, Doklady Akad. Nauk. S.S.S.R., 1975,222, 606. A. N. Asanov, Doklady Akad. Nauk S.S.S.R., 1975, 225, 838. A. Fujishima, K. Kohayakawa, and K. Honda, J. Electrochem. SOC.,1975, 122, 1487. W. Gissler, P. L. Lensi, and S. Pizzini, J. Appl. Electrochem., 1976, 6, 9. P. Clkhet, F. Juillet, J. R. Martin, and R. Olier, J. Chim. phys., 1976, 73, 396. D. M. Shub, A. A. Remnev, and V. I. Veselovskii, Soviet Electrochem., 1975, 11, 1021. V. E. Henrich, G. Dresselhaus, and H. J. Zeiger, Phys. Rev. Letters, 1976, 36, 1335. E. C. Dutoit, F. Cardon, and W. P. Gomes, Ber. Bunsengesellschaftphys. Chem., 1976, 80, 475. J. G. Mavroides, J. A. Kafalas, and D. F. Kolesar, Appl. Phys. Letters, 1976, 28, 241. M. S. Wrighton, A. B. Ellis, P. T. Wolczanski, D. L. Morse, H. B. Abrahams, and D. S. Ginley, J. Amer. Chem. SOC.,1976, 98, 2774. T. Watanabe, A. Fujishima, and K. Honda, Bull. Chem. SOC.Japan, 1976, 49, 355. A. B. Ellis, S. W. Kaiser, and M. S. Wrighton, J. Phys. Chem., 1976, 80, 1325.

580 Photochemistry Antimony-doped SnO, electrodes 53 and oxide layers on W,54Nb, Ta, and Li 55 all show effects similar to those observed in TiO,, but smaller. It has long been known that anodic photocurrents at n-ZnO are spectrally sensitized by rose bengal, and that only dye molecules or ions adsorbed on the surface are effective. The use of thick dye layers to increase the absorbance is

Figure 4 Current-potential characteristics of irradiated SrTiO, single-crystal electrode (Reproduced by permission from Bull. Chern. Soc. Japan, 1976, 49, 355)

unsuccessful, partly because such films are highly resistive. Tsubomura et 57 have circumvented this problem ingeniously by the use of sinter discs of ZnO powder, dyed by immersion in a concentrated rose bengal solution. A large total absorption of light can thus be obtained in conjunction with a fairly low inner resistivity (ca. 100 ohm cm). A cell containing this electrode, aL5,9

dyed ZnO sinter I 0.13M-KI, 10-SM-I,, 0.2M-Na,S04 I Pt

had a power conversion efficiency at 563 nm of 1.5%. Cadmium SuZphide Electrode. Gerischer and Gobrecht 58 have reported more fully their investigation of thin-layer cells containing one CdS electrode, one quasi-metallic SnO, electrode, and a redox couple in solution. Cell characteristics for illumination of the CdS electrode through the SnO, electrode and the electrolyte are shown in Figure 5. The fill factor is good for Fe(CN),4-/Fe(CN),3-, as this couple is reversible at the SnO, electrode, but it is poor for the S,0,2-/ S,Oa2- couple, which exhibits charge transfer polarization at SnO,. The optimum conversion efficiency for Fe(CN),4-/Fe(CN)63- in sunlight is ca. 5.5%. However, this high value decreases drastically within minutes because the CdS electrode 5a

M. S. Wrighton, D. L. Morse, A. B. Ellis, D. S. Ginley, and H. B. Abrahams, J. Amer. Chem. SOC.,1976, 98, 44.

64 66

66

57 68

G. Hodes, D. Cahen, and J. Manassen, Nature, 1976, 260, 312. P. Clkhet, J. R. Martin, R. Olier, and C. Vallouy, Compt. rend., 1976,282, C, 887. H. Tsubomura, M. Matsumura, Y.Nomura, and T. Amamiya, Nature, 1976, 261, 402. M. Matsumura, K. Yamamoto, and H. Tsubomura, Nippon Kagaku Kaishi, 1976, 403. H. Gerischer and J. Gobrecht, Ber. Bunsengesellschaftphys. Chem., 1976, 80, 327.

Photochemical Aspects of Solar Energy Conversion

58 1 decomposes anodically, producing a superficial layer of sulphur. Anderson and Chai 6g encountered similar trouble in the cells CdS I O.lM-CH,COOH I Pt and GaAs I O.1M-HCl I Pt They observed no significant difference in the behaviour of the 0001 CdS face (Cd-rich) and the OOOT (S-rich) face.38 389

Photopotential

-U ( V )

Figure 5 Output power characteristics for the cell 0.2M-K4Fe(CN), 0.01M-K,Fe(CN), 0.M-KCl CdS or 1M-Na,S,O, 1M-NaI O.lM-I, Irradiation with 40 mW cm-2 light from xenon lamp (Reproduced by permission from Ber. Bunsengesellschaft phys. Chem., 1976, 80, 327)

1

+ +

+

+

The decomposition of the CdS can, however, be prevented by the use of sulphide-rich solutions.so~ The behaviour of the irradiated cell n-CdS or CdSe I NaOH, Na,S, S (all ca. 1M) I Pt

has been reported.so At the irradiated semiconductor, sulphur is formed by oxidation of S2- in solution. At the counter-electrode, hydrogen is initially evolved, but if sufficient elemental sulphur is added, this ceases, and the electrode process is presumed to be the reduction of some polysulphide species. Etched electrodes maintained power conversion efficiencies of ca. 5% for CdS at 500 nm, and up to 9% for CdSe at 633 nm. Similar behaviour occurs in organic sulphide/sulphur solutions from which air is rigorously excluded.,l With a CdSe electrode and an activated charcoal counter-electrode, short-circuit currents of 7-10 mA cm-2 and open-circuit potentials of ca. 0.5 V were obtained in AM1 sunlight. The inclusion of a third electrode of porous Ag/Ag,S can convert the device into a storage battery, with a charging current efficiency of up to 90%, provided that sulphur is excluded from the storage electrode compartment. 69

6o

61

Y. G. Chai and W. W. Anderson, Appl. Phys. Letters, 1975, 27, 183. A. B. Ellis, S. W. Kaiser, and M. S. Wrighton, J . Arner. Chern. Soc., 1976,98, 1635. G. Hodes, J. Manassen, and D. Cahen, Nature, 1976,261, 403.

582

Photochemistry

Miscellaneous. p-Silicon in aqueous solution exhibits cathodic photoeffects 62 as expected. However, the electrode behaviour deteriorates because a superficial oxide layer is formed. Bard has instigated an examination of the photoelectrochemistry of semiconductor electrodes in non-aqueous soIventsYB3 in which stability is generally improved. Both n- and p-Si in acetonitrile 64 show behaviour that strongly depends upon the electrode pretreatment, and the absence of ideal semiconductor behaviour suggests that surface states and surface degradation are important even under non-aqueous conditions. A contribution to the problem of stabilizing semiconductor electrodes of small band gap has been made by Tsubomura and c o - w ~ r k e r s ,who ~ ~ have shown that n-GaP electrodes coated with thin gold films (of transmittance 5-30%) behave on illumination like n-Gap, rather than Au, electrodes. Photondriven charge injection continues, but it seems possible that this occurs on regions of the semiconductor surface not covered by gold. Lyons has reported the first stage of a project aimed at photoelectrochemical energy conversion based on hole-electron pair generation in thin anthracene crystals by absorption of light. Tris-(1,10-phenanthroline)iron(iI)2+and p-anilinobenzenesulphonate are oxidized with 100% current efficiency by positive holes emerging from an anthracene crystaLBs

4 Photochemistry in Vesicles, Micelles, and Artificial Membranes There is a large electrical double layer at the aqueous-organic interface of micelles and vesicles of ionic surfactants. This produces marked effects on the rates of ionic reactions in the vicinity of the interface, and may greatly prolong the lifetime of the primary products of a photoelectron transfer r e a ~ t i o n . ~Thus ~-~~ the interface provides a means for spatial separation and stabilization of photogenerated oxidant and reductant. Tomkiewicz and Corker have examined photoredox reactions of chlorophyll a in egg yolk lecithin vesicles.7o With [Fe(CN),I3- or Sm3+as electron acceptors, the cation radical Chlt was produced, but they could not detect electron transfer to ubiquinone or iron-sulphur proteins (which are involved in the primary in vim photosynthetic process) except in the case of one protein isolated from Rhodospirillum rubrum. Boguslavskii et aL7137 2 have observed the photoreduction of chlorophyll at a decane-water interface. Mange173has made an interesting observation on the relative efficiency of energy conversion of chlorophyll-containing bilayer membranes (BLM) and liposomes. Although BLM-containing chlorophylls and carotenes are photoR. M. Candea, M. Kastner, R. Goodman, and N. Hickok, J. Appl. Phys., 1976,47,2724. S . N. Frank and A. J. Bard, J. Amer. Chem. SOC.,1975,97,7427. 13' A. J. Bard, J. Phys. Chem., 1976, 80, 459. eL Y. Nakato, T. Ohnishi, and H. Tsubomura, Chem. Letters, 1975, 883. I6L. E. Lyons and K. G. McGregor, Austral. J. Chem., 1976,29, 21. (' S. A. Alkaitis, G. Beck, and M. Gratzel, J. Amer. Chem. Soc., 1975, 97, 5723. a* D. J. W. Barber, D. A. N. Morris, and J. K. Thomas, Chem. Phys. Letters, 1976,37,481. K. Kano, Y.Takada, and T. Matsuo, Bull. Chem. Soc. Japan, 1975,48, 3215. 7 0 M. Tomkiewicz and G. A. Corker, Photochem. and Photobiol., 1975, 22, 249. 71 L. I. Boguslavskii, A. G. Volkov, and M. D. Kandelaki, F.E.B.S. Letters, 1976, 65, 155. L.I. Boguslavskii and A .G. Volkov, Proc. Acad. Sci. U.S.S.R., 1975, 224, 1201. 78 M. Mangel, Biochim. Biophys. Acta, 1976, 430, 459. ea

63

Photochemical Aspects of Solar Energy Conversion

583 sensitive, the quantum yield (electrons transported per photon absorbed) is only ca. In photosynthetic membranes, by contrast, it is unity. It has been proposed 74 that the lack of chlorophyll aggregates in BLM, which always retain some organic solvent, is responsible for their low efficiency. Mangel’s study supports this, in that quantum yields of electron transfer in liposomes containing carotene and aggregated chlorophyll have quantum yields of ca. 0.075. It appears that pigment aggregates may play an important role in the conversion of photonic into electronic energy. The retinal protein complex bacteriorhodopsin forms the major constituent of the purple membrane in Halobacterium halobium. In vivu, this protein acts as a light-driven proton pump;75its absorption maximum is at 570 nm. It has been suggested 76 that oriented purple membranes from halobacterial cells could be made into stabilized sheets containing a H+/Na+ antiporter. The whole would then act as a light-driven dialysis unit. Skulachev77 has shown that planar artificial sheets formed from purple membrane are very stable : sheets supported on millipore filters may be boiled for 1 min without inhibiting photochemical activity. 5 Photosynthesis

A series of papers under the general title ‘Bioenergetics of Tomorrow’ has appeared in the April 1976 issue of F.E.B.S. Letters. These include a general review of photobiological energy conversio~i,~~ and three papers on bacteriorhodopsin. Calvin has reviewed photosynthesis as a resource for energy and materials. 79p

The Structure and Function of Photosynthetic Membranes.-As in previous years, some of the recent findings in photosynthesis are reported in the hope that better understanding of photosynthesis in vim may lead to more efficient in vitro processes. Anderson 81 has reviewed the molecular organization of chloroplast thylakoids. One of the intriguing aspects of the primary processes of photosynthesis is the high efficiency with which energy migrates from the light-harvesting antenna of pigment molecules to the trap, for this process has not as yet been achieved at anything like the same efficiency in vitro. Solutions of chlorophyll in vitro, whether in fluid solvents, rigid matrices, monolayers, multilayers, or bilayer vesicles, exhibit the phenomenon of concentration quenching of the excited state at concentrations much lower than those which are present in the chloroplast ( 10-1M). To account for this, Porter 82 has proposed that the mechanism of concentration quenching in non-biological chlorophyll systems is Forster energy transfer between chlorophyll molecules followed by capture at a non-fluorescent N

74 76 76

77 70

81 8a

A. Ilani and D. S . Berns, J. Membrane Biol., 1972,8, 333. R. H. Lozier, R. A. Bogomolni, and W. Stoeckenius, Biophys. J., 1975, 15, 955. D. Oesterhelt, F.E.B.S. Letters, 1976, 64, 20. V. P. Skulachev, F.E.B.S. Letters, 1976, 64, 23. D. 0. Hall, F.E.B.S. Letters, 1976, 64, 6. M. Calvin, Photochem. and Photobiol., 1976, 23, 425. M. Calvin, Amer. Scientist, 1976, 64, 270. J. M. Anderson, Biochim. Biophys. Acta, 1975, 416, 191. G . S. Beddard and G . Porter, Nature, 1976,260, 366.

584

Photochemistry

trap, which is merely a pair of chlorophyll molecules whose separation, although part of the equilibrium statistical distribution, is less than a critical distance Rt. A Monte Carlo method was used by Beddard 82 to calculate fluorescence yields as a function of chlorophyll concentration, and excellent agreement was obtained with experimental data from a wide variety of systems for a trap distance Rb of 1 nm. These calculations indicate that efficient energy transfer would be impossible in the photosynthetic unit if it resembled a randomly distributed solution of chlorophyll molecules containing, as such a distribution would, a high proportion of chlorophyll a molecules with nearest neighbours within 1 nm. It is therefore inferred that the chlorophyll molecules are separated from each other, probably by co-ordination of lipids or proteins to the magnesium atom of the chlorophyll. The first direct structure determination of a chlorophyll-containing protein by X-ray crystallography has been The protein concerned, extracted from the green photosynthetic bacterium Prusthecochloris Aestuarii 2K, consists of three identical subunits, each containing a core of seven bacteriochlorophyll molecules confined within an ellipsoid of axial dimensions 4.5 x 3.5 x 1.5 nm. The orientation of the porphin rings does not conform to any repetitive pattern, although their planes do lie roughly parallel to one another. The average nearneighbour distance is cn. 1.2 nm, which should allow efficient energy transfer whilst inhibiting concentration quenching. Absorption and c.d. spectra of the same protein show evidence of exciton interaction between the seven bacteriochlorophyll molecules in each of the three The co-ordination properties of the central magnesium atom in chlorophyll have for some time now been recognized to play a decisive role in chlorophyll function in photosynthesis. Computer deconvolution of absorption spectra has provided evidence that both five-and six-co-ordinations occur in bacteriochlorophyll in viuo and in uitru.8S Self-aggregated bacteriochlorophyll is five-co-ordinate. The magnesium atom is also five-co-ordinate in ethyl chlorophyllide a and b dihydrates, which contain cross-linked one-dimensional polymers of translationally equivalent molecules.s6 Features in the absorption spectra bear a striking correspondence to those observed in vivu and it is suggested that aggregated chlorophyll in vivo has a similar structure. Chlorophyll a aggregation in the presence of small amounts of water has been investigated spectroscopically.s7 The structure of the ‘special pair’ chlorophyll P700 in green plants, which acts as a trap for excitons from the chlorophyll antenna, has been reconsidered by Katz and co-workers.88 The apparently dimeric species (Chl a. H20)z, A700, obtained by slow cooling of M chlorophyll a in hydrocarbon, may have a similar structure as it has an oxidation potential of ca. 0.5 V, similar to that of P700 in v i v ~ . ~ ~ 83

86 86

88

R. E. Fenna and B. W. Matthews, Nature, 1975, 258, 573. J. M. Olson, B. Ke, and K. H. Thompson, Biochim. Biophys. Acta, 1976,430, 524. T. A. Evans and J. J. Katz, Biochim. Biophys. Acta, 1975, 396, 414. H. C. Chow, R. Serlin, and C. E. Strouse, J. Amer. Chern. SOC.,1975, 97, 7230. F. K. Fong and V. J. Koesler, Biochim. Biophys. Acta, 1976, 423, 52. L. L. Shipman, T. M. Cotton, J. R. Norris, and J. J. Katz, Proc. Nut. Acad. Sci. U.S.A., 1976, 73, 1791.

88

V. J. Koesler, L. Galloway, and F. K. Fong, Naturwiss., 1975, 62, 530.

Photochemical Aspects of Solar Energy Conversion

585

Primary Photochemical Events in Photosynthesis.-The transient state termed P F , which is formed within 10 ps of excitation of the bacteriochlorophyll reaction centre special pair P870, was described last year. Picosecond kinetics of the 1250nm band of the reaction centre of Rhodopseudomonas spheroides provide some evidence that this state is produced by electron transfer from P870 (i.e. BChl,) to a bacteriopheophytin (BPh) molecule in the reaction centre.Q0Fajer et aLgl have measured the optical spectrum of BPhr in vitro, and have demonstrated a good fit between the PF data and the sum of the spectral data for BChl,? and BPhs. However, optical data on the primary acceptor, chemically trapped in the reduced form in the bacterium Chromatium vinosum, suggest that the electron may be shared between BChl and BPh.Q2It is now established that the electron is passed on to a ubiquinone (UQ) molecule closely coupled to an iron(@ protein, whose role may possibly be to facilitate electron transfer between the ‘primary’ UQ accepter and a second, more loosely bound UQ in the bacterial reaction Photosynthetic Hydrogen and Oxygen Evolution.-It has been known for some years that photosynthetic organisms which contain the enzymes nitrogenase and hydrogenase will, under suitable conditions, produce molecular hydrogen instead of fixing nitrogen or reducing carbon dioxide to carbohydrate. Interest in the possibility of large-scale hydrogen production by this means continues. Some intact plants have this ability: the water fern Azolla is an example.Q4 Formation of hydrogen by irradiation of chloroplast suspensions containing hydrogenase has been examined both in the absenceQ5and the presenceQsof exogenous electron donors. In the former case, it appears that H,O itself is reduced. Packer Q7 has examined the possibility of stabilizing the in vitro photochemical activity of chloroplasts used for hydrogen production by immobilization through cross-linking with the bifunctional reagents dimethylsuberimidate and glutaraldehyde, and has shown that performance is somewhat improved by this measure. The role of manganese in photosynthetic oxygen evolution is an intriguing and challenging question. The oxidation states involved, and even the number of manganese atoms in the reaction centre of photosystem 11, are not known with certainty. The dimer of the manganese@) gluconate complex [Mnl1(GH,),l2- in basic aqueous media has been reported to undergo redox chemistry which parallels much that is observed of manganese in photosystem II.D8The dimer can be chemically or electrochemically oxidized in an overall four-electron process to a product which, under low oxygen partial pressures, evolves molecular oxygen and regenerates the starting material.

B2 g3

g4 O6

8e O7

P. L. Dutton, K. J. Kaufmann, B. Chance, and P. M. Rentzepis, F.E.B.S. Letters, 1975, 60, 275. J. Fajer, D. C. Brune, M. S. Davis, A. Forman, and L. D. Spaulding, Proc. Nat. Acad. Sci. U.S.A., 1975,72,4956. D. M. Tiede, R. C. Prince, G. H. Reed, and P. L. Dutton, F.E.B.S. Letters, 1976, 65. 301. M. Y. Okamura, R. A. Isaacson, and G. Feher, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 3491. J. W. Newton, Science, 1976, 191, 559. K. K. Rao, L. Rosa, and D. 0. Hall, Biochem. Biophys. Res. Comm., 1976, 68, 21. A. A. Krasnovskii, V. V. Nikandrov, G. P. Brin, I. N. Gogotov, and V. P. Oschchopkov, Doklady Akad. Nauk S.S.S.R., 1975, 225, 711. L. Packer, F.E.B.S. Letters, 1976, 64, 17. D. T. Sawyer and M. E. Bodini, J. Amer. Chem. SOC.,1975,97, 6588.

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Photochemistry

6 Photovoltaic Cells There has been a noticeable trend this year towards the investigation of inorganic heterojunction cells, some of which now exhibit efficiencies approaching those of the well-established silicon p-n junction cell. This is an encouraging development, as the fabrication of heterojunctions requiring the deposition of a thin semiconductor layer on a fairly thin substrate of a different semiconductor is simpler, and therefore potentially less costly, than fabrication of diffused junction cells. A book of general interest and an introduction to the theory of photovoltaic cells loohave appeared. Silicon.-The proceedings of a two-day meeting on high efficiency silicon solar cells have appeared,lol as has a report on the feasibility of producing silicon ribbon suitable for solar cell manufacture by ‘edge-defined film-fed growth‘ (EFG).lo2 Distinct progress has been made with the EFG technique; solar cells with 7% efficiency (in simulated AM0 sunlight) are now routinely made. Crystals grown by EFG on the usual graphite dies tend to incorporate some S i c crystallites, which act as centres for the accumulation of other impurities in the crystal.loS Cells with p-n junctions grown epitaxially on silicon EFG ribbon have substantially higher efficiencies than is achieved by diffusion.lo4 Silicon p+-p-n-n+ solar cells of thickness ca. 35 pm, grown epitaxially onto n+ material by deposition from SiH2C12,with ASH, and B2Hs dopants, have good performance (Kc 0.636 V, fill factor 0.79, AM1 efficiency 12.6%).lo6 Polycrystalline n+-p-p+ silicon layers grown on steel were not successful, but n+-Si Ip-Si Ip+-Si I graphite cells performed rather better (Kc 0.35 V, AM0 efficiency 1.5%).lo8 Some MIS (metal-insulator-semiconductor) silicon cells have been investigated. The open-circuit voltage of a simple SBSC is generally lower than that of ap-n junction cell, because the simple metal-semiconductor junction has a large reverse saturation current. Inclusion of a thin insulator layer decreases this and can improve cell performance, provided the resistance is not too much increased. The open-circuit voltage of MIS cells on n-Si was optimized by a SiO, layer of thickness ca. 2 nm.lo7 H F etching of the SiO, layer of a single-crystal MIS silicon solar cell raised the AM1 efficiency above 10%.lo8 Thin film (ca. 1 pm) p-i-n silicon solar cells of AM1 efficiency up to 2.4% J. A. Merrigan, ‘Sunlight to Energy: Prospects for Solar Energy Conversion by Photovoltaics’, MIT Press, 1975. l o oP. T. Landsberg, Solid State Electronics, 1975, 18, 1043. lol ‘High Efficiency Silicon Solar Cell Review’, NASA Technical Memorandum, NASA TM X-3326. loa B. Chalmers, A. I. Mlavsky, T. Surek, J. C. Swartz, R. 0. Bell, D. N. Jewett, D. A. Yates, K. V. Ravi, and F. Wald, ‘Continuous Silicon Solar Cells: Final Report’, N.T.I.S. accession no. PB-247-228. lo3 C. V. Hari Rao, H. E. Bates, and K. V. Ravi, J. Appl. Phys., 1976,47,2614. lo’ H. Kressel, R. V. D’Aiello, and P. H. Robinson, Appl. Phys. Letters, 1976,28, 157. lo6 R. V. D’Aiello, P. H. Robinson, and H. Kressel, Appl. Phys. Letters, 1976,28,231. lo6 T. L. Chu, J. C. Lien, H. C. Mollenkopf, S. C. Chu, K. W. Heizer, F. W. Voltmer, and G. F. Wakefield, Solar Energy, 1975, 17. 229. lo’ J. P. Ponpon and P. Siffert, J. Appl. Phys., 1976,47, 3248. lo8 A. H. M. Kipperman and M. H. Omar, Appl. Phys. Letters, 1976, 28, 620. 99

587

Photochemical Aspects of Solar Energy Conversion

have been fabricated from amorphous silicon deposited from a Aow discharge in silane and dopant gases.l0@ ll1 Two general analyses of high-voltage vertical multijunction solar cells indicate that these devices, illustrated schematically in Figure 6, promise efficiencies greater than those predicted for any other structure. 1109

Figure 6 High-voltage vertical rnultijunction solar cell with concentrating cover

Cadmium Sulphide Heterojunction Cells.-Interest in the physics of the CdS-Cu2S heterojunction continues, as the efficiency of cadmium sulphide solar cells 113 and the improves. The current-voltage characteristics of the junction photoconductivity of junctions produced by forming layers of Cu2S on single crystals of CdS 114 have been investigated. Some very promising cells based on CdS heterojunctions with III-V and other II-VI semiconductors have been reported. Shay et aZ.llb have provided a fuller description of the preparation and properties of the very successful p-InP/n-CdS cell. Previously reported solar conversion efficienciesof 12.5% can be improved to 14% by annealing the cell for 15 min at 600 "C in forming gas (15% H2 + 85% N2), which improves the open circuit voltage. It is estimated that efficiencies of 17.2% (AM2) and 14.0% (AMO) will be achieved.l12 Fabrication of rather less efficient (4.1%) epitaxially grown single-crystal InP/CdS cells has been reported.lls Polycrystalline thin film InP/CdS solar cells of 2.8% (AM1) efficiency have been made.117 Based on their current-voltage characteristics, it is estimated that l l 2 y

D.E. Carlson and C. R. Wronski, Appl. Phys. Letters, 1976,28, 671.

log

R. J. Soukup, J. Appl. Phys., 1976,47, 555. P. Shah, Solid State Electronics, 1975, 18, 1099. ll2K. W. Boer, Phys. Rev. (B), 1976, 13, 5373. llS D. L. Vasilevskii, V. V. Serdyak, and G. G . Chemeresyuk, Soviet Physics-Semiconductors, 1975, 9, 1351. 11' B. G. Caswell, G. J. Russell, and J. Woods, J. Phys. ( D ) , 1975, 8, 1889. l16. J. L. Shay, S. Wagner, K. J. Bachmann, and E. Buehler, J. Appl. Phys., 1976,47, 614. 116 K. H o and T. Ohsawa, Jap. J. Appl. Phys., 1975,14, 1259. n7 K. J. Bachmann, E. Buehler, J. L. Shay, and S. Wagner, Appl. Phys. Letters, 1976, 29, 121. ll1

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Photochemistry

development of an improved contact to the InP would result in substantially higher efficiencies without any improvement in the InP-CdS interface. A n-CdS/n-CdTe/p-CdTe polycrystalline device fabricated by deposition of 30 pm of the In-doped CdS on P-doped CdTe polycrystals has been reported to have good photovoltaic properties. (6,0.55 V, isc 14 mAcm-2 under 80 mW cm-2 sunlight, efficiency 4.5%.)11* However, Martinuzzi has shown that the n-CdS/p-CdTe junction is unstable because of diffusion of excess Cd from CdS to CdTe.ll9 Efficient CuInSe,/CdS cells have also been reported by Shay et aZ.120 Cell characteristics measured for 92 mW ern+ incident sunlight are V,, 0.5 V, isc 38 mA cm-2, fill factor 0.6, giving an overall efficiency of ca. 12%. n-CdS layers grown epitaxially on a (111) n-GaAs face exhibit photovoltaic properties, with a current collection efficiency of 0.69 at 633 nrn.121 Gallium Arsenide.-High-performance solar cells have been prepared by vapourphase epitaxial growth of n-AlAs on p-GaAs.122 Cells with measured sunlight A

Graded p-type

n - type

Figure 7 Graded band gap solar cell

conversion efficiencies of 13-18% and areas of several cm2 have been prepared by this method, which appears to produce abrupt junctions rather than the gradual ones characteristic of liquid-phase epitaxial growth. Relatively good GaAs solar cells can be made by the latter technique from poor quality substrates by making the junction deep (> 1 pm) instead of shallow and by ‘leaching’ impurities out of the substrate in a Ga-As melt.123 GaAs has an undesirably large surface recombination velocity (S = lo6lo7cm s-l) for use as a superficial layer in a solar cell. The usual heteroepitaxial system p-Ga,_,Al,As/p-GaAs/n-GaAs represents one way of overcoming this property. Another is the use of a graded band gap, illustrated in Figure 7. Two theoretical analyses 124s 125 have shown that grading the composition of the 11*

N.Nakayama, H. Matsumoto, Y. Hioki, and S. Ikegami, Jap. J. Appl. Phys., 1975,14, 1387.

S. Martinuzzi, Phys. Status Solidii ( A ) , 1976, 34, K21. J. L. Shay, S. Wagner, and H. M. Kasper, Appl. Phys. Letters, 1975, 27, 89. lZ1 A. Yoskikawa and Y. Sakai, Jap. J. Appl. Phys., 1975, 14, 1547. laa W. D. Johnston and W. M. Callahan, Appl. Phys. Letters, 1976, 28, 150. laS H. J. Hovel and J. M. Woodall, Appl. Phys. Letters, 1975, 27, 447. lZ4 J. A. Hutchby and R. L. Fudurich, J. Appl. Phys., 1976, 47, 3140. la6 M.Konegai and K. Takahashi, Solid State Electronics, 1976, 19. 259. lle

lao

Photochemical Aspects of Solar Energy Conversion

589

superficial layer (i.e. varying x in a controlled manner) should lead to substantially improved efficiency. The first report has appeared of the fabrication of such a cell, which had the expected good characteristics (Kc 0.89 V, isc 18 mA ern-,, fill factor 0.8, efficiency 14%).12g James and Moon127have constructed a GaAs concentrator solar cell which produced a power of 4.52 W cm-, at a concentration of x 312 suns, an overall conversion efficiency of 17.5% Other Semiconductor Heterojunctions.-Bachmann et have considered the general requirements for good efficiency in polycrystalline heterojunction solar cells. These include a good lattice match between the two components and an easy method of substrate preparation. They have called attention to the fact that none of the 11-IV-V2 compound semiconductors such as InSiP, have been evaluated as solar cell components, and only CuInSe, of the I-111-VI, compounds has been investigated. The latter seems a promising material: the calculated theoretical conversion efficiency for CuInS, p n homojunct ions is 27.4-32.1%, taking a realistic minority carrier lifetime of 10-8-10-6 s.12@ p-Cu,Se/n-AgInSe, structures exhibit photovoltaic effects, with a peak current efficiency of 0.18 at 1150 nm.130 n-CdSe/p-ZnTe heterojunctions also have photovoltaic properties, though solar cell performance is limited by the short diffusion length of holes in CdSe (GC0.55-0.61 V, fill factor 0.37-0.56, current efficiency 0.14--0.25%).131 Very small photovoltaic effects have been reported in the heterostructures Au/Ge,Se,-,/SnO, and Ge,Sel-,/Se/Sn0,.133 N

Schottky Barrier Solar Cells.-Theory. McOuat and Pulfrey lS4 have extended their general model of metal-semiconductor solar cells, taking into account all likely loss mechanisms. Comparison of experimental and computed data for Auln-GaAs yielded good agreement. Green 196 has compared the depletion layer collection efficiency for p n junction, Schottky barrier, and surface insulator solar cells. He has shown that the collection efficiency of a Schottky barrier is generally smaller than for a p-n junction, and that previous approaches, which have generally assumed a collection efficiency of unity for a Schottky barrier, are therefore overoptimistic in their conclusions. Green135and several other workers136-138have shown that an MIS Schottky barrier solar cell is capable of better performance than is a comparable MS Schottky diode. An optimum thickness of ca. 2 nm for the insulating layer has lZe

M. Konagi and K. Takahashi, J. Appl. Phys., 1975,46, 3542,

lZ7

L.W.James and R. L. Moon, Appl. Phys. Letters, 1975,26,467. K. J. Bachmann, E. Buehler, J. L. Shay, and S. Wagner, 2. phys. Chem., 1975, 98, 365.

lee 12*

J. M. Meese, J. C. Manthuril, and D. R. Locker, Bull. Amer. Phys. SOC.,1975,20, 696. B. Tell, P. M. Bridenbaugh, and H. M. Kasper, J. Appl. Phys., 1976,47,619. F. Buch, C. L. Fahrenbuch, and R. H. Bube, Appl. Phys. Letters, 1976,28,593. M. Okuda, T.T. Nang, T. Matsushita, and S. Yokota, Jap. J. Appl. Phys., 1975,14,1597. T.T.Nang, M. Okuda, T. Matsushita, S. Yokota, and A. Suzuki, Jap. J. Appl. Phys., 1976,

lZ0 131

lSB lS3

IS, 383. 13'

lSs lS7 13*

R. F. McOuat and D. L. Pulfrey, J. Appl. Phys., 1976,47,21 13. M.A. Green, J. Appl. Phys., 1976,47, 547. R. L. Anderson, Appl. Phys. Letters, 1975,27, 691. H.C. Card and E. S. Yang, Appl. Phys. Letters, 1976,29,51. R. Singh and J. Shewchun, Appl. Phys. Letters, 1976,28, 512.

20

Photochemistry

590

been Because this layer increases the open-circuit voltage at the expense of the short-circuit current, an MIS cell would be unsuitable for use in focused, intense s ~ n 1 i g h t . l ~ ~ Green139has described SBSC with non-planar geometry that should be 50% more efficient than the ordinary planar type. Inorganic Materials. Thin (ca. 10 nm) metal films on silicon substrates have been examined for photovoltaic effects.140 The best result was obtained with 5 nm Cr overlaid by 5 nm Cu, the entire metal film having a transmission of k 55% and a resistance of 20 ohm square-l. SBCS with 9.5% sunlight conversion efficiency, and better short wavelength sensitivity than a conventional p-n silicon cell, were prepared. Silicon MOS solar cells : Silicon monoxide I 8-12 nm A1 I 2-4 nm SiO, I p-Si AR coating

also perform well (isc 26.5 mA cm-2, efficiency 8% in simulated sunlight).141 Au/GaAs,-,P, SBSC can be significantly improved by formation of a 3-5 nm oxide layer of GaAs,-,P, before the gold is deposited; efficiencies of up to 15% can be achieved thereby.142 Organic Materials. Photovoltaic effects in Cr/Chl a/Hg and Cr/Chl a/Hg-In sandwich cells have been The Cr/Chl a interface is a Schottky barrier, the Hg/Chl a interface is ohmic. The best power conversion efficiency (ca. was observed in Cr/Chl a/Hg cells at 740 nm. Chl a-coated Cr in contact with Chl a in non-polar solutions showed a quantum yield of charge injection of ca. 0.1 .144 Small photovoltaic effects have been observed in Au/copper(ii) phthalocyanine/Al cells 145 and in Au/naphthalene 1 a ~ e r s . l ~ ~ Rosseinsky and co-workers 14' have reported the dark and photoconductivity of several novel adducts of methylene blue with anionic transition-metal complexes of PFs- or dithiolate. An estimate of the photovoltaic conversion efficiency for an organic film system based on the (1 : 1) complex of poly-N-vinylcarbazole and trinitrofluorenone has been made.148 It is predicted that the limiting factor in films of thickness greater than 1 pm will be space-charge-limited conductance. In thinner films, the efficiency will be determined by the efficiency of hole-electron pair generation. While ultra-thin films could in principle approach 1% efficiency, such high values have not yet been observed in practice. M. A. Green, Appl. Phys. Letters, 1975, 27, 287. W. A. Anderson, A. E. Delahay, and R. A. Milano, Appl. Optics, 1976, 15, 1621. 141 E. J. Charlson and J. C. Lien, J. Appl. Phys., 1975,46,3982. 14* R. J. Stirn and Y. C. M. Yeh, Appl. Phys. Letters, 1975, 27, 95. 143 C. W. Tang and A. C. Albrecht, J. Chem. Phys., 1975,63,953. lQ4C . W. Tang, F. Douglas, and A. C. Albrecht, J. Phys. Chem., 1975,79, 2723. 145 A. Goswami and S. Radhakrishnan, Indian J. Pure Appl. Phys., 1975, 13, 439. l 4 6 M. Matsumura, H. Uohashi, M. Furasawa, N. Yamamoto, and H. Tsubomura, Bull. Chem. SOC. Japan, 1975,48, 1965. 14? D. R. Rosseinsky, K. Kite, R. E. Malpas, and R. A. Hann, J. Electronnnlyt. Chem. Interfacial Electrochem., 1976,68, 120. 148 P. J. Reucroft, K. Takahashi, and H. Ullal, J. Appl. Phys., 1975, 46, 5218. 130

140

Part VI CHEMICAL ASPECTS OF PHOTOBIOLOGY By

G. BEDDARD

1 Introduction This Report deals with the photochemical and photophysical aspects of the primary processes in photosynthesis and vision. Because of the overwhelming importance of chlorophyll (Chl) to photosynthesis, and the retinals to vision, the photochemistry and photophysics of these molecules are discussed in relation to their photobiological roles. The review of photosynthesis is concerned with processes which occur in the light-harvesting arrays and reaction centres.l Work dealing with oxygen evolution and with the electron-transport chains following the secondary acceptor to the reaction centre is not covered. Likewise, in the section which covers visual processes, only the chromophore isomerization and resulting conformational changes induced in the protein are reviewed. 2 Photosynthesis Chlorophylls in vivo and in vitro.-Several studies have in the past made it clear that while both singlet and triplet chlorophyll (ChlT) states are quenched in the presence of quinones, Chl+' is only detected when produced from the triplet. This failure to observe Chl+' in singlet quenching may be explained by a rapid decay of the excited singlet charge-transfer complex (Chl+' Q-*) rather than by the ion-pair separation which occurs from the triplet complex. Lamola et aZ.,2 using the ENDOR technique, claim that photo-oxidation of Chl-a and -b occurs only from the triplet, but they are unable to exclude the possibility that an ion-pair formed from the singlet recombines rapidly. Bacteriopheophytin (BPh), quenched by p-benzoquinone (BQ), produces BPh+' from a triplet complex (BPh+' BP-') > 1 ns after excitation when the singlet is largely decayed. It is suggested that the singlet complex decays faster than the ions can ~ e p a r a t e , ~ as the radical-cation is not detected when this state is quenched. BChl and BPh

a

'Bioenergetics and Photosynthesis', ed. Govindjee, Academic Press, New York, 1975; R. Malkin, Photochem. and Photobiol., 1975, 22, 292; W. Cramer and P. Horton, ibid., 1975, 22, 304; J. Norris, ibid., 1976, 23, 449; L. Forster, ibid., 1976, 23, 445; P A . Song and R. Fugate, ibid., 1975, 22, 277; W. Parson and R. Cogdell, Biochim. Biophys. Actu, 1975, 416, 105; J. Anderson, ibid., 1975, 416, 191; H. T. Witt, Nuturwiss., 1976, 63, 23; R. Radmer and B. Kok, Ann. Reu. Biochem., 1975,409; M. Calvin, Amer. Sci., 1976,64,270; Proceedings International Congress on Photosynthesis, Jerusalem, 1975, ed. M. Avron, vol. 1-3; A. Takamiya and E. Yakushyi, 'Tampakushitu Kakusan KOSO'Bessatu, Japan, 1976, p. 386. A. Lamola, M. Manion, W. Roth, and G. Tollin, Proc. Nut. Acud. Sci., 1975,72, 3265. D. Holten, M. Gouterman, W. Parson, M. Windsor, and M. Rockley,Photochem. and Photobiol., 1976, 23, 415.

593

594

Photochemistry

radical-cations, detected by e.s.r., are produced in the quenching by phenylhydrazine or tri~henylamine.~T-T absorption spectra and kinetics of a 'triplet exciplex' formed between Chl-a and BQ are observed during photo-oxidation.6 Difference spectra show a large negative band at 430 nm and a positive band from 465 to 600nm. This species does not appear to have been observed by others. Photoconductivity measurements on Ch1+',6the effect of dielectric constant on its formation in the presence of BQ,' and Chlf' produced by quenching by nitronaphthalenes have all been reported. Photo-galvanic, -voltaic, and -electric effects of Chl are reported in Part V. The redox properties of Chl excited states and of Zn uroporphyrin triplet states, which are similar to those of Chl, have been measured.n Triplet oxidation is charge-controlled and electron transfer is found to occur over distances as large as 20-30 A.lo Chlorophyll T-T annihilation in polar solvents is found to be the main source of Chl cations and anions.ll The Chl+' threshold formation energy in MeCN is 1.8 eV compared with 5 eV for direct photoionization in tetramethylsilane. The wavelength dependence of Chi+' destruction in propanol was measured in an unsuccessful attempt to form the Chl dication.12 The absence of this species is explained by the short lifetime of excited Chl+'. The irreversible photooxidation of bacterioviridin in the presence of BQ results in the production of a doubly oxidized non-radical species of bacteri~viridin.~~ Electron-transport between Chl-a and c a r o t e n e ~l5 , ~Chl-a ~~ bleaching with digitoxin l6 and its photochemical hydrogenation have been rep0~ted.l~The observation that plastoquinone phosphorescence is quenched by Chl suggests an in vivo role to protect the quinone from photoreduction.lB Chl fluorescence and that of its heavy metal analogues is quenched by quinones and by dynamic and static processes depending on the viscosity of the medium.ln Reversible photo-oxidation of BChl-a and -b occurs via BChl+' formation from -160 to 0 ° C in aqueous detergents with ubiquinone acceptor and dichlorophenolindophenol donor.20 BChl-b is more reactive than BChl-a and the oxidation rate depends on the This type of redox system attempts to mimic

' lo

l1 la l8

l4

l6 l6 l7

V. Voznyak, V. Kim, and V. Evstigneev, Biofizika, 1975,21,54. N. Andreeva and A. Chibisov, Biofizika, 1976,21, 24. N. Gudkov, Yu. Stolovitskili, and V. Evstigneev, Biofizika, 1975, 20, 807. V. Garrilova, BioJizika, 1975, 20, 996. V. Voznyak, V. Kim, and V. Evstigneev, Biofizika, 1975,20,406. B. Kiselev, Yu. Kozlov, and V. Evstigneev, Doklady Akad. Nauk, S.S.S.R., 1976, 226,310. P. Carrapellucci and D. Mauzerall, Ann. New York Acad. Sci., 1975, 244, 214. J. Imura, T. Kurutsuka, and K. Kawabe, Photochem. and Photobiol., 1975, 22, 129. N. Andreeva, A. Peshkin, and A. Chibsov, Biofizika, 1975,21, 24. V. Kim, E. Elfinov, and V. Voznyak, Biofizika, 1975,21,50. J. Lafferty, E. Land, and T. Truscott, J.C.S. Chem. Comm., 1976,2,70. S . Lebedev, V. Chepelev, and I. Aleinikov, Fiziol. Biokhim, Kul't Rast., 1975, 7 , 356. H. Jonas, Planzenphysiol., 1977, 1, 42. G. Gurinovich, A. Losev, and M. Sarzhevskaya, Doklady Akad. Nauk ( B ) S.S.S.R., 1975,19, 1129.

N. Bunce, M. Hadley, A. Mellors, and W. Sandford, Photosynthetica, 1975, 9, 220. lB E. Kapinus, I. Ivnitskaya, and I. Dilung, Biojizika, 1975, 20, 41 1. 2O N. Drozdova, A. Umrikhina, E. Pushkina, and A. Krasnovskii, Doklady Akad. Nauk S.S.S.R., la

1975,225, 1198.

A. Krasnovskii, E. Pushkina, N. Drozdova, N. Bublichenko, and A. Umrikhana, Doklady Akad. Nauk S.S.S.R., 1975, 221, 1457.

Chemical Aspects of Photobiology

595

the photosynthetic reaction centre. In this, a donor D, a Chl complex P, and an acceptor A react according to DP*A -+ DP+A- -+ D+PA-. Chlf' is observed by e.s.r. in lecithin vesicles 22 and liposomes 23 when Fe(CN),3-, SmC13,22or iodide 23 are in the aqueous phase. Tomkiewicz finds that neither ubiquinone nor ferredoxin in the water oxidizes the Chl in the lipid. A kinetic analysis of 24 Fe(CN),3- quenching is given which accounts for diffusion of the Oettmeier et observe reversible electron-transfer within the liposome via Chl from NNN'N'-tetramethyl-p-phenylenediamine donor to ubiquinone acceptor. The Chl+' signal, observed in the light, takes 2 min to reach full intensity, but no signal was observed with Fe(CN),3- acceptor, in contrast with Tomkiewicz and Gorker,22possibly due to different instrumental sensitivities. A good model of photosynthetic charge separation is described by Mange1.25 Charge transport occurs across Chl-carotene-containing liposomes in the presence of a potential gradient produced by encapsulating FeC1,-FeCl, + buffer inside the liposome and adding ascorbate to the aqueous phase. The absorbance of Fe3+ produced on illumination is measured. The calculated efficiency charge found in Black Lipid transport of 0.075% is considerably in excess of the Membranes (BLM's). The mechanism of the electron transport is undetermined. Chl dimers may be involved but their presence in the bilayer is not certain. Lipid peroxide formation causes Chl-containing bilayers to become permeable to protons in the presence of the Fe3+-Fe2+couple;26the effect on liposomes is unknown. BLM's made of Chl-containing chloroplast fragments have their photosensitivities enhanced by added biliproteins, e.g. p y c ~ c y a n i n28, ~ ~or~ amino-acid~.~~ The membranes separate Fe2+-Fe3+and ascorbate, and the electron transport is aided by Fe3+-biliprotein complexes. Water-soluble Chl derivatives, chlorophillin and chlorin E, act as sensitizers for electron transfer from methyl-red to ascorbate on poly(~inylpyrrolidene).~~ Chlorophyll incorporation into hydrated lipid bilayers perturbs the lipid structure. The porphyrin ring resides in the polar head region of the lipid at 55" to the b i l a ~ e r in , ~ ~agreement with earlier determinations. Spin probes also locate Chl in the same region in l i p o s o m e ~ .In ~ ~these lipid systems Chl is able to exist only as the monomer,22p23but Chl-containing liposomes can be made to contain Chl aggregates that absorb at 685 nm and quench the Chl monomer 34 An increase in fluorescence intensity in these scattering solutions fluores~ence.~~~ at the liquid-crystalline phase transition has led to the suggestion that disaggregation occurs at the phase 34 However, fluorescence intensity measurements do not account for the solution transmittance changes. a2

23 24 25 20

27 28

31 32 33 34

M. Tomkiewicz and C. Gorker, Photochem. and Photobiol., 1975, 22, 249. W. Oettmeier, J. Norris, and J. Katz, Z.Naturforsch., 1976,31c, 163. M. Tomkiewicz and C. Corker, Chem. Phys. Letters, 1976, 37, 537. M. Mangel, Biochinz. Biophys. Acta, 1976,430,459. L. Boguslavski, B. Lozhkin, and B. Kiselev, Doklady Akad. Nauk S.S.S.R., 1975,221, 228. C. Chen and D. Berns, Proc. Nat. Acad. Sci. U.S.A., 1975,72, 3407. D. Berns and C. Chen, Jerusalem Symposium on Quantum Chemistry and Biology, 1975,547. M. Mangel, Biochim. Biophys. Acta, 1976, 419, 404. V. Evstevneev and I. Nazarova, DokIady Akad. Nauk S.S.S.R., 1975, 224, 964. F. Podo, J. Cain, and J. Blasie, Biochim. Biophys. Acta, 1976, 419, 19. W. Oettmeier, J. Norris, and J. Katz, Biochem. Biophys. Res. Comm., 1976, 71, 445. A. Lee, Biochim. Biophys. Acta, 1975, 413, 11. A. Lee, Biochemistry, 1975, 14, 4397.

Photochemistry

596

Transmittance changes have been shown to be the cause of increased pyrene cation formation at the phase change;35the cation yield is unchanged, however. In uiuo, Chl molecules (and a number of accessory pigments such as carotenes) occur in photosynthesizing organisms in an efficient light-harvesting network to transport the energy absorbed to the reaction centre. The mechanism of the energy migration and the Chl structure in the harvesting network have been discussed.’ Energy migration, by repeated Forster transfers, among Chl-a molecules on polyvinylpyridine, sensitizes BChl fluorescence at several Chl : BChl The Chl-a fluorescence is quenched by Chl-Chl, BChl-BChl, and most effectively by Chl-BChl ‘dimers’. Modification of the Forster equation for single-step transfers to account for multiple steps among donor molecules before quenching unfortunately gives only fair agreement with the data, the best fit being where migration is Chl-c fluorescence is quenched by Chl-a at Strouse 37 has determined the crystal strucvarious pigment ture of ethyl chlorophyllide-a and -b dihydrate and finds linear and twodimensional polymers with the Chl molecules hydrogen-bonded together. He proposes that these linear polymers are of the type used in vioo in the antennae, as the low-temperature absorptions correspond closely to those in the chloroplast. While exciton interaction at low temperature can lead to efficient energy transport, it is difficult to see why this should occur at physiological temperatures where thermal motions would be expected to destroy coherence. Norris et aLS8 propose that the light-harvesting antennae are anhydrous oligomers of Chl. This implies that the Chl environment must be anhydrous, that no breaks or impurities occur in the oligomer as these would stop energy transfer, and that (Chl), must be synthesized in an anhydrous environment. However, Chl has been shown to be in the hydrophilic part of bilayers 31s sB and Chl-Chl dimers, in the little photophysics of them that is known, appear to quench energy 36a Indeed, self-quenching of the fluorescence, caused by transfer to weakly fluorescent molecular associations, is present in all model systems of the light harvesting yet made. Beddard and Porter 40 have shown by a Monte Carlo calculation that the fluorescence concentration quenching of Chl can be described by energy migration (Forster) about a random array of molecules, with quenching occurring when two molecules are closer than 1.0 nm. Both fluorescence yields and fluorescence quenching rate, which is time de~endent,~’ have been calculated at various Chl concentrations. It is suggested that galactosyldiglycerides hold the Chl apart in vivo to prevent quenching, but allow fast energy migration. These lipids, which may bond to the Chl Mg or to ring-IV or -V ester or carbonyl groups, have been shown to be necessary for photochemical activity in c h l o r o p l a ~ t s . ~Not ~ - ~ all ~ the Chl is contained in the lipid, however, 34s

8s 3e

37

39 40 41 4a

49 44

D. Barber, D. Morris, and J. Thomas, Chem. Phys. Letters, 1976, 37, 481. (a) G. Seely, J . Phys. Chem., 1976, 80,441,447 (b) D. Wrobel, Z . Solomon, and D. Frankiowiak, Acta Phys. Polon., 1976, A49, 269. C. Strouse, J. Amer. Chem. Soc., 1975,97, 7230, 7237; C. Strouse, Prog. Inorg. Chem., 1976, 21, 159. J. Norris, H. Scheer, and J. Katz, Ann. New York Acad. Sci., 1975, 244, 260. A. Hoff, Photochem. and Photobiol,, 1974,19, 51. G. S. Beddard and G. Porter, Nature, 1976,260, 366; G . Porter, Nuturwiss., 1976, 63, 207. G. S. Beddard, unpublished results. Z. Krupa and T. Baszynski, Biochim. Biophys. Actu, 1975, 408, 26. B, Shaw, Diss. Abs. ( B ) , 1975, 36, 2784. A. Shaw, M. Anderson and R. McCarty, Plant Physiol., 1976,57,724.

Chemical Aspects of Photobiology

597

and the antennae may contain up to half of the Chl in proteins 45* 46 in which the Chl must be positioned so as to prevent quenching configurations from being present. The absorption spectra of Chl-lipid or protein interactions do not completely reproduce the in vivo absorptions as do the Chl oligomers. A systematic study of Chl-lipid-protein spectra has yet to be made. Chlorophyll and its analogues form a variety of oligomers in non-polar solvents and polymers in polar ones. In ionic and non-ionic detergents both Polymers of (Chl-dioxan), in decane Chl monomer and dimers are (Ch12-pyrazine),, (Chl-bipyrimidine),, and (Chl-H,O-pheophytin), in dodecane have been described.48 Fong et ~ 2 1 propose . ~ ~ a structure for (Chl-2H20), which differs from that of its ethylchlorophyllide analogue.37 A structure for (Chl), is also proposed 6o and Chl-H20 is found to be remarkably stable, only losing the water at 120 0C.61 In aqueous acetone, polymers of Chl-a, -b, and epi-Ch1,62 and in aqueous ethanol those of pyropheophytin have been described.63 The latter may be related to pheophorbide-H,O polymers.37 1.r. spectra of the polymers of Chl-a,-b, proto-Chl, bacterioviridin, protopheophytin, and pheophytin 64 and c.d. and fluorescence-polarization of protochlorophyllide polymers have been measured.66 The properties of Chl and phycobiliproteins in cellulose nitrate films66 and the aggregation of a number of porphyrins in aqueous solutions have been detailed.67 Interestingly, some mixed polymers of Chl, proto-Chl, BChl, and pheophytin in dioxan-H20 show energy migration from short- to long-wavelength absorbing species, but the fluorescence yield and transfer efficiency are low (10-4).68 Fourteen forms, in two major families, of Chl aggregates have been observed at various Chl concentrations in CCI4 solution and at - 196 "C in solid films.59Proto-Chl aggregates and Chl phosphorescence at - 196 "C at 960 nm have been noted.6o It is remarkable that Chl phosphorescence can be seen in cellulose acetate films at 22 "C (at 960 nm) as well as at - 196 "C (940 nm).61 From the phosphorescence of octaethylchlorin and its bacterio- and metal derivatives it is suggested that the Chl triplet level should be 895 nm above that of the ground state.62 46

IS 47 dB 49

I1

J. Thornber, Ann. Rev. Plant Physiol., 1975, 26, 127. K. Kan and J. Thornber, Plant Physiol., 1976,57,47. A. Krasnovskii and A. Luganskaya, Zzvest. Akad. Nauk S.S.S.R.,Ser. Biol., 1976, 2, 182. J. Norris, H. Scheer, and J. Katz, Ann. New York Acad. Sci., 1975, 244, 260. F. Fong and V. Koester, Biochim. Biophys. Acta, 1976,423, 52; V. Koester, L. Galloway, and F. Fong, Naturwiss., 1975,62, 530; V . Koester, J. Polles, J. Koren, L. Galloway, R. Andrews, and F. Fong, J. Luminescence, 1976, 12/13, 781; F. Fong, Appl. Phys., 1975,6, 151. F. Fong and V. Koester, J. Amer. Chem. Soc., 1975, 97, 6888. N. Winograd, A. Shepherd, D. Karwick, V. Koester, and F. Fong, J. Amer. Chem. Soc., 1976, 98,2369.

A. Prischepov and G. Gurinovich, Zhur. priklad. Spektroskopii, 1975, 23, 458; Izvest. Akad. Nauk S.S.S.R., Ser. fz., 1975, 39, 1962. I 8 A. Prischepov, Zhur. priklad. Spektroskopii, 1975, 22, 857. 64 M. Bystrova, T. Mal'gusheva, and A. Krasnovskii, Mol. Biol. (Moscow), 1976, 10, 193. M. Brouers, Photosynthetica, 1975, 9, 304. I 6 A. Rotolo, Diss. Abs. (B), 1975,36,1692. C . Brown, M. Shillcock, and P. Jones, Biochem. J., 1976, 153, 279. E. Zen'Kevich, A. Losev, and G. Gurinovich, Mol. Biol. (Moscow), 1975,9, 516. I @ F. Litvin, V. Shobin, and V. Sineshchekov, Biofizika, 1975,20, 202. 6 o A. Krasnovskii, N. Lebedev, and F. Litvin, Doklady Akad. Nauk S.S.S.R., 1975, 225, 207. 61 N. Lebedev, J. NaGs, and A. Krasnovskii, Biofzika, 1975,21, 382. 6a A. Gradyushko, K. Solov'ev, A. Turkova, and M. Suivko, Biofizika, 1975,20, 602. 62

21

598

Photochemistry

Circular dichroism of Chl oligomers and dimers in CC14are markedly different from the spectra in chloroplasts;s3 only in rigid polystyrene was this difference less marked. Chl dimers in decalin interact with carotene l ~ t e i n .Disaggrega~~ tion does not occur, however, but it is suggested that the lutein fits between the two porphyrin rings which are tilted at 35" to one another.64 A quantummechanical formalism, applicable to aggregates in general, has been used to predict the properties of Chl aggregate^,^^ and MO calculations on Chl have also been presented.66 Maug7has shown that some unusual Chl fluorescence bands are not due to dimers6*or hot-band emission6Das has been suggested, but may be explained by self-absorption and impurities respectively. Chl-a and -b fluorescence has been vibrationally resolved at 10 K in an organic glass using the site-selection technique,70and the two-photon excited singlet and triplet cross-sections have been determined from the non-linear intensity dependence of the fluorescence and transmittance with laser intensity.71 Large photoelectron quantum yields of electron photon-l at 180 nm from Chl-a and -b mean that in electron microscopy of chloropfasts, the chlorophylls are the main source of the ~ in pure Chl-a or -b monolayers the Crystalline Chl is reported in B L M ' s , ~and fluorescence intensity changes non-uniformly with intermolecular distances (15-4OA range), possibly reflecting a packing change which occurs at about 35 A.74 Triplet-state zero-field splittings and intersystem crossing rates of Chl-a and -b and Zn-Chl show that the metal has little effect on the decay rates: these are very different from each sub-level of the triplet.75 With strong electron-withdrawing groups, the keto-group in ring v of Chl-a has much stronger binding energy than the two ester carbonyl groups, but for weak interactions the donor properties are similar.76 1.r. spectroscopy detects four types of electron-accepting abilities in Chl related to the different binding of the Mg to the ring.77 Co-ordination to the 5 and 6 positions on the Mg in BChl in dry solvents produces absorption bands at 580 and 610 nm re~pectively.~~ Some interesting new peripheral compounds formed from pheophytin in pyridine have absorptions to the red of p h e ~ p h y t i n .In ~ ~these compounds Mg2+,Zn2+,but not Cu2+,Ni2+, Mn2+,bind with the /3-ketoester of ring v instead of in the centre of the porphyrin ring. 63 64

O7

70

71

79 74

76

76

77 78

A. Gafni, H. Hardt, F. Schlessinger, and I. Stienberg, Biochim. Biophys. Acta, 1975,387, 256. A. Arnoff, Ann. New York Acad. Sci., 1975,244,320. J. Katz, J. Phys. Chem., 1976,80, 877. C. Weiss, Ann. New York Acad. Sci., 1975, 244, 204. A. Mau, Chem. Phys. Letters, 1976, 38, 279. M. Kaplanova and K. Vacek, Photochem. and Photobiol., 1974, 20, 371. E. Menzel and J. Polles, Chem. Phys. Letters, 1974,24,545. J. Funfschillung and D. Williams, Photochem. and Photobiol., 1975, 22, 151. R. Arsenault and M. Denariez-Roberge, Chem. Phys. Letters, 1976, 40, 84. R. Dam, K. Kongslie, and 0. Griffith, Photochem. and Photobiol., 1975, 22, 265. I. Csorba, J. Szabad, L. Erdei, and C. Fajszi, Photochem. and Photobiol., 1975, 21, 377. R. Aoshima, K. Iriyamu, and H. Asai, Biochim. Biophys. Acta, 1975, 406, 362. R. Clarke, R. Connors, T. Schaffsma, J. Kleibeuker, and R. Plantenkamp, J. Amer. Chem. SOC., 1975,98,3674. L. Shipman, T. Jansen, G. Ray, and J. Katz, Proc. Nat. Acad. Sci. U.S.A., 1975,72, 2873. J. Leickmann, M. Henry, R. Plus, R. Gillet, and J. Kleo, Nuclear Sci. Abs., 1975,31, no. 29 040. T. Evans and J. Katz, Biochim. Biophys. Acta, 1975, 396, 414. H. Scheer and J. Katz, J. Amer. Chem. SOC.,1975, 97, 3273.

Chemical Aspects of Photobiology

599

The photoreduction of H,-tetraphenylporphyrin to chlorin is found to be dependent on the amine used.80 Free-base porphyrin radicals are produced from a charge-transfer complex of the amine and porphyrin. The kinetics of the photoreduction accounts for the equilibrium between the radicals and their dimer which can also photodissociate to the chlorin. Photosystem I (PS I).-A small reversible bleaching of a Chl band at 703 nm is the result of system I reaction centre oxidation and is linked quantitatively to a reversible e.s.r. signal with g = 2.0025.81 This signal, with a linewidth 1/42 of that of normal Chl, and ENDOR spectra having hyperfine splittings almost half that for monomeric Chl, both indicate that P700 exists as a dimer.s8 It is proposed 38 that P700 has the structure (Chl-H,O-Chl) with bonding; Mg--O(H)H-*O=C ring v ester group of the other Chl. The species (Chl-ROH),; R = H, Et, protein, also has low-temperature absorption and e.s.r. properties similar to those of P700.82 It has been proposed that this symmetrical dimer has bonding Mg*-O(R)-H -*O=C-ring v carbonyl from one Chl molecule to the other. A similar dimer (Chl-H,O), had previously been suggested by Fong:s3 in this dimer bonding was made to -C=O in the ester group of ring v. At low temperatures in hydrocarbon solvents, the absorption of this dimer at 700nm was attributed to the equilibrium 2Chl-Hz0 + (Chl-H,O),. This species is also reversibly oxidized by I, in a reaction analogous to that of P700.49,60 The Chl-water ‘dimers’ described only have properties similar to P700 at low temperatures in hydrocarbon solvents. If these ‘dimers’ are good models of the reaction centre, then the effect of temperature should only be to restrict the relative motions of the Chl molecules. This may be achieved in vivo by the lipidprotein environment of the Chl.s2 There is still considerable controversy over the nature of the primary acceptor for P700. An iron-sulphur protein acceptor has been proposeda4 because of the stoicheiometry between its e.p.r. signal when photoreduced and P700 oxidaValues t i ~ n86. ~ ~ ~ for g of 2.05, 1.94, 1.86 are reported for blue-green algae and similar values have been given for subchloroplast particles.88 McIntosh et aLS9 and Evans disagree with this assignment and by freezing PS I particles under extreme reducing conditions, for example, illumination in the presence of methylviologen and dithionite, they observed a fully reversible e.s.r. signal, from a species X, at g = 1.76 on illumination. They propose that this signal is from the primary acceptor. The complete spectrum has g values 2.07, 1.86, and 1.76.89-92Bearden Y.Harel, J. Manassen, and H. Levanon, Photochem. and Photobiol., 1976, 23, 337. 82

83 84

86

J. Warden and J. Bolton, J . Amer. Chem. SOC.,1973, 95, 6435. L. Shipman, T. Cotton, J. Norris, and J. Katz, Proc. Not. Acad. Sci. U.S.A., 1976, 73, 1791. F. Fong, Proc. Nat. Acad. Sci. U.S.A., 1974, 71, 3692. R. Malkin and A. Bearden, Proc. Nut. Acad. Sci. U.S.A., 1971, 68, 16. A. Bearden and R. Malkin, Biochim. Biophys. Acfa, 1976,430, 538 (and references therein). A. Shuvalov, V. Klimov, and A. Krasnovskii, Mol. Biol. (Moscow), 1976,10, 326. R. Malkin, A. Bearden, F. Hunter, R. Alberte, and J. Thornber, Biochim. Biophys. Acfa, 1976, 430, 389.

R. Malkin, Arch. Biochem. Biophys., 1975, 169, 77. A. McIntosh and J. Bolton, Biochim. Biophys. Acta, 1976,430,555; A. McIntosh, M. Chu, and J. Bolton, ibid., 1975, 376, 308. M. Evans, C. Shira, J. Bolton, and R. Cammack, Nature, 1975,256, 668. M. Evans, Biochem. SOC.Trans., 1975,492. E. Evans, R. Cammack, and M. Evans, Biochem. Biophys. Res. Comm., 1976, 68, 1212.

600 Photochemistry and co-workers 87 did not observe signal X in their e.s.r. spectra under conditions similar to those used by Evans et d.90-91 The reasons for this are unclear. Experiments using 67Feindicate that X probably contains no Fe.92 Flash excitation of chloroplasts at room temperature produces a transient e.s.r. emission at g = 2.0037.93 This e.s.r. signal is thought to arise from the primary acceptor, although it differs in g value from the signals observed by Bearden and co-workers87 and Evans.g1 The e.s.r. emission results from a chemical reaction favouring the population of one spin state over the other, and slow spin-lattice relaxation allows the emission to be detected, with microsecond resolution, before the spin populations are equalized. The theory of the CIDEP phenomenon described indicates that the precursor of the e.s.r. signal is a triplet state or a radical pair. For the chloroplasts, the former possibility is favoured but not By analogy with bacterial reaction centres, the scheme (P700*)SXA + (P700')~xA -+ (P700f)X-A

-+

(P700+)XA-

describes the primary processes. A is an iron-sulphur protein 87 and P700 is the Chl dimer complex in which intersystem crossing may be fast since little fluorescence is seen.l#81 E.s.r. spectra also show that the iron-sulphur ferredoxin has a box-like 4Fe-4S, rather than a 2Fe-2SY Purification of the reaction centres to contain only one instead of five or six polypeptides removes the bound iron-sulphur e.s.r. signaLQ6In fact two iron-sulphur proteins are present at potentials of - 540 and - 590 mV 91 and are close to species X as electron transfer occurs at liquid helium temperatures and in <20 ps at room t e m p e r a t ~ r e .Bolton ~~ and Warden 94 have suggested that these two proteins are coupled to the cyclic electron flow (- 540 mV species) and to the non-cyclic flow to NADP+. Two candidates for the donor to P700+ have been proposed, they are cytochrome F (cyt-f) and plastocyanin (pc).' The precise relationship between these two remains to be established. Haehnil 98 found that when both cyt-f and pc are reduced P700+ decays in 20ms, but when both are oxidized two decays of P700f of 20 ps and 300 p are observed. Warden 9g shows that the 300 p s component represents P700+ oxidation with a component at a potential of 375 & 10mV, indicating that cyt-f is located next to P700. Other experiments suggest, however, that cyt-f and pc act in The kinetics of electrontransfer to P700+ from several cytochromes and proteins have been reported.lol Biphasic kinetics of P700+ reduction at high light intensities is thought to be incongruous with a linear electron transport chain.lo2 Further details on PS I are given in references 1 and 103. R. Blankenship, A. McGuire, and K. Sauer, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 4943. J. Bolton and J. Warden, Ann. Rev. Plant Physiol., 1976, 27, 375. R. Cammack and M. Evans, Biochem. Biophys. Comm., 1975, 67, 544. 88 M. Nelson, C. Bengis, B. Silver, D. Getz, and M. Evans, F.E.B.S. Letters, 1975, 58, 363. Q7 B. Bouges-Bocquet, Biochim. Biophys. A d a , 1975, 396, 382. 88 W. Haehnil, G. Doring, and H. Witt, Z . Naturforsch., 1971, 26b, 1171. 9n B. Warden, Biochim. Biophys. Acta, 1976, 440, 89. l o o W. Haehnil, 'Third International Congress Photosynthesis Research', ed. M. Avron, Elsevier, Amsterdam, Vol. 1, 1975, p. 557. loL P. Wood and D. Bendall, Biochim. Biophys. Acta, 1975, 387, 115. lo2 S. Khangulov and M. Gol'dfel'd, Biofizika, 1975, 20, 652. lo5 J. Bolton and J. Warden, Ann. Rev. Plant Physiol., 1976,27, 375. Q3

94

Chemical Aspects of Photobiology

601

Photosystem I1 (PSII).-It is now generally accepted that PS I1 reacts analogously to P S I although the species involved are not so clearly defined. The reaction centre Chl complex (P680) bleaches at 682nm, and an absorption at 320 nm called X320 is identified with the radical-anion of the primary acceptor, possibly plastoquinone. At low temperatures the absorption at 550 nm (C500) has kinetics similar to X320, but as it is not formed in the first flash it is not involved in the primary act but reflects the state of the Carotenoids may be involved in the 500 nm A flash-induced absorption change at 825 nm is attributed to P680 ChlChl+-.105-107This species undergoes a back reaction at 200 s-l or is reduced by the secondary donor at 50s-l. Similar processes have been detected by flash e.s.r.loS At low temperatures the scheme is: P680* Q +-~ 5 6 P680+ 9 Q- + ~ 5 5 9 +P680 Q-

~559

where c559 is a cytochrome and donor to P680+.10BIt may not be the only donor, as P680+ reacts in a similar manner if the cytochrome is oxidized with FeCN63-.105* lo8 Little is known of the formation rate of P680f except that it occurs in < 50 ps.lo53lo6 X320 forms in < 1 ps in tris washed chloroplasts.110 This high turnover rate is mediated by charge recombination or fast cyclic electron Delayed fluorescence has been used to monitor reversible charge separation in PS 11,111and at low pH, light-induced absorption and e.s.r. changes may be caused by a Chl dimer in P680.ll21113 Microwave-induced transitions in zero-field in dithionite-treated chloroplasts at 2 K are observed as changes in fluorescence at 735 nm. The triplet state parameters are very similar to those for monomeric Chl, and Hoff and Van der Waalsl1* suggest that the electron may not be delocalized over the Chl dimer in the reaction centre. This observation is remarkable also in that the changes in PS I1 are seen at 735 nm, a wavelength normally ascribed to PS I emission. McIntosh and Bolton 115 have recently seen a CIDEP emission from room temperature chloroplasts which they tentatively attribute to a triplet state of a Chl dimer. A light-induced transient e.p.r. signal is observed in chloroplasts at room temperature when oxygen evolution is inhibited.ll6S117 This signal is from the physiological donor to P680+, termed 2. As Z+' is formed stoicheiometrically with the P S I signal, this indicates that the donor to P680 is different at room (a) R. Radner and B. Kok, Ann. Rev. Biochem., 1975, 409; (6) H. Van Gorkom, Biochim. Biophys. Acta, 1974, 347, 439. loo P. Mathis and A. Vermeglio, Biochim. Biophys. Acta, 1975, 396, 371. lo4

P. Mathis, A. Vermeglio, and J. Haveman, in 'Lasers in Physical Chemistry and Biophysics', ed. Joussot-Doubien, Elsevier, Amsterdam, 1975, p. 465. lo' J. Haveman, P. Mathis, and A. Vermeglio, F.E.B.S. Letters, 1975, 58, 259. l o * R. Malkin and A. Bearden, Biochim. Biophys. Acta, 1975, 390, 250. l o g P. Mathis and A. Vermeglio, Biochim. Biophys. Acta, 1975, 368, 130. C. Renger and Ch. Wolff, Biochim. Biophys. Acta, 1976, 423, 610. ll1 H. Van Gorkom, M. Pulles, M. Haveman, and G. den Haan, Biochim. Biophys. Acta, 1976, lo(

423, 217. lla 113

114

116 11'

H. Van Gorkom, M. Pulles, and J. Wessell, Biochim. Biophys. Acta, 1975, 408, 331. M. Pulles, H. Van Gorkom, and C. Verschoor, Biochim. Biophys. Acta, 1976, 440,98. A. Hoff and J. Van der Waals, Biochim. Biophys. Acta, 1976, 423, 615. A. McIntosh and J. Bolton, Nature, 1976, 263, 443. G. Babcock and K. Sauer, Biochim. Biophys. Acta, 1975,376, 315, 329; ibid., 1975,396,48. R. Blankenship, G. Babcock, and K. Sauer, Biochim. Biophys. Acta, 1975,387, 165.

602 Photochemistry and low 117 A further rapid transient signal IId is detected at 3381 G in oxygen-evolving chloroplasts from species Z+'.118e 119 It is suggested that this is formed by electron tunnelling to P680+.ll8*119 When Mne+ is extracted,lZ0Z+' decays in 1 s compared with 400-900 ps when bound Mn2+ associated with oxygen evolution is p r e ~ e n t . l l ~Two -~~~ pathways compete for P680+ reduction. Mn2+lowers the intensity of signal IIfl, but not the half-life, so this hydrophilic reaction site acts in parallel with the unknown lipophilic 117 Delayed fluorescence from PS I1 particles also supports the assignment of signal IId to Z+'.121 The rates of reaction of Z+' with the oxygen-evolving precursor states So***S,have been reported.lZ2 Many other aspects of PS I1 have been reviewed.l, 104a Photosynthetic Bacteria.--The photochemically active reaction centre of the carotenoidless mutant of the purple bacteria Rhodopseudomonas sphaeroides contains only three polypeptides, four BChl, two Bacteriopheophytins (BPh), one ubiquinone, and a non-haem iron, and is free of antennae Chl.lZ3 When the were able to detect two excited states, Pf and acceptor is reduced, Parson et aZ.lZ4 Pr at low redox potential. Their spectra generally resembled those of triplet states of BChl and BPh. The state Pf which formed immediately after the flash decayed with a lifetime of 6 ns, whereas Pr decayed in 6 ps, both at 295 K. Excitation with 6 p s laser pulses at 530nm have revealed that Pf decays with a half-life of between 150 125 and 250 ps lZ6 when the suspension is at +220 mV. At - 200 mV the absorbance changes persist, thereby indicating that this state is responsible for charge separation. Pf is formed within the laser flash at almost unity quantum yield when the acceptor is oxidized. A bleaching decaying with 30 ps lifetime is observed at 800 nm and measurements at loo&--1250 nm show the 1250 nm band to be formed in ca. 10 ps.lZ7 This band is characteristic only of (BChl), and indicates that this is present as part of the species Pf. Pf is not a pure singlet state as the decay is too fast.125,l z 6 ,128 Fajer et aZ.lZ9have generated the anion-radical of BPh by a chemical method, and have determined its structure by e.s.r. spectroscopy and MO calculations. They have proposed that BPh acts as a transient electron-acceptor between the primary donor and ubiquinone 'primary' acceptor. A composite spectrum of ((BChl),+'-(BChl),} plus (BPh-'-BPh) is qualitatively similar to the Pf transient spectrum from 350 to 1250 nm except for a small band near 800 nm. The 118

J. Warden, R. Blankenship, and K. Sauer, Biochim. Biophys. Acta, 1976,423,462. R. Blankenship, G . Babcock, J. Warden, and K. Sauer, F.E.B.S. Letters, 1975,51, 287. T . Wydrzynski, N. Zumbulyadis, P. Schmidt, and Govindjee, Biochim. Biophys. Acta, 1975, 408, 349.

lal lZ2

J. Haveman and J. Lavorel, Biochim. Biophys. Acta, 1975,408,269. G . Babcock, R. Blankenship, and K. Sauer, F.E.B.S. Letters, 1976, 61,286; B. Velthuys and

J. Visser, ibid., 1975, 55, 109. W. Parson and R. Cogdell, Biochim. Biophys. Acta, 1975, 416, 105. lZ4 W. Parson, R. Clayton, and R. Cogdell, Biochim. Biophys. Acta, 1975, 387, 265. lZ6 K. Kaufmann, P.Dutton, T. Netzel, J. Leigh, and P. Rentzepis, Science, 1975,188, 1301. 126 M. Rockley, M. Windsor, R. Cogdell, and W. Parson, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 2251. P. Dutton, K. Kaufmann, B. Chance, and P. Rentzepis, F.E.B.S. Letters, 1975, 60,275. l a g M. Windsor, J, Luminescence, 1976, 12/13, 893. l a @ J. Fajer, D. Brune, M. Davies, A. Forman, and L. Spaulding, Proc. Nut. Acad. Sci. U.S.A., 1975. 72, 4956. 123

603

Chemical Aspects of Photobiology

discrepancy is due to the rapid bleaching of this band and is involved with the disruption of the centre as charge separation 0ccurs.l C.d. spectra also monitor the effects of charge separati0n.l The primary processes may now be written as P*870 c,(BChl+'-BChl BPh)X-

-

c,(BChl+'-BChl BPh-')X c,(BChl+'-BChl BPh)X-

-

with Pf as the initially excited species, c2 a cytochrome and X the Fe2+ubiquinone acceptor complex. When this complex is reduced, species Pf12*decays to Pr with a lifetime of 30 ns. Pr is (BChl+'-BChl-' BPh X-) and this decays to (BChl-BChl BPh X-) in 120 p at 298 K. BPh has also been implicated in the transfer process by Shulalov et a1.l3O and photoinduced changes in the reaction centre bleaching have been observed to be temperature independent from - 160 to 20 "C.131 The nature of the Chl triplet state produced with the primary acceptor in the The state Pf is, of reduced form has been examined by e.p.r. course, too short to be observed, but any spin selection is preserved and the unusual polarization produced indicated that the initial act is a radical pair formation. Triplet zfs parameters and intersystem crossing rates have been measured in BChl and reaction 134 The two BChls in uiuo are inclined at 48" and rotated at 78" to one another. Resonance raman spectra135indicate that the four BChl are initially strongly coupled. Presumably the charge subsequently becomes localized on two of these BChls. The involvement of four BChl in the reaction centre is also proposed from linewidth measurements of e.p.r. spectra,126but ENDOR spectral linewidths indicate the presence of dimers in the reactive species of the reaction centre 136, 13' in line with other evidence.129 A photochemical quantum yield of unity (k 15%) for Pf formation from 5-300K has been The absorption due to BPh splits into two components at low temperatures. As Pf also shows a peak at one of these wavelengths (542nm), the two BPh are in different environments with only one directly involved in photo~hemistry.~~~, 138-140 Ubiquinone is considered to be the acceptor for BPh-• as its removal by detergents from the reaction centre results in Pf lasting 1 ns instead of 250 ps.138,140 In fact, two ubiquinones are present;141the first is removed easily and is a secondary acceptor, but removal of the other results in the loss of photochemistry. Low-temperature absorption measurements also implicate ubiquinone as the V. Shulalov, V. Klimov, I. Krakhmaleva, A. Moskalenko, and A. Krasnovskii, Doklady Akad. Nauk S.S.S.R., 1976, 227, 9847. 131 N. Karapetyan and A. Kononenko, Mikrobiologiya, 1975,44,422. 13% M.Thurnauer, J. Katz, and J. Norris, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 3270. 13s R. Clarke, R. Connors, and H. Frank, Biochem. Biophys. Res. Comm., 1976,71, 671. lS4 R. Clarke, R. Connors, J. Norris, and M. Thornauer, J. Amer. Chem. SOC.,1975,97, 1789. 136 M. Lutz and J. Kleo, Biochem. Biophys. Res. Comm., 1976,69, 711. 136 C. Feher, A. Hoff, D. Isaacson, and D. Ackermann, Ann. New York Acad. Sci., 1975, 244, 239. 13' P. Loach, M. Kung, and B. Hales, Ann. New York Acad. Sci., 1975,244,297. lS8 R. Clayton and T. Yamomoto, Photochem. and Photobiol., 1976,24, 67. 139 K. Kaufmann and P. Rentzepis, Accounts. Chem. Res., 1975, 8, 407. 140 K. Kaufmann, K. Petty, P. Dutton, and P. Rentzepis, Biochem. Biophys. Res. Comm., 1976, 70, 839. 141 M. Okamura, R. Isaacson, and C. Feher, Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 3491. 130

604

Photochemistry

a ~ c e p t 0 r . lFe2+ ~ ~ and ubiquinone may interact by magnetic exchange; the process facilitates electron transfer from one ubiquinone to the As the photochemistry of the bacterial reaction centre is different after the first flash it is suggested that the electron moves to the second acceptor as the next photoelectron appears.143 A double-flash laser technique144 has been used to measure the transfer rate between acceptors of ca. lo3s-l in two species of bacteria. At low temperatures this process may occur by electron tunnelling. When the primary acceptor is reduced and carotenes are present in the reaction centre, the triplet absorption of Pr is not seen but absorption due to triplet carotenes appears instead.14s,14* The formation of this triplet parallels the decay of the pf species. Carotenes excited with polarized light transfer energy to BChl from which the polarized fluorescence is in the same plane as the excitation, indicating that the two dipoles may be ~ a r a l l e 1 . lThe ~ ~ fluorescence polarization is almost zero when BChl is directly excited. The triplet carotene is present to prevent degradation of the centre, and the triplet may be seen at light intensities in excess of those needed for photochemistry to occur.147-149The carotene sphaeriodene is observed in resonance raman spectra but interacts only weakly with the reaction-centre pigments.lS0 Two reaction centres are proposed for Chromatium, only one of which contains a ubiquinone Delayed fluorescence in R. sphaeroides arises from a back-reaction between the oxidized centre and the reduced as with green plants. The state of the acceptor is reflected in absorption changes at 450 nm.142 Two cytochromes having redox potentials of + 255 and 390 mV act as electron donors to the oxidized reaction centre.153 Cyclic electron flow and the role of cytochromes have also been discussed.lS4 The structure of the water-soluble antennae protein from the green photosynthetic bacterium C. ZimicoZa is found to consist of three subunits each containing seven BChl molecules arranged irregularly within an ellipsoid of 45 x 35 x 15 A.1s5*lS6 BChl nearest-neighbour distances are 12-15 A and 24 A to the nearest subunit of the trimer. These distances are ideal for fast energy transfer without self-q~enching.~~, 41 C.d. spectra resulting from the interaction of the close BChl molecules show six components arising from seven molecules.157 The arrangements of the BChl in each trimer may be slightly different.16' The

+

J. Romijn and J. Amsez, Biochim. Biophys. Acta, 1976,423, 164. C. Wraight, R. Cogdell, and R. Clayton, Biochim. Biophys. Acta, 1975, 396, 242. 144 S. Chamorousky, S. Reminnikov, A. Kononenko, P. Venediktov, and A. Rubin, Biochim. Biophys. Acta, 1976, 430, 62. 146 R. Cogdell, T. Monger, and W. Parson, Biochim. Biophys. Acta, 1975, 408, 189. 14* R. Cogdell, W. Parson, and M. Kerr, Biochim. Biophys. Acta, 1976, 430, 83. 147 I. Ozolina and A. Mochalkin, Izvest. Akad. Nauk S.S.S.R., Ser. Biol., 1975,3,387. ua M. Kung and D. de Vault, Photochem. and Photobiol., 1976,24, 87. Ire M. Kung, Diss. Abs. (B), 1976,36, 3358. M. Lutz and J. Kleo, Biochem. Biophys. Res. Comm., 1976,69, 711. 161 K. Takamiya and M. Nishimura, Biochim. Biophys. Acta, 1975, 396, 93. Isa R. Carithers and W. Parson, Biochim. Biophys. Acta, 1976, 440,215; ibid., 1975, 387, 194. lS3 V. Sanders and 0. Jones, Biochim. Biophys. Acta, 1975,396, 220. 164 R. Prince and P. Dutton, Biochim. Biophys. Acta, 1975, 387, 609; R. Prince and P. Dutton, Arch. Biochem. Biophys., 1976, 172, 329; P. Dutton, K. Petty, H. Bonner, and S. Morse, Biochim. Biophys. Acta, 1975, 387, 536. 166 R. Fenna and B. Matthews, Nature, 1975,258, 573. 166 A. Lee, Nature, 1975, 258, 568. 167 J. Olson, B. Ke, and K. Thompson, Biochim. Biophys. Acta, 1976,430, 524.

14a

Chemical Aspects of Photobiology

605

mid-point potentials for reaction centres of this bacteria have been measured.15* Two cytochromes act as electron donors, and the electron acceptor is at a very low potential, much lower than in purple bacteria and sufficient for direct reduction of NAD+.lS8 Fast Fluorescence from the Light-harvesting Pigments.-Picosecond fluorescence from ChloreZla, chloroplasts, and subchloroplast particles has been measured by a number of groups, most using 530 nm excitation from either single laser pulses or whole-pulse trains of between about 6 and 10 ps per pulse. The time profile of the Chl fluorescence from the light-harvesting antennae arrays in the chloroplast is measured. The quenching observed arises from the capture by the reaction centre of the migrating excitation energy and migration occurs by multiple Forster-type single-step transfers among the Chl molecules. As several different pigments are present, the energy preferentially transfers to the longer wavelength-absorbing species of which the reaction centre is the longest. The possibility of transfer to a shorter wavelength-absorbing species has also been Shapiro et aZ.lSo~ lS1measured the Chl-a decays in ChZoreZla (41 ps), Anasystis (74ps), and in concentrated solution and concluded that the short decay times in vivo are due to concentration quenching. This suggestion was criticized by Beddard et aZ.,40,41 who pointed out that concentration quenching would prevent energy migration to the reaction centre. They measured longer decays (in ChZureZZa) than Shapiro and found a long component of low intensity in the decay, comparable with earlier nanosecond measurements.'* lS2 When the reaction centre is blocked the decay times rise to 1.5 ns.lS2 Spinach sub-chloroplast particles of systems I and I1 were found to have exponential fluorescence decays of 60 and 200 ps respectively 163 in agreement with yield measurements. Pashenko et aZ.lS4using both 530 and 694 nm excitation observed fluorescence decays very different from those of other groups. With 530 nm excitation and observing at 730 and 650nm, a fluorescence grow-in of 200ps and decays of 80ps and 300 4500 ps respectively were seen. The long grow-in time before fluorescence maximum is reached is probably an instrumental artifact and not due to energy transfer to carotenes as suggested.lS4,lS5 The clear picture of the fluorescence properties of the light-harvesting array was changed by an observation by Mauzerall,les who, using 7 ns long laser pulses, showed that the fluorescence intensity was a decreasing function of light intensity. He proposed that second hits on the excited reaction centre resulted in rapid quenching of the fluorescence. As all previous measurements using picosecond lasers were made with lo1' to at least 10l6photons cm-2 per pulse,

+

R. Prince and J. Olson, Biochim. Biophys. Acta, 1976,423, 357. us P. Bennoun and H. Jupin, Biochim. Biophys. Actu, 1976,440, 122.

168

180 161

V. Kollmann, S. Shapiro, and A. Campillo, Biochem. Biophys. Res. Comm., 1975,63,917.

16'

S. Shapiro, V. Kollman, and A. Campillo, F.E.B.S. Letters, 1975, 54, 358. G.Beddard, G.Porter, C. Tredwell, and J. Barber, Nature, 1975, 258, 166. W. Yu, P. Ho, R. Alfano, and M. Siebert, Biochim. Biophys. Acta, 1975, 387, 159. V. Paschenko, S. Protasov, A. Rubin, K. Timoteev, L. Zamazova, and L. Rubin, Biochim.

m6

Biophys. Actu, 1975,408, 143. V. Paschenko, A. Rubin, and L. Rubin, Kuantouayu. Electron., 1975, 6, 1336. D. Mauzerall, Biophys. J., 1976, 16, 87.

lea

606

Photochemistry

the accuracy of these measurements to measure the true decay time is brought into doubt, as the threshold for the onset of intensity quenching is < 1014photon cm-2. In fact, recent picosecond measurements demonstrate the decrease of lifetime and intensity of fluorescence as the laser intensity is i n c r e a ~ e d . l ~ ~ - l ~ ~ The quenching may be due to singlet-singlet annihilation162 or to triplet states162s170to which energy is transferred, the triplet being formed from a previous Iaser pulse when a whole pulse train is used. At low light levels (< 1013photons cm-2), the laser measurements produce a lifetime of 450 ps in agreement with previous nanosecond technique measurement^.^^^ The decays are still not single exponentials, however, and this fact can be attributed to timedependent quenching rates caused by energy migration in a process analogous to diffusion-controlled quenching.41,171 Decay times of chloroplasts and PS I type particles have been measured from 25 to -200 "C, and at low temperatures emission from free Chl is also seen.172 An unidentified species in Rhodospirillum rubrum absorbing at 420 nm has been observed to emit at 596 and 654 nm with a lifetime of 5.7 ns.173 Decay times of excited BChl and BPh in the reaction centres of R. rubrum may also have contributions from bulk pigments.174 Theoretical calculations have been made on the rate of energy migration and fluorescence yield 175, 176 using the 'master equation' approach and also using the formalism previously developed by Kenkre and Knox,17' with the master e q ~ a t i 0 n . lThe ~ ~ techniques used assume that the molecules are all fixed in regular arrays or lattices, but slight imperfections in regularity make the equations unsolvable. As it seems most reasonable to assume that the PSU is irregular, this approach should only be used to predict trends rather than details of energy migration. The fluorescence polarization has also been calculated for chloroplasts in many geometrical A principle of instant action has been formulated, viz. that energy migration must be complete in ca. 10-lo s for high photochemical yields to occur and to minimize photodegradation of the chloroplast.lB0 Carotenes.-The triplet states of spheriodene and sprilloxanthin efficiently protect photosynthesizing bacteria from O2 lAg, but carotene does this less well, and phytoenes not at all.lS1 Since these molecules have low triplet energies 107

A. Campillo, V. Kollman, and S. Shapiro, Science, 1976, 193, 227.

G. Porter, J. Synowiec, C. Tredwell, and J. Barber, Biochim. Biophys. Acta, in press. L. Harris, G. Porter, J. Synowiec, and C. Tredwell, Biochim. Biophys. Acta, 1976, 449, 329. 170 R. Knox and V. Ghosh, Photochem. and Pbotobiol., 1975,22, 149. 171 A. Campillo, S. Shapiro, V. Kollman, K. Winn, and R. Hyer, Biophys. J., 1976,16,93. G. Hervo, C. Pailliton, and J. Thiers, J. Chim. phys., 1975,72, 761;A. Brenze and A. Pelvas, Onde Electr., 1975,55, 518. 173 B. Silverstein, S. Malkin, and A. Haas, F.E.B.S. Letters, 1976, 63, 299. lI4 Govindjee, J. Hammond, W. Smith, R. Govindjee, and H. Merkelo, Photosynthetica, 1975, 16*

1°0

9, 216. G. Pailliton, J. Theor. Biol., 1976, 58,219. G . Pailliton, J. Theor. Biol., 1976,58,237. 177 V. Kenkre and R. Knox, Phys. Rev. (B), 1974,9,5279; Phys. Rev. Letters, 1974,33,803. 178 K. Colbow and R. Danyluk, Biochim. Biophys. Acta, 1976,44Q, 107. 170 M. Michel-Villas, J. Theor. Biol., 1976, 58, 113. la0 A. Borisov and W. Yu, Mol. Biol. (Moscow), 1976, 10,460. 181 R. Bensasson, E. Land, and B. Maudinas, Photochem. and Photobiol., 1976, 23, 189. 175

Chemical Aspects of Photobiology

607

they are also good energy acceptors for Chl triplet states in photosynthetic organisms, and so protect against photodegradation.lsl, lsa The spectra of carotene radical-cations are red-shifted by 150 nm from the anions, and also exhibit a solvent shift.ls3 Photochemical and spectral properties of carotenes in deuteriated and protonated solvents have been reported, and decomposition has been observed to be faster in d e ~ t e r i o - ~ 0 1 ~ e Fluornt~.~~~ mol 1-1 have been escence lifetimes of 55 ps for a- and fl-carotene at reported.ls6 Self-quenching is possibly a cause of this short lifetime, but a fast decay may be expected in view of the low fluorescence quantum yield. The temperature-dependent fluorescence yield of phytochrome has been ls7as has the yield (0.17) for phototransformation to the far-red absorbing species.188

3 Vision Retinals and Retino1s.-In contrast with aromatic species, the excited singlet states of diphenylpolyenes, retinol, and retinal do not show any heavy atominduced quenching. The intersystem crossing rates are the same with and without quenchers, but the triplet states are quenched.lss*ls9 There seems no straightforward explanation for this effect. An interaction between Lif and the double bonds in all-trans retinal causes an increase in its fluorescence quantum yield, and a similar effect is seen with some coumarins.lsS At low temperatures, the temperature-dependent quantum yields of alltrans-retinal and retinol ls6,ls9may be caused by the microviscosity of the solvent which restricts certain conformations of the excited state. Wavelength-dependent non-exponential fluorescence decays of retinollao are thought to be due to conformational relaxation of the excited singlet initially produced with groundstate geometry. A similar but unrelated effect is the wavelength dependence (354 vs. 265 nm) of intersystem crossing yields @isc in 11-cis- and 13-cis-retinals, where @)iac 353 > (Disc 265.1Q1This is due to increased photochemistry or internal conversion to So from the ‘cis’ band at 265 nm compared with internal conversion to S1. 9-cis- and 11-trans-Retinal have a constant (Disc but a wavelengthdependent fluorescence yield as described above. A common triplet state produced by sensitization for dienes and trienes of the vitamin A series has been invoked to explain the high yields (up to 100%) of the formation of the sterically hindered 7-cis-isomer;Isz g-cis-, 7,9-di-cis-, and all-trans-isomers are also produced, depending on the sensitizer energy. I. Ozolina and A. Mochalkin, Izuest. Akad. Nauk S.S.S.R., Ser. Biol., 1975, 3, 387. E. Dawe and E. Land, J.C.S. Faraday I, 1975, 11,2162. lS4 D. Frankowiak and G. Bialet, Bull. Acad. Polon. Sci., S b . Math. Astron. Phys., 1975,23,355. A.Campillo, R. Hyer, V. Kollman, S. Shapiro, and H. Sutphin, Biochim. Biophys. Acta, 1975, 387,533. lE8 P A . Song and Q. Chae, J. Luminescence, 1976,12113,831. lS7 P . 4 . Song, Q. Chae, and W. Briggs, Photochem. and Photobiol., 1975,22, 77. lE8 L.Pratt, Photochem. and Photobiol., 1975,22,33. lSo P . 4 . Song, Q. Chae, M. Fujita, and H. Baba, J. Amer. Chem. SOC.,1976,98,819. loo S. Georghiou, Nature, 1976,259,423; S. Georghiou and J. Churchich, International Quantum Chemistry and Quantum Biology Symposium, 1975, 2, 331. R. Bensasson, E. Land, and T. Truscott, Photochem. and Photobiol., 1975,21,419. lea (a) V. Ramamurthy and R. Liu, J. Amer. Chem. SOC.,1976, 10, 2935; (b) V. Ramamurthy, C. Tustin, C. Yan, and R. Liu, Tetrahedron, 1975, 31, 193.

lS2

ls3

608

Photochemistry

Non-classical behaviour (similar to that of the styrenes) is observed with endothermic sensitizers.la2 Higher members of the vitamin A series have planar triplets which, except the 7-cis-tripletYcan equilibrate at room temperature producing a mixture of isomers although not the 7-cis-isomer.la2 On direct irradiation, only singlet state isomerization occurs but in 11-cis-retinal both sub-nanosecond singlet and slower triplet isomerizations occur.la3 The 11-cis-Schiff's base isomerizes in <10 ns from the singlet, but no isomerization is seen from the protonated Schiff's base.ls3 It is not known whether these singlet isomerizations occur as fast as the ca. 10 ps reported in vivo by Busch et aZ.lS4 Theoretical calculations on polyene isomerization provide a mechanism for hydrogen-transfer reactions, and indicate that one photon can only isomerize one bond.ls5 This is apparently in conflict with experiments where a two-photon effect is claimed to occur in 9,13-di-cis-rhodopsinlQ6* IQ7and in the isomerization of 7,9-di-cis- to 13-ci~-retinal.~~~ It will be most difficult to demonstrate this two-bond isomerization conclusively because of the possibility of the second isomerization occurring thermally lQ9or via the absorption of a second photon by an intermediate. The natures of the polyene excited states are discussed in references 200-205. Visual Pigments.-The visual pigment rhodopsin is composed of an 1l-cisretinyl chromophore covalently bound to a protonated Schiff's base linkage of the &-amino group of a lysine residue in the apo-protein opsin.206 Upon absorption of a photon the 11-cis-chromophore isomerizes to the all-transretinal with a quantum efficiency of ca. 0.7. At room temperature opsin and all-trans-retinal are the final products of this reaction, but only at low temperatures have all the (dark) intermediates been identified. A number of retinal isomers besides the 9-cis- and 11-cis-isomers206 form rhodopsins. These are the methyl derivatives of 1l - c i ~ - , ~9,l O ~3-di-cis-,la79-cis-, and 9,13-di-cis-6,7-didehydro-5-dihydroretinals. The latter were used as stereoisomeric mixtures.208 With 7-cis-, 7,9-di-cis-, and 7,9,13-tri-cis-retinal~,~~~~ 210 lgS lg4

E. Menger and D. Kliger, J. Amer. Chem. SOC.,1976, 98, 3975. G. Busch, M. Applebury, A. Lamola, and P. Rentzepis, Proc. Nut. Acud. Sci. U.S.A., 1972, 69, 2802.

A. Kushnick and S. Rice, J. Chem. Phys., 1976,64, 1612. lee W. Waddell, A. Yudd, and K. Nakanishi, J. Amer. Chem. SOC.,1976, 98, 238. lg7 R. Crouch, V. Purvin, K. Nakanishi, and T. Ebrey, Proc. Nut. Acad. Sci. U.S.A., 1975, 72, 1538. lQ9 2oo 201

2oa

203 204 206 206

207

208

209 210

R. Liu, quoted in ref. 192u (ref. 19 therein). M. Applebury, D. Zuckerman, A. Lamola, and T. Jovin, Biochemistry, 1974, 13, 3448. A. Szabo, J. Langlet, and J. Malrieu, J. Chem. Phys., 1976, 13, 173. B. Mallik, K. Jain, and K. Mandal, Indian J. Pure Appl. Phys., 1975,13,699. A. Capparelli and 0. Sorrain, 2.phys. Chem., 1976, 256, 479. T. Moore, Diss. Abs. (B), 1976, 36, 5007. V. Komarov and L. Kayushin, Studies in Biophys., 1975,52, 107. N. Tyutyulkov, I. Petkov, 0. Polansky, and J. Fabian, Theor. Chim. Actu, 1975, 38, 1. E. Land, Photochem. and Photobiol., 1975, 22, 286; H. Shichi, ibid., 1975, 21, 457; T. Ebrey and B. Honig, Quart. Reo. Biophys., 1975, 8, 129; S. Ostroumov, Privoda, 1975, 3, 58 (in Russian); E. Menger, Accounts Chem. Res., 1975, 8, 81. T. Ebrey, R. Govindjee, B. Honig, E. Pollock, W. Chan, A. Yudd, and K. Nakanish, Biochemistry, 1975, 14, 3933. K. Nakanashi, A. Yudd, R. Crouch, C. Olson, H. Cheung, R. Govindjee, T. Ebrey, and D. Patel, J. Amer. Chem. SOC.,1976,98,236. W. De Grip, R. Liu, V. Ramamurthy, and A. Asato, Nature, 1976, 262,416. A. Asato and R. Liu, J. Amer. Chem. SOC.,1975, 97, 4128.

Chemical Aspects of Photobiology

609

the retinals formed show wavelength shifts caused by secondary interactions with the o p ~ i n 9-cis-Retinal .~~~ also forms an isoiodopsin with iodopsin, the opsin of chicken-retina cones,211 and /3-ionone and some derivatives preferentially react with opsin in the presence of ll-cis-retinal.211 Only the 13-cis- and alltrans-isomers do not form rhodopsins. The shape of the cleft in the opsin which contains the retinal is thus far less restrictive than had previously been supposed. It has been suggested that 9,13-di-cis-rhodopsin undergoes a single photoninitiated two-bond isomerization when the rhodopsin is detergent-s01ubilized.l~~ However, both cis- and trans-isomers can be produced using different detergents, so it is possible that the detergents affect the dark reactions after the initial isomerization. Protein-chromophore interactions are affected, but not chromophore c ~ n f o r m a f i o n212 ~.~~~~ A new Raman technique, which eliminates interference by photoproducts, shows 11-cis- and 9-cis-retinals to have spectra very similar to, although not identical with, rhodopsin and isorhodopsin respectively. The sample flows at 660 cm s-1 through an Ar+ laser beam which is sufficiently fast to prevent any photolysed sample contaminating the spectrum.212,213 The spectra are thus dissimilar to those previously reported in static solution. In solution, 1l-cisretinal has a predominantly 12-sym-trans conformation, but in rhodopsin this is distorted at C-ll.e13 The use of laser Raman spectroscopy is reviewed by Lewis.214 From a comparative study of the spectra of several retinals and their rhodopsins, Ebrey et aL207have concluded that 11-cis-retinal has the twisted 12-sym-cis configuration. This differs from previous suggestions and is based on a 30 nm blue shift observed in the absorption of 14-methyl-11-cis-retinal. The shift would not occur if the conformation were 12-sym-trans. That the rhodopsins formed by both retinals are similar suggests a 12-sym-cis-conformation in U ~ U O . ~ O ~ Crystals of 11-cis-retinal have the 12-sym-cis-configurationand similar Raman lines to those from rhodopsin, but unlike those in The half widths of visual pigment spectra as a function of wavelength216and the anion-induced changes in absorption of some retinylidene analogues have been A theoretical study of the rhodopsin spectrum using the Pariser-Parr-Pople method has been used to describe the chromophore properties.218 A vibrational analysis of the polyene chain has also been made.21e Absorption of a photon by rhodopsin results in the appearance of a transient species called bathorhodopsin (preluminrhodopsin) within a few This intermediate is converted in a series of dark reactions through lumi-, meta-I, and meta-I1 rhodopsin, and then to free opsin and 11-cis-retinal. Bensasson et aLzZ0 using 530 nm excitation observed a transient absorption at 330, 450, and 550 nm with a 125 ns decay time, and a transient at 470 nm with 211

21a

213

216 219

H. Matsumoto and T. Yoshizawa, Nature, 1975,258,523; H. Matsumoto, F. Tokunaga, and T. Yoshizawa, Biochim. Biophys. Acta, 1975, 409, 300. P. Mathis, A. Oseroff, and L. Stryer, Proc. Nut. Acad. Sci. U.S.A., 1976,73, 1. R. Callender, A. Doukas, R. Crouch, and L. Nakanishi, Biochemistry, 1976, 15, 1621. A. Lewis, Fed. Proc., 1976,35, 51. R. Cookingham, A. Lewis, D. Collins, and M. Marcus, J. Amer. Chern. SOC.,1976,98, 2759. A. Greenberry, B. Honig, and T. Ebrey, Nature, 1976, 257, 823. P. Blatz, L. Lane, and J. Aumiller, Photochem. and Photobiol., 1975,22,261. V. Komarov and L. Kayushin, Studies in Biophys., 1975, 52, 107. F. Inaguki, M. Tasumi, and T. Migazawa, J. Raman Spectroscopy, 1975, 3, 335. R. Bensasson, E. Land, and T. Truscott, Nature, 1975, 258, 768.

610

Photochemistry

a 5 ps decay time which corresponds to lumirhodopsin. Since the short-lived transient cannot be rationalized in terms of the dark intermediates, these workers suggest that the absorption is caused by a charge-transfer complex of rhodopsin possibly with tryptophan. This proposal receives support from the observation of tryptophan 222 and phosphorescence 222 in rhodopsin as a result of energy transfer from the chromophore. The c.d. spectra of rhodopsin also ~~~, indicates an interaction of retinal with aromatic a m i n o - a c i d ~ . 224 Goldschmidt et aZ.226have observed transients similar to those of Bensasson et aZ.220except that no peak at 330 nm was seen. They take a more conventional viewpoint and suggest that the short-lived transient absorption is due to the presence of isorhodopsin, the 9-cis-isomer. At low temperatures, the equilibrium rhodopsin + batho + is0 exists and is favoured by high light intensities. It is suggested 226 that the transient arises from contributions from iso-, batho-, and meta-I1 rhodopsins. But it is difficult to reconcile this proposal with the observation of the same decay rate across the transient spectrum225unless the interconversion rates are faster than the other decay rates. On the basis of resonance Raman spectra and H-D exchange, Fransen et aZ.22Ssuggest that bathorhodopsin is a hexaeneamine of 1l-cis-retinal in the 12-sym-cis-configuration. The spectra indicate the presence of an exomethylene double bond at C-5 to C-18. In addition, the 6% H-D exchange which occurs on illumination of lyophylized opsin in D 2 0 in the presence of ll-cis-retinal indicates that a hydrogen-shift reaction is taking place. No exchange is observed in unphotolysed samples or on photolysis of 1l-cis-retinal alone. Fransen and co-workers have implicated a hydrogen-shift rather than isomerization as the primary photoprocess. Eventually a hydrogen atom is replaced in the thermal reactions.22s Salem and Bruckmann 227 suggest that a twist about the ll-cis double bond in the excited m r * singlet state of an N-retinylidene chromophore triggers a short-lived electrical signal as charge moves from one end of the molecule to the other. They suggest that this is the first step of the isomerization. A transient dipole moment of 3&40 Debye is produced and this could, by electrostatic interaction, affect permeability of the disc membrane to Na+ or cause a conformational change in the protein. The molecular dynamics performed by retinal, initially as the 1 l-cis- (12-symtrans-) isomer, during photoisomerization in a restrictive site have been simuIsomerization occurs by concerted motion about parallel pairs of double bonds ('bicycle pedal' motion) and a cis-conformation can propagate from different double bonds to the terminal C-N bond. As both ends of the chromophore are constrained, vibrational modes of the molecule are restricted to this bicycle pedal motion and the conformation changes only along a 221 222

K. Shirane, Nature, 1975, 254, 722. S. Alekseev, N. Vselolodov, A. Kayushin, M. L'vov, and M. Ostrovskii, Studies in Biophys., 1976,54,85.

228 224 226

226

227 228

K. Azuma, M. Azuma, and T. Suzuki, Biochim. Biophys. Acta, 1975,393,520. J. Parkes, J. Rockley, and K. Liebmann, Biochim. Biophys. Acta, 1976, 428, 1. C. Goldschmidt, M. Ottolenghi, and T. Rosenfeld, Nature, 1976, 263, 169. M. Fransen, W. Luyten, J. Vanthujl, J. Lugtenberg, P. Jansen, P. Van Breugel, and F. Daemen, Nature, 1976,260, 726. L. Salem and P. Bruckmann, Nature, 1975,258, 526. A. Warshel, Nature, 1976,260, 679.

Chemical Aspects of Photobiology

611

one-dimensional pathway. The 90" twisted excited singlet produced about C-11, C-12 crosses to the ground state and the all-trans-isomer is formed in a series of dark reactions.228Not all of these highly plausible suggestions about the nature of the primary and subsequent events are compatible with one another and we shall have to await further experimental results to confirm these suggestions. The photoisomerization in vivo produces a protein conformational change that triggers the release of some transmitter substance, and it is of interest that neutron diffraction studies show that photolysis causes a small but significant outward shift of the protein in the membrane.229 Hydrogen-tritium studies show that exchange occurs from hydrogens near the membranewater surface, but illumination promotes further exchange involving hydrogens deep in the membrane. This finding indicates that the chromophore acts as a plug in the membrane in the unbleached state and is distorted by light to open a channel for some transmitter.230 Conformational changes are also reflected by photodissociation of the two halves of an enzymatically cleaved and detergentsolubilized r h o d o p ~ i nand , ~ ~by ~ the increased rate of reaction of p-benzoquinone with S-H groups in rhodopsin on i l l ~ r n i n a t i o n . ~ ~ ~ is the result of the This increased availability of sulphydryl groups from the hydrophobic part of the membrane on bleaching.232Three pairs of S--H groups have been identified in r h o d o p ~ i n .Structural ~~~ changes have been detected through an increase in magnetic anisotropy on illumination,234as an increase in rotational relaxation time of detergent-rhodopsin m i c e l l e ~ and ,~~~ from a study of the reactions of lumi-, meta-I, and meta-I1 rhodopsin intermediate^.^^^ Photochromism at -22 "C in frog retina rhodopsin is caused by illumination at 579 nm and reversed by 435 nm light.237The yields and rates for this process have been measured and the species produced have been shown to differ from those of bovine r h o d o p ~ i n .Cephalopod ~~~ rhodopsin undergoes transformations very similar to those of vertebrate rhodopsins. A combined study by flash p h o t o l y ~ i s steady-state ,~~~ and c.d. spectral measurements 223 at low temperatures (- 80 to - 15 "C) has identified a new transient at 465 nm termed P465 or mesorhodopsin. P465 is in equilibrium with lumi- or metaI-isomers and is photodecomposed to meta-II-rhodopsin from 0 to 10 "C. It is thermally decomposed to opsin and all-trans-retinal. The structure of this intermediate is unknown, but the fact that it does not react with opsin to form a visual pigment suggests a trans but some other evidence has suggested a cis form.239 C.d. spectra also indicate interactions with a m i n o - a c i d ~ . ~ ~ ~ The c.d. spectra of rhodopsins and porphyropsins 224 and the linear dichroism spectra of the photoreceptors of several species have been 228 230

231 232

233 234

236

236 237 238 a39

240

H. Saibil, M. Phabre, and D. Worcester, Nature, 1976, 262, 266. N . Downer and S. Englander, Nature, 1975, 254, 625. T. Power and L. Stryer, J. Mol. Biol., 1975,5,477. S. Alekseev, K. L'vov, and M. Ostrovsky, Biojizika, 1975,20,371. W. De Grip, S. Bonting, and F. Daemen, Biochim. Biophys. A m , 1975, 396, 104. R. Chagneux and N. Chalazonitis, Compt. rend., 1976, 283, D, 1049. A. Wright, Biophys. Chem., 1976, 4, 199. S. Stewart, B. Baker, and T. Williams, Nature, 1975,258, 89. V. Krongauz, R. Shifrina, I. Fedorovich, and A. Ostrovskii, Biojizika, 1975,20,219,419,426. Y . Ebina, N . Nayasawa, and T. Nobukta, Japan. J . Physiol., 1975, 25, 217. T. Suzuki, K. Uji, and Y. Kito, Biochim. Biophys. Acta, 1976,428, 321. F. Harosi, J. Gen. Physiol., 1975, 66, 357.

Author Index

Aaron, J. J., 42 Abakumov, G. A., 10 Abbas, M. A., 150 Abdul-Baki, A., 238, 426 Abe, I., 79 Abeles, R. H., 214 Abella, I. D., 39 Abello, F., 315, 438 Abjean, R., 139 Abkin, A. B., 273 Abrahams, H. B., 579, 580 Abram, I. I., 39, 65 Abramovitch, R. A., 207, 387, 519, 521, 531 Abuin, E. B., 98, 120 Achiba, Y.,89, 100 Ackerman, M., 151, 152, 188 Ackerman, R., 550 Ackermann, D., 603 Adalbert, A., 226 Adam, G., 241,270,276 Adam, W., 101 Adamovics, J. A., 278 Adams, A., 27 Adams, G. W., 148 Adamson, A. W., 176, 190 Adar, F., 225 Addison, J., 56 Adiwidjaja, G., 303 Aerts, A., 549 Afans’eva, N. V., 153 Afonichev, D. D., 195 Aft, H., 443 Agaki, H., 192 Agapiou, A., 200 Agata I., 266 Ageev: B. G., 152 Agosta, W. C., 242, 275 Ahaja, R. C., 548 Ahlborn, B., 9 Ahmad, M., 194 Ahmed, M. G., 132, 155 Ahumada, J. J., 29 Aikawa, M., 42, 84, 93, 96 Aikens, R. S., 26 Ajello, J. M., 157 Ajmera, M. P., 154 Akahori, Y., 227 Akazawa, H., 47, 88 Akermark, B., 385, 433 Akhalkatsi, E. G., 381 Akhtar, M. H., 339, 460, 507 Akiba, K., 528 Akins, D. L., 101 Akiyama, T. K., 227 Akopyan, M. E., 123 Akulin, V. M., 146, 147 Al-Ani, Kh., 112 Alberte, R., 599 Albertin, G., 202 Albini, A., 210, 473

Albrecht, A. C., 53, 56, 590 Albrecht, F. X., 67, 325 Albritton, D. L., 153, 157, 161 Al-Chalabi, A. O., 18, 89 Alcock. A. J.. 15 Alcock; N. W., 195 Alcock, W. G., 126 Alder, A. P., 279, 340 Aldridge, F. T., 135 Aleinikov, I., 594 Aleksandrov, A. P., 193 Aleksandrov, E. I., 31 Aleksandrov, G. G., 211 Alekseev, S., 610, 611 Alexander, E. C., 95,248,429, 479 Alexander, R. D., 19 Alexander, W. B., 16 Alfano, R. R., 49, 605 Alferov, G. A., 225, 576 Alfimov, M. V., 98,322 Alicata, L., 455 Aliev, I. Ya., 296 Alimniev, S. S., 146, 147 Aliwi, S. M., 171, 541 Alkaitis, S. A., 99, 170, 582 Allamandola, L. J., 41 Allan, M., 81, 115 Allana, D. L., 20, 149 Allen, D. M., 202 Allen, J. K., 229 Allen, N. S., 546, 547, 551 Allen, R. O., 20 Aller, L. H., 148 Alman, D. H., 17 Almlof, J., 207 Alobaidi, T. A. A., 20 Aloisi, G. G., 92, 227, 472 Alper, H., 220 Al’Shits, E. I., 42 Altland, H. W., 525 Alt, H., 220 Alt. H. G., 197 Altman, L. J., 330 Alway, D. G., 201 Alwin, E., 94 Amamiya, T., 580 Amand, B., 47,93 Amano, A., 155 Amano, K., 71 Amato, D. V., 22 Ambartsumyan, R. V., 146, 147,229 4meen, S., 48 4merik, V. V., 551 Ames, D. E., 469 Amit, B., 477 Amme, R. C., 154 4mouya1, E., 422 4mphlett, J. C., 125 Ampulski, R. S., 211

Amrein, W., 31, 97, 237, 238 Amsez, J., 604 Anacreon, R. E., 34 Anand, N., 243 Anastasi, C., 159 Anastassiou, A. G., 346 Andersen, N., 15 Andersen, T., 144 Anderson, C. D., 343,458 Anderson, D. E., jun., 152 Anderson, D. J., 356, 392 Anderson, H. R., jun., 549 Anderson, J., 593 Anderson, J. E., 442 Anderson, J. G., 152, 158, 159 Anderson, J. M., 583 Anderson, L. W., 10 Anderson, M., 596 Anderson, R., 163 Anderson, R. L., 202, 589 Anderson, R. S., 8 Anderson, R. W., 24, 49, 144, 147 Anderson, T., 139 Anderson, W. A., 590 Anderson, W. W., 577, 581 Anderson, H. F., 139 Ando, H., 162 Ando, W., 438,439, 512 Androsik, A., 101 Andre, J. C., 418 Andreeva, N., 594 Andreoni, A., 39, 196 Andrews, D. J., 544 Andrews, G. H., 25, 107 Andrews, L., 20, 233,238 Andrews, L. J., 56 Andrews, R., 597 Andrist M.,‘49 Anerlaih, R. A., 65, 10.5 Anfalt, T., 19 Angus, A. M.,5 , 22 Anisimov, K. N., 211 Anisimova, E. K., 552 Anpo, M., 237 Anson, M., 46 Antal, M. J., 571 Anthenius, N., 108 Antipov, A. B., 152 Anufrieva, E. V., 545 Aoki, J., 104 AokI, M., 225 Aoki, S., 39 Aono, T., 266 Aoshima. R.. 598 Aoyagi, R.,240 Aoyagi, S., 462 Aoyama, H., 273, 274, 278, 480. 491 Appefby, A., 149 Appelman, E. H., 132

613

Author Index Applebury, M., 608 Applequist, D. E., 408 Arad-Yellin, R., 299, 403 Aragon, S. R., 37, 60 Arakawa, S., 309 Araniu, F., 23, 37 Archer, M. D., 571 Arens, J. F., 30 Aretz, J., 370 Arguello, C. A., 43, 44 Arimitsu, S., 98, 314, 424 Aristov, A. V., 194 Armstrong, R. L., 151 Arnoff, A., 598 Arnold, D. R., 314 Arnold; I., 133 Arnold, S. J., 56, 105 Arnoldi, D., 146 Arnould. J. C.. 274. 456.. 457 Aronson, J. R.‘, 20 ’ Arora, P. C., 495 Arrington, C. A., jun., 156 Arsenault, R., 57, 598 Artemyev, A. V., 149 Arthur, J. C., 179 Arthur, N. L., 125 Artusy, M., 6 Arzhankov, S. I., 188, 189 Asaba. T.. 162 Asada; S.,. 202 Asai, H., 598 Asai, M., 191, 543 Asai, T., 550 Asakura. S.. 574 Asami, S., 542 Asanov, A. N., 579 Asanuma, T., 88, 351, 375 Asao, S., 92, 429, 485 Asato, A., 608 Asher, S., 225 Ashford, R. D., 158,438 Ataullakhanov, F. I., 226 Athale, A. M., 547 Atkins, D. H. F., 151 Atkins, J. R., 39 Atkins, P. W., 418 Atkins, R. J., 369 Atkins, R. L., 11 Atkinson, G. H., 54, 58, 117 Atkinson, J. B., 38, 142 Atkinson, J. G., 349, 481 Atkinson, R., 40,156,157, 159 Atsumi, K., 454 Atsumi, M., 548 Attwood, D. T., 13 Atzmon, R., 39 Auerbach, R. A., 39 Ault, B. S., 233 Ault, E. R., 7, 8, 135 Aumann, R., 212 Aumiller, J., 609 Aurich, F., 38 Aurich, H. G., 470 Ausabel, R., 138 Ausloos, P. J., 138 Austin, R. H., 46 Auston, D. H., 13, 26 Avakian, P., 546 Avila, M. J., 128 Avila, V., 120, 244 Avram, M., 512 Awatsuji, T., 310 Axelrod, H., 152 Ayer, W. A., 443

22

Ayscough, P. B., 49 Azuma, C., 542 Azuma, K., 610 Azuma, M., 610 Azumi, T., 92 3aba, H., 42,81,84,92,93,95, 96, 114, 130,602

3abaeva, A. V., 191 3abcock, G., 601 3abcock, R. V., 8 3absch, H., 347, 348 3abu, S. V., 152 3achmann, H. R., 24,146,229 3achmann, K. J., 587, 589 3ack, R. A., 107, 129, 504 3acon, E., 249 3adea, M. G., 53 3aeckstrom, P., 385,433 3aes, M., 395 3aeva, V. P., 489 3aeyens-Volant, D., 546 3agdasarya1-1,K. S., 532 3agdasaryan, R. V., 552 3agratashvili, V. N., 152 3agus, P. S., 207 Bailey, G. J., 152 3ailey, R. T., 41 3aker, B., 611 3aker, C., 254 3aker, H. J., 44 3aker, R. L., 176 3alakhinn, V. P., 156 3alchunis, R. J., 290, 483, 484 3aldis, H. A., 9 3ald0, J. H., 48 3aldry, P. J., 336 3aldwin, B., 150 3aldwin. G. D., 12 3aldwin, R. R., 155 3aldwin, S., 250, 266,416 3allardini, R., 173 3allik, E. A., 151 3alling, L. C., 142 3almain. A., 276 Balny, C., 226 Balykin, V. P., 496 Balzani, V., 167, 169, 170, 174, 180. 573. 574

Bambergec C. E., 574 Bamford, C. H., 171,204,218, 541

Band, Y. H., 138 Banerjee, R., 226 Bangia, T. R., 193 Bannerman, C. G. F., 514 Bansal, W. R., 448 Baraldi, I., 321, 322 Barat, M., 153 Barber, D. J. W., 582, 596 Barber, J. R., 185, 605, 606 Barcelo, J., 5 18 Bard, A. J., 40,84,90, 101,582 Bargeron, C. B., 6 Bargon, J., 104, 259 Barile, G. C., 295, 414 Barker, D. B., 152 Barlow, M. G., 363, 364, 365 Barltrop, J., 364,366,373,385, 430,431,433,465

Borod’ko, Y. G., 184 Baron, B., 231 Barnard, C. F. J., 213 Barnasconi, C., 533

Barnett, K. W., 201 Barr. T. L.. 154 Bariadas, I., 54, 61 Barson, C. A., 541 Barth, C. A., 152 Bartlett, P. D., 351, 439 Bartocci, C., 187, 222 Bartocci, G., 99, 322 Barton, D. H. R., 297, 298, 441,488, 527, 530

Barton, I. J., 150 Barton, J., 102, 542 Barton, T. J., 232, 330, 499 Barwise, A. J. G., 98 Bashkin, A. S., 131 Basile, L. J., 20 Basinski, J. E., 306 Basov, N. G., 8, 146 Bass, A. M., 124, 125 Basselier, J. J., 442 Bassewitz, K. V., 552 Bassler, H., 104 Basting, D., 13 Basu, N. K., 488 Basu, S., 83, 86 Baszynski, T., 596 Bateman, R. J., 58 Bates, H. E., 586 Bates, M. L., 441 Bateson, J. H., 298 Batten, R. A., 541 Baudinet-Robinet, Y., 148 Bauer, E., 149 Bauer, S. H., 24, 56 Baughman, R. H., 549 Baum, B., 37 Baum, G., 142 Baumann, N., 73,277 Baumstock, A. L., 101 Bauschlicher, C. W., jun., 123 Bayley, P., 46 Bayliss, N. S., 228 Bazhin, N. M., 178 Bazhulina, N. P., 77 Beadle, P. C., 125 Bean, B. L., 9 Bearden, A., 599, 601 Beattie, I. R., 233 Beauchamp, J. B., 99 Beauchamp, J. L., 115, 144 Beavan, S. W., 121, 547 Bechtel, J. H., 22 Beck, G., 28, 58, 99, 170, 545, 582

Beck, R. A., 149 Becker, D., 264 Becker, H.-D., 326, 458 Becker, H. P., 219 Becker. K. H.. 39. 57. 161 Becker; R. S.,’83 . Becker, W., 54 Beddard, G. S., 52,54,86,583, 596. 605

Bedie;, S , 155 Beeson, K. W., 46 Begleiter, A., 507 Begley, R. F., 44 Behrens, R., jun., 145 Behringer, K., 29 Beilke, S., 164 Beinert, G., 545 Bekkers, G. W., 13 Bekowies, P. J., 435 Belair, R. J., 191

614 Belenov, E. M., 146 Bell, A. N., 321 Bell, J. T., 195, 196 Bell, M. I., 43 Bell, R. O., 586 Bellamv. F.. 191. 471 Bellobono, I. R.; 277, 398 Beloshitskii, N. V., 179 Bellows, J. C., 146 Belousova, I. M., 135 Bel'tyokova, S. V., 193, 194 Belyi, M. U., 229 Bemand, P. P., 19, 132, 133, 159

Benard, D. J., 9 Benci, S., 27 Bendall, D., 600 Bender, C. F., 123 Bender. C. 0.. 327 Benderskii, V: A:, 579 Benedetti, P. A., 27 Benedict, J. J., 21 1 Ben-Efraim, D. A., 477 Benesch, N. M., 161 Benetti, P., 57, 196 Benezra, C., 507 Bengis, C., 600 Beniska, J., 101 Ben-Lakhdar-Akrout, Z., 139 Benmair, R. M. J., 24, 146 Bennett, R. A., 157, 163 Benoist D'Azy, O., 54, 162 Ben-Reuven, A., 108 Bensasson, R., 47, 72, 93, 94, 180,422, 606, 609 Ben-Shoshan, R., 201 Benson, P., 47, 195 Benson, R. C., 6 Benson, S. W., 145 Benson, W. W., 17 Bensoussan, P., 142 Bent, D. V., 66,76, 95, 322 Bentley, J., 154 Bentley, P., 417, 551 Bentz, 5. P., 545 Bercaw, J. E., 217 Berchtold, G. A., 441 Bercovici, T., 102, 412 Bereiter, M., 103 Berens, G., 306 Berezin, I. V., 189 Berger, M., 237 Bergman, A., 78 Bergman, J. G., 13 Bergman, R. G., 101,217 Bergmann, E. E., 8 Bergmann, K., 133, 147 Bergquist, D. A., 229 Berman, M. R., 34,260, 527 Bernard, M., 226 Bernasconi, C., 360 Bernassau, J.-M., 246 Berns, D., 595 Berns, D. S., 583 Bernstein, R. B., 139 Berridge, J. C., 372 Berry, M. J., 138 Berry, R. S., 161 Bersohn, R., 58, 138, 142 Bertoniere, N. R., 254 Bertran, J., 271 Bertrand, L., 6 Besecke, S., 223 Besselievre, R., 461

Author Index Bettinetti, G. F., 473 Betts, C. E., 190 Betty, K. R., 59 Beugdmans, R., 68, 247, 254, 315, 336, 372, 383, 411, 447 Bevan, M. J., 46, 144 Bevan. P. C.. 201 Beyer,'R. A.; 57, 117, 157 Beyl, J. P., 545 Bhacca, N. S., 324, 359, 390 Bhalla, C. P., 153 Bhandari, K. S., 308 Bhaskar, N. D., 153 Bhaumik, M. L., 7, 8, 135 Bialet, G., 607 Bida, G. T., 156 Bidan, G., 240 Bidlingmaier, G., 214 Bieber. L.. 508 Bied-Chaireton, C., 217 Biedenkapp, D., 157 Biedrzycki, J., 231 Biemont, E., 148 Bierzynski, A,, 33 Biggins J 16 Bigwoob,'M., 335 Billen, D., 32 Billings, M. R., 195 Billmeyer, F. W., 17 Bimberg, D., 58 Binkley, R. W., 345, 402, 527 bin Samsudin, M. W., 369 Binur, Y., 45 Biondi, M. A., 153 Biraben, F., 153 Birch, D. J. S., 58, 65 Birchall, J. M., 138 Birely, J. H., 147, 155 Birge, R. R., 39, 65 Birke, R. L., 101 Birks, J. B., 65, 66, 77, 108 Birnbaum, M., 40, 151, 264 Birot, A., 154 Bisacchi G. S 450 Bischel, 'W. K:: 108 Biswas, M., 543 Bjnrrgo, J., 470 Black, C., 159 Black, G., 58, 116, 131, 134, 156, 157, 161, 163 Blair, D. P., 59 Blakeley, R. V., 179 Blank, B., 69,417 Blanke, J. F., 20 Blanken, R. A., 20 Blankenship, R. M., 280, 324, 600, 601, 602 Blasie, J., 595 Blass, W. E., 16 Blat, B. M., 26 Blatz, P., 609 Blayden, H. E., 233 Blazejowski, J., 231 Blickensderfer, R. P., 142 Blidner, B. B., 220 Bloch, M., 56 Bloom, D. M., 13 Blossey, E. C., 434 Blount, J. F., 475 Blume, C. W., 139 Boar, R. B., 527 Bobrovskii, A. P., 229 Bock C. R., 202 Bockrath, B., 80

Bockris, J. O'M., 571 Boden, R. M., 434 Bodini, M. E., 585 Bodor, N., 123 Bohnisch, V., 473 Boer, K. W., 587 Boesl, U., 57 Boettcher, R. J., 327, 506 Bogan, D. J., 122, 438 Bogdanov, G. A., 177 Bogdanov, V. L., 114 Bogomolni. R. A.. 583 Bo&slavskii, L. I., 582, 595 Bohme, D. K., 127 BOIX,J., 315, 438, 488 Bokii, N. G., 211 Bokun, V. Ch., 155 Boldvrev. V. V.. 229 Boll&ta, F., 1671 170, 174,573, 574 Bollinger, W., 159 Bolotko, L. M., 114 Bolsman, T. A. B. M., 454 Bolton, J. R., 48,215,599,600, 60 1 Bonczvk. P. A.. 21 Bond, -A:, 209 Bondybey, V. E., 233 Bonet, J. J.,273, 315,438,481, 488. 490 Bonfrkri-J. M. G., 341 Bonneau, R., 95,233,400,434, 435,494 Bonnet, R. M., 148 Bonnett, R., 79 Bonsang, R., 151 Bonsevich, N. A., 145 Bontchev, P. R., 172 Bonting, S., 611 Borden, W. T., 338 Borgeson, G. C., 542 Boring, M., 196 Borisevich, N. A., 114 Borisov, A., 606 Borisov, E. N., 139 Borodkina, M. S., 548 Borovkov. Yu. V.. 440 Borovsky,'J., 319,*331 Bortolus, D., 99 Bortolus, P., 81, 90, 227, 322 Borucki, W. J., 149 Borys, R. D., 149 BOS,H. J. T., 276 Bos. K. D.. 231 Boschung, A. F., 454 Bott, C. C., 546 Bott, J. F., 135 Bottcher, C., 142, 144 Botten, D. S., 19 Bottenheim, J. W., 25, 58, 164 Bouas-Laurent, H., 84, 409, 500 Bouchet, P., 477 Bouchy, M., 418 Boudjouk, P., 145, 229 Boue, S., 68, 112,231,335,362 Bouges-Bocquet, B., 600 Boule, P., 306 Bourgois, J., 507 Bourimov, V. N., 57, 146 Bowd, A., 95, 398 Bowen, J. R., 452 Bowen, M. W., 360,420, 534 Bower, D. I., 546

615

Author Index Bower, N. W., 29 Bowers, M. T., 115 Boxall, M. J., 58, 139, 153 Boyd, A. W., 52, 77 Boyd, D. R., 470 Boyer, J. H., 239, 515, 534 Boyer, R. F., 446 Boyson, R. A., 138 Bozhevol'nov, E. A., 177 Brach, E. J., 37 Bracokova, V., 48 Bradbury, D., 373, 385, 430, 43 3

Bradford, R. S., 7, 135 Bradley, A. B., 34 Bradley, D. J., 6, 29, 52, 154 Bradley, M. G., 213 Bradshaw, J. S., 484 Brady, S. E., 20 Brainard, R., 249 Brambilla, E., 19 Bramwell, F. B., 93 Brand, L., 54, 77 Braslavsky, S. E., 120, 244 Bratt, S. W., 204 Brau, C. A,, 7 Brau, C. N., 135 Brauer, H. D., 446 Brault, J. W., 151 Brauman, J. I., 81, 123 Braun, H. P., 493, 531 Braun, W., 133, 159 Brechignac, P., 20 Breckenridge, W. H., 19, 45, 141, 142, 156

Brede, O., 58, 81, 416 Bredekamp, J. H., 28 Bree, A., 104 Breheret, E., 251 Breiland, J. G., 152 Brember, A. R., 342 Brenot, J. C., 153 Brenze, A., 606 Breslow, R., 250, 330, 416 Breuer, G. M., 122 Breuze, G., 80 Brewer, R. G., 100 Bricks, B. G., 8 Bridenbaugh, P. M., 589 Bridges, J. W., 42 Bridges, T. J., 14 Bridoux, M., 32 Briggs, J. P., 164 Briggs, W. R., 80,607 Brightwell, N. E., 358,359,390 Brin, G. P., 585 Brine, G. A., 402, 536 Brink, G. O., 144 Brinker, U. H., 523 Brinkman, D. W., 17,45 Brinkworth, B., 571 Brislow, R., 94 Britayev, A. S., 150 Britten, A., 55, 60 Brkic, D., 438 Broadbent, A. D., 389 Broadbent, T. W., 45, 141 Brocker, U., 446 Brocklehurst, B., 83 Brod, H. L., 38 Broda, E., 571 Brodman, R., 39 Brodsky, L.,242 Broida, H. P., 38, 144, 145

Broissier, P., 20 Bromberg, A., 56 Bromfield, J., 20 Bron, J., 147 Bronstert, B., 292 Brook, A. G., 240, 5 0 0 Brooks, C. C., 443 Brooks, D. W., 327 Brooks, J. N., 152 Brophy, J. H., 57, 155 Brouers, M., 597 Brouwer, A. C., 497 Brown, A., 144 Brown, C. W., 25, 164, 597 Brown, D. H., 498 Brown, G. M., 184 Brown, H. A,, 7 Brown, J. H., 162 Brown, L. C., 125 Brown, M. J., 190 Brown, R. D. H., 133 Brown, R. E., 69 Brown, R. F., 53 Brown, R. G., 121 Brown, R. H., 280 Brown, T. L., 203 Brownbridge, P., 321 Browne, P. F., 9 Broyer, M., 57, 137 Brubaker, C. H., 196 Bruce, M. J., 310 Bruckmann, P., 610 Brumbach, S. B., 233 Brune, D. C., 585,602 Bruner, E. C. jun., 148 Brunet, H., 154 Bruneteau, J., 163 Bruni, M. C., 321, 322 Brunow, G., 447 Brus, L. E., 58,233 Bruscato, F. N., 456 Bryant, F. D., 76 Bryant, M. F., 37 Bryce-Smith, D., 311,362,368, 369, 372, 380, 384, 389

Bryn, S. L., 16 Brzozowski, J., 159 Bube, R. H., 589 Bublichenko, N.. 594 B#ucat,R. B.; 228 Bsuch, F., 589 B'uchanan, D. N., 292 B'uchardt, O., 481 BNuchholz. G.. 387.529 Buchholz; V. -L.,15 Buchi, G., 294 Buchkremer, J., 212 Buchler, H., 39 Buchler, N. E., 506 Buchwalter, S. L., 504, 505 Buckman, A. B., 231 Budesinsky, M., 454 3udkevich, B. A,, 188 3udnik,.R. A., 205 hdzwait, M.,200 juecher, H., 196 3uehler, E., 587, 589 3uehler, N. E., 327 Bufalini, G., 94 Sugrim, E. D., 137 3uh1, D., 148 3uhler, R. D., 44 Bukta, J. F., 155 Bulgakov, R. G., 195

Bull, D. C., 83 Bullik, E. A., 21 Bullivant, M. J., 322, 323 Bulos, B. R., 142 Bulten, E. J., 231 Bunce, N. J., 311, 385, 412, 433,473, 594

Burch, D. E., 21 Burchill, C. E., 389 Burde, D. H., 46, 133 Burdett, J. K., 170, 198 Burdett, K. A., 333, 335 Burger, F., 4 Burgess, C., 19 Burimov, V. N., 229 Burkhardt, R. D., 43, 548 Burks, T. L., 124 Burlamacchi, P., 9 Burnett, J. N., 18 Burnett, N. H., 15 Burnham, R., 6, 7 Burns, D. T., 42 Burns, G., 46, 133 Burns, P. A., 101, 444,445 Burnside, R. G., 152 Burrows, B., 577 Burrows, H. D., 170 Burshtein, M. L., 139 Burson, R. L., 348 Burton, C. S., 18,140,156,226 Busch, C., 609 Busch, G. E., 51 Buschmann, H. W., 440 Busetta, B., 409 Bushaw, B. A., 51 Bushey, D. F., 268 BUSO,O., 192, 453 Butaux, J., 139 Butcher, R. J., 6 Butler, I. S., 220 Butler, J. F., 20 Byer, R. L., 14, 44, 144 Byers, B. H., 203 Byers, G. W., 454 Byrne, B., 265 Byron, S. R., 9 Bystritskaya, E. V., 553 Bystrova, M., 597 Caccamese, S., 551 Cacciatore, M., 157 Cadle, R. D., 150, 164 Cadman, P., 133, 138, 509 Cadogan, J. I. G., 491, 501, 514

Cahen, D., 580, 581 Cahnmann, H. J., 447 Caimi, F., 32 Cain, J., 595 Cairns, T. L., 149 Calabrese, P. R., 37 Calafell, M., 524 Caldwell, M. M., 16, 150 Caldwell, R. A., 53, 88, 381 Callahan, R. M., 184 Callahan, W. M., 588 Callear, A. B., 124, 127 Callender, R., 609 Callis, P. R., 79 Calvert, J. G., 25, 150, 164 Calvin, M., 583, 593 Cammack, R., 599 Camp, D., 15 Campbell, I. M., 156, 157, 159

616 Campbell, J. D., 127 Campbell, J. M., 292 Campbell, R. M., 470 Campbell, T. C., 327 Campillo, A., 52, 605,606,607 Candea, R. M., 582 Cannon, J., 225 Canonica, L., 515 Canters, G. W., 223 Cantrell, T. S., 268, 301, 372, 374 Cantu, A. M., 45 Capek, I., 542 Capelle, G. A., 22 Capitelli, M., 157 Gaplain, S., 427 Capomacchia, A. C., 80 Capone, L. A., 148 Capparelli, A., 608 Capron, E., 554 Carassiti, V., 167, 173 Card. H. C.. 589 Carder, R., 364 Cardon, F., 579 Carey, J. H., 177, 178, 578 Cargill, R. L., 268 Carithers, R., 604 Carless, H. A. J., 255 Carli. €3.. 151 Carlin, J: R., 530 Carlin, S. E., 86 Carlock, J. T., 484 Carlone, C., 160 Carlsen, L., 530, 531 Carlsen, P. H. J., 356, 465 Carlson, D. E., 587 Carlson, R. W., 54, 163 Carlsson, D. J., 412, 547, 551 Carr, R. V., 64, 123, 127, 159 Carrapellucci, P., 594 Carrington, T., 58, 126 Carroll, F. A., 62, 63 Carroll, S. E., 517 Carter, G. M., 19 Carturan, G., 218 Casagrande, C., 515 Casals, P.-F., 310 Casasent, D.; 32 Casey, C. P., 202 Castelli, F., 57, 176 Castellan. A.. 84. 408. 409 CastellanoI A.. 387. 427. 428, Caswell. B. G., 587 Caton, R. B., 164 Catteau, J. P., 387, 427, 428, 485

CaLdle, G . F., 9 Cauguis, G., 509 Caumartin, F., 442 Cava, M. P., 438 Cavenett, B. C., 23 Cehelnik, E. D., 33 Cehelnik, E. F., 36 CekoviC, Z., 218, 453, 488 Celotta, R. J., 159, 163 Cerefice, S. A., 359 Cerfontain, H., 238, 293, 338, 419

Ceinia, E., 551 Chabay, I., 22, 44 Chae, Q., 92, 80, 607 Chagneux, R., 611 Chai, A. T., 32

Author Index Chai, Y. G., 577, 581 Chaikin, A. M., 155 Chakrabarti, K. P., 542 Chakrabarty, S., 218, 541 Chakroun, A., 163 Chalapati Rao, Y. V., 152 Chalazonitis, N., 61 1 Challand, S. R., 519 Chalmers, B., 586 Chaloner, C. P., 152 Chamberlain, G. A., 39, 122, 137, 525 Chamberlain, J. W., 152 Chamberlain, T. R., 375 Chambers, R. D., 365, 469 Chameides, W. L., 149, 150 Chamousky, S., 604 Chan, C. K., 52 Chan, G. Y. C., 549 Chan, L.-M., 75 Chan, M., 115 Chan, P. P., 128 Chan. S. S.. 46 Chan; W., 608 Chan, W. H., 20 Chance, B., 585, 602 Chance, R. R., 58, 549 Chandrasekaran, S., 253 Chandler. D. W.. 5 5 Chandros's, E. A:, 81 Chang, C.-C., 303, 360, 573 Chang, C. K., 59,225 Chang, D. W. L., 93, 322 Chang, H. W., 46, 133, 330 Chang, I. C., 16 Chang, J.-C., 121 Chang, J. S., 149 Chang, P. L., 85, 220 Chang, S. H., 262 Chang, T. Y., 108 Chantepie, M., 141 Chantrell, S. J., 79, 221 Chapelon, R., 100 Chapman, C. J., 5 8 , 139, 153 Chapman, 0. L., 25,237,268, 292, 303, 307, 360, 513 Chapovsky, P. L., 153 Charlson, E. J., 590 Charlson, R. J., 150 Charlton, J. L., 42, 389 Charneau, R., 41 Charney, D. R., 415 Chatfield. R., 150 Chatt, J.,. 201 Chattopadhyay, A. K., 543 Chaudrasekhar, S., 469 Chauhan, M. S., 495 Chauser. E. G.. 215 Chauvin, Y., 210 Chawla, 0. P., 228 Chawla, H. M., 443 Chekalin, N. V., 229, 146, 147 Chekhmataeva, G. D., 130 Chekhov, V. O., 77 Cheltsova. T. V.. 548 Chemeresyuk, 6. G., 587 Chen, c.,-595 Chen, C. H., 138, 153 Chen, C. J., 8 Chen, C. K.. 45 Chen, C.-P.,'251 Chen, C. T., 25, 106, 148 Chen, E., 467, 571 Chen, H. E., 418

Chen, H. L., 142 Chen, K.-N., 212 Chen, S.-C., 345,402,486, 527 Chen, S.-N., 179, 189,283, 575 Chen, S.-Y., 518 Chen, T. H.,387, 532 Chen, W. Y., 475 Cheng, C.-C., 148 Cheng, Y. S., 438 Chepelev, V., 594 Cherkasov, A. S., 407 Cherkasov, Y. A., 548 Chernaya, L. I., 197 Cherton, J. C., 442, 516 Chervinsky, S., 128 Cheshnovsky, O., 39 Cheu, E.-L., 552 Cheung, H., 608 Cheung, L. D., 225 Chiang, Y.-L., 226 Chiaroni, A., 217, 405, 461 Chibber, S. S.,443 Chibsov, A., 594 Chihal, D. M., 324 Chihara, K., 114 Childs, W. J., 19 Chilton, J., 94, 413 Ching, T.-Y., 435, 449, 470 Chipman, E., 148 Chiraleu, F., 512 Chiu, C.-L., 128 Chiu, L.-Y. C., 153 Choi, L. S. L., 530 Chong, U. C., 262 Choo, K. Y., 125, 155,421 Chor, S., 37 Chottard, J. C., 218 Choudbury, B. J., 154 Chow, H. C., 584 Chow, Y. L., 486, 487,489 Chrisman, R. W., 12 Christensen, J. J., 481 Christianson, M., 145, 230 Chrlstje, J. R., 138 Christie, K. O., 230, 233 Chrysochoos, J., 193, 194 Chu, B., 18 Chu, J. Y. C., 501 Chu, M., 599 Chu, S. C., 586 Chu, S.-Y., 94 Chu, T. L., 586 Chudacek, I., 548 Chujo, Y., 447 Chum, K., 89 Chung, K. T., 25, 154 Chung, T. J., 50 Chung, V. V., 254, 358 Churchich, J., 607 Churchill, M. R., 212 Cicerone, R. J., 149, 150 Cieslik, S., 149 Cihonski, J. L., 204, 219 Cimino, G. D., 22 Cimolino, M., 202 Clardy, J., 329 Clark, J. H., 24, 57, 147, 162 Clark, K. P., 389 Clark, W. D. K., 575 Clark, W. E., 151 Clarke, B. P., 78, 104 Clarke, M. T., 380 Clarke, R., 598, 603 Claspy, P. C., 22

Author Index Claudel, B., 195 Clauss, G., 310 Claxton. T. A.. 231 Clayton; R., 602, 603, 604 Clear, R. D., 137 Clechet, P., 579, 580 Clement, G., 26 Clements. A. D.. 122 Clennan,’E. L., 364 Clerc, M., 45, 105 Cleveland, W. S., 150 Clin, B., 409, 500 Cline, J. E., 19 Cline Love, L. J., 60, 61 Clinton, N. A., 120 Clive, D. L. J., 297 Closs, G. L., 504 Clyne, M. A. A., 19, 39, 132, 133, 153, 156 Cmiel, E., 426 Coates, P. B., 27, 28 Cocivera, M., 418 Coden. F. C. M.. 144 Cogdell, R., 593,602, 604 Cohen, H., 507 Cohen, M. D., 83,237,406 Cohen, N., 135 Cohen, R., 104 Cohen. S.. 228. 423. 425 Cojan,‘J.-L., 141 ’ Colas, A., 198 Colbow, K., 606 Coleman, L. W., 13, 29 Coleman, W. F., 177 Colles, M. J., 5, 22, 52 Collin, P. J., 410 Collingwood, J. C., 23 Collins, A. T., 15, 20 Collins, C. B., 142, 153 Collins, D., 609 Collins, G. J., 5, 144 Collins, P. M., 478 Colson, E. C., 93 Colussi, A. J., 156, 157 Commereuc, D., 210 Commons, T. J., 317 Compernolle, F., 303 Condel, A., 15 Condorelli, G., 455 Connors, R., 598, 603 Conze E. G., 19 Cook,’J. D., 153, 154 Cook, W. R., 13 Cooke, M., 152 Cookingham, R., 609 Cookson, P. G., 230 Cookson, R. C., 262 Cool, T. A., 159 Coolen, F. C. M., 19 Coombe, R. D., 135 Cooper, G., 405,461 Copp, D. E., 125 Coquelet, C., 477 Corbally, R. P., 365, 469 Corcoran, V. J., 12 Corey, E. J., 442 Corker, G. A., 582 Corkum, R., 24 Cormier, R. A., 275 Cornelisse, J., 382 Comes, F. J., 133 Corradini, G. R., 322 Corrigan, S. J. B., 164 Corrons, A., 30

617 Cortese, C., 23 Costa, L. F., 17, 40 Costanzo, L. L., 455 Cotter, D., 14 Cottier, L., 297 Cotton, F. A,, 177 Cotton, T., 599 Cotton, T. M., 584 Cottrell, D. M., 329 Counturie, Y.,6 Courseille, C., 409 Courtin, A., 396 Courtot, P., 68, 218, 303, 339, 340, 455 Coutts, R., 15 Couture, A., 393,432, 495 Coveleskie, R. A., 117 Covert, D. S., 150 Cowan, D. O., 91,92,352,353, 406,407 Cowley, D. J., 67 Cox, A.. 47, 179, 194, 195,202, 400,454,494 Cox, D. J., 156 Cox, G. B., 384, 389 Cox, R. A., 127, 150, 151 Cox, S. K., 150 Coyle, J. D., 492 CrabbC, P., 299 Crabtree, J. H., 150 Cradock, S., 116 Craig, J. T., 527 Cramer, F., 521 Cramer, W., 593 Crane, G. R., 13 Cravens, T. C., 142 Cravitt, S., 38 Crawford, J. R., 157 Creed, D., 53,88,310,381,423 Creel. C. L.. 162 Crellin, R. A., 543 Cremel, H., 574 Creutz, C., 169, 182, 575 CriDDs. H. N.. 546 Cristbl; S. J., 254,3 17,318,430 Crosby, G. A., 167, 180 Crosby, P. C., 157, 163 Crosby, P. M., 111, 321 Crosley, D. R., 39, 160, 161 Cross, R. J., 232, 498 Crouch, R., 608, 609 Crouch, S. R., 17 Crouse, D. N., 442 Crowder, J. R., 554 Crowe, H. R., 95 Crowley, M. G., 40 Cruickshank, F. R., 41 Cruikshank, D. P., 148 Crumrine, D. S., 510 Cruse, H. W., 57 Crutcher, R. M., 148 Crutzen, P. J., 149, 150 Csorba, I., 598 Cu, A., 426 Cubeddu, R., 57, 196 Cullen. W. R.. 220 Cundall, R. B:, 331, 362,, 370, 541, 547 Cuong, N. B., 151 Cuppen, T. J. H. M., 391 , 392 Curl, R. F., 39, 163 Eurran, A. H., 39 Lurrv. S. M.. 142 Curtis, M. D:, 203, 213

Curzon, F. L., 9 Cutten, D. R., 150 Cvetanovic, R. J., 19, 156, 157, 159 Czajkowski, M., 140 Czerlinski, G., 48 Dabrio, M. V., 361, 533 Daccord, G., 265 Dadson, W. M., 339,460 Daehler, M., 16 Daehne, S., 54 Daemen, F., 610, 611 Dagdigian, P. J., 54, 57, 144 D’Agostino, J., 93 da Graca Miguel, M., 170 Dahl, L. F., 197 Dahlberg, J. A., 139 Dahnke, K. F., 46, 94 D’Aiello, R. V., 586 Dale, R. E., 77, 545 Dalgaard, L., 498 Dalgarno, A., 142 Dalton, J. C., 69, 71, 79, 90, 237,239, 244, 245, 249, 264, 415,426 Dalvi, A. G. I., 193 Daly, J. J., 209 Dam, R., 598 Dana, M. T., 151 Danahy, E., 26 Dance, I. G., 19 Daniels, J. A., 213 Daniels, R. B., 6 Danilina, L. T., 551 Danilov, 0. B., 135 Danilov, V. I., 288, 480 Danilychev, V. A., 8 Danissi, F., 551 Dannoehl-Fickler, R., 176 Danyluk, R., 606 Darby, B., 446 Darinskii, A. A., 545 Darling, T. R., 101, 122 Darnall, K. R., 159 Darshutkin, A. A., 102 Dashevskaya, E. I., 142 da S. Pereira, M. O., 273 Datta, S., 24, 147 Datta, S. C., 298 Daub, J., 326 Dauben, W. G., 257,281,347, 534 1Daum, P. H., 59 1Davenport, J., 161 IDavid, C., 546, 547 IDavid, F., 177 IDavid, P. G., 177 IDavidson, E. R., 162 IDavidson, J. A., 58, 157 IDavidson, J. L., 220,221 1Davidson, R. S., 101, 447 IDavidsson, A., 22 IDavies, A. G., 230, 419 IDavjes, A. K., 433 IDavies, C., 153 IDavies. M., 602 IDavis, ‘A., 554 IDavis, C. C., 135 IDavis, D. D., 14, 56, 159, 161 IDavis, J. A., 9 IDavis, J. H., 65, 101 IDavis, L. D., jun., 142, 152 IDavis, L. I., jun., 159

618 Desvergne, J. P., 408 De Toma, R. P., 91, 92 Detzer, N., 299, 524 Deura, H., 551 Devaquet, A., 338 de Vault, D.. 604 Devi, U.; 147 Devonshire, R., 259 Dewar, M. J. S., 123,436, 437 Dewey, C. F., 12, 13 Dewey, H. J., 13, 23, 24, 30 de Witte. 0..78 Dezauzier, P.,45 Dharmarajan, V., 153 Dhawan, S. N., 311 Dhuicq, D., 153 Diamantis, A. A., 201 Diamond, M. J., 553 Dias de Silva, J., 61, 109 di Bartolo, B., 228 Dice, D. R., 132, 527 Dickel, J. R., 148 Dickinson, D. A., 292 Dickinson, R. E., 148 Diels, J.-C., 12 Diemente, D., 435 Dienes, A., 10 Dieng, C., 405, 461 Dieter, R. K., 265 Diffey, B. L., 554 Dignam, M. J., 20 Dillemuth, F. J., 158 Dilung, 1. I., 222, 594 d’Incan, J., 161 Dingle, T. W., 161 Dingwall, J. D., 365 Dishington, R. H., 44 Disteldorf, W., 5 11 Dittmer, D. C., 220 Dixon, J. E., 231, 260 Djeu, N., 6, 7 Dobashi, S., 444 543 Docken, K. K., 155 Demas, J. N., 31, 85, 435 Dodiuk, H., 72, 74, 75 de Mauriac, R. A., 518 Doehler, J., 43 De Mayo, P., 42, 131, 294, Dopp, D. O., 426,476 400,434, 494, 495, 497 Doerr, A. B., 242 Dementiev, A. P., 156 Dolan, G., 48 De Mets, M., 37 Dolbier, W. R., 466 Demoulin, D., 353 Dolby L. J., 284 Demtroder, W., 57, 162 Dolce, D. L., 534 de Murcia, M., 37 Doleiden, F. H., 439 Demuth, M. R., 441 Denariez-Roberge, M. M.,57, Dolgikh, V. A., 8 Doll. R. J.. 266 598 D’Olieslager, J., 188 Denault, G. C., 21 Dolphin, D., 214 Deneke, C. F., 436 Dolzhikov, V. S., 229 den Haan, G., 601 Den Hollander, J. A., 418, 503 Domey, J., 9 Donahue, T. M., 149,150,161 DeNiro, J., 429 Donatsch. J.. 64 Denisoff, O., 200 Donchi, K. F., 125 Denning, R. G., 23 Donnet, J. B., 545 Dennis, E., 93 Donohue, T., 46, 133 Denniston, M. L., 232 Donovan, R. J., 116, 125, 157 Denson. R.. 40 Donzel, A,, 6 De Pena, R: G., 151 Dorer, F. H., 130 Derr, V. E., 10, 39, 137 Denvent, R. G., 127, 150, 151 Dorfman, L. M., 80 Doring, G., 600 Desbene, P. L., 516 Dorofeenko, G. N., 360, 533 De Schryver, F. C., 303, 479 Dorr, F., 36, 54, 92 Descotes, G., 360, 533 dos Santos, R., 37 Deshayes, H., 259, 433 Dose, V., 157 Desorby, V., 268 Doskotch, R. W., 439 Dessaux, B., 32 Doty, J. C., 88, 306, 381 Dessey, R. E., 59

Davis, M. S., 585 Davis, R., 209, 220 Davis, T. I., 550 Davy, J. R., 322 Davydov, E. Y., 179, 550 Dawe, A., 607 Day, A. C., 364, 366,465 Day, M. J., 488 Day, P., 23 Day, V. W., 201 Dean, D., 254 Dean, F. M., 309 Deane, G. H. W., 554 de Boer, B. G., 212 Debrunner, P. G., 46 Debuch, G., 36 de Cian, A., 210 Decker, C. D., 11 Dederichs, B., 542 Dedinas, J., 415 Dee, D., 154 DeFonzo, A. P., 18 DeGraff, B. A., 518 De Grip, W., 608, 611 Degtyareva, A. A., 550 DeGuzman, J. S., 104, 335 de Haas, N., 156 de Heer, F. J., 164 Dehrn, D., 278 Deinum, T., 113 De Jaegere, S., 188 Dekker, R. H., 486 Delaby, A., 112, 231, 362 De La Cruz, D., 307 Delahay, A. E., 596 DeLap, J. H., 38 Dellinger, J. A., 547 Dellonte, S., 90, 227 Delvaux, J., 138 De Maeyer, L., 48 De Mark, G. R., 108, 120, 139,

Author Index Doucerain, H., 461 Douglas, A. E., 118 Douglas, F., 53, 590 loukas, A,, 609 Douzou, P., 226 Dowd, P., 214 lownall, H. J., 92 Downer, N., 611 Downey, G. D., 163 Downing, J. W., 65 Downing, R., 46 Drabe, K. E., 94 Drake, 5. M., 10, 11 Dreibelbis, R. L., 504 Dresselhaus, G., 579 Dressler, K., 161 Drewer, R. J., 492 Dreyer, J. W., 161 Dreyfus, R. W., 6 lrexhage, K. H., 11 lrickamer, H. G., 102 Drisko, R., 406 lrobyazko, V. N., 193, 194 lrogobytskii, V. M., 156 Dronov, V. I., 497 lrozdova, N., 594 Drummond, J. R., 152 lube, G., 13 lubinsky, R. N., 38, 156 Dubois, J.-E., 18, 200 Dubost, H., 41 lucas, T. W., 19 Duce, R. A., 149 Ducuing, J., 50 ludeney, A. W. L., 19 Diirr, H., 507, 509 lufayard, J., 141 luff, J. M., 240, 500 Dumartin, G., 84 Dumont, P. D., 148 Dunbar, R. C., 115 Duncan, F. W., 26 Duncan, W., 116 Duncan, W. G., 517 Dunford, H. B., 226 h n k i n , I. R., 104, 494 Dunn, J., 515 Dunn, 0. J., 161 Dunning, F. B., 153, 154 Dupuy, F., 104 Duquesne, M., 37 Duran, N., 101 Durani, S., 243 Durante. V. A., 183 Duren, R. R., 486 Dusek K., 549 Duthaier, R. O., 243, 288 Dutoit. E. C.. 579 Dutsch, H. U., 150 Dutton, F. E., 454 Dutton, P., 585, 602, 603 Duzy, C., 161 . Dvorak, V., 65, 83 Dvornikov, S. S., 223, 224 D’yachkovskii, F. S., 197 Dvke. T. R.. 42 Dikska, C.’E., 123 Dyllick-Benzinger, R., 379 Dymerski, P. P., 115 Dzhabiev, T. S., 197 Dzhagarov, B. M., 224 Eachus, R. S., 191 Eadon, G., 249

Author Index Earl, B. L., 164 East, D. S. R., 284 Ebara, N., 97, 193 Eberbach, W., 326, 344 Ebina, Y., 611 Ebrey, T., 608, 609 Eckerle, K. L., 30 Eckert. J. A.. 575 Eckert; P., 515 Eckstrom, D. J., 145 Edelson, M., 104 Edelstein, S. A., 145 Ederer. D. L.. 30 Edman, J. R.; 329 Edmond, J. M., 150 Efimov, I. P., 193 Egan, W. G., 30 Egbert, W. C., 23 Egert, D., 292 Eggerding, D., 276 Eggleton, A. E. J., 150, 151 Egorov, V. I., 156 Eguchi, K., 225 Ehhalt, D. H., 149, 150 Ehrenberg, B., 60 Ehrl, A., 412 Eibling, J. A., 572 Eicher, T., 303, 331 Eisenstein, L., 46 Eisenthal, K. B., 50 Eisinger, J., 545 Ekwal, S. R., 20 Eland, J. H. D., 161 Eldik, R. V., 191 El-Feraly, F. S., 439 Elfinov, E., 594 Elguero, J., 477 Eliseeva, N. V., 551 Elix, 5. A., 402 Ellerman, J., 221 Elliot, R. A., 14 Ellis, A. B., 579, 580, 581 Ellis, J. E., 219 Ellis, J. W., 19 Elnesr, M. K., 150 El Sanadi, N., 218, 337 El-Sayed, M. A,, 93 Elsherbiny, M. M., 21, 151 Eltsov, A. V., 383, 390, 491 Emge, D. E., 435 Enanoza, R. M., 492 Encina, M. V., 69,98, 120,244 Endicott, J. F., 184, 185, 187, 190 Endo, Y., 90, 548 Endriz, J. G., 32 Enescu, L., 512 Engel, P. S., 283, 284, 455 Engelbrecht, W. J., 509 Englander, S., 611 Engleman, R., 29 Engler, P. E., 37 English, T. H., 49 Enikeeva, M. N., 489 Ennen, G., 144 Enoki, A., 442 Epiotis, N. D., 327 Epling, G. A., 284 Eppeldauer, G., 18 Epstein, M. S., 4 Eptink, M. R., 87 Eranian, A., 78 Erb, W., 30, 370 Erdei, L.,598

619 Erecinska, M., 521 Erez, G., 144 Erickson, J. O., 17 Eriksen, J., 299 Erikson, G. R., 11 Erkelens, J., 20 Ermakov, A. Y., 226 Erman, P., 159 Ermolaev, V. L., 192, 194 Ershov, Y. A., 5 15 Ertl, H., 308, 526 Esfandiari, S., 468 Eubanks, A. G.. 28 Evans, D. E., 9,' 11 Evans, J. V., 152 Evans, M., 83, 599, 600 Evans, N. A., 553 Evans, S., 205 Evans. T. A.. 584. 598 Even, 'U., 45' ' Evenson, K. M., 159 Evstaf'eva, 0. N., 191 Evstevneev, V., 595 Evstigneev, V. B., 576, 594 Ewart, P., 139 Ewing, G. W., 22 Ewing, J. J., 7, 135, 144 Eyler, J. R., 19, 58 Eyley, S. C., 339 Eyring, E. M., 47 Eyring, H., 112, 207 Fabbro, R., 163 Fabian, J., 608 Fabian, P., 150 Fabish, T. J., 546 Fabre, E., 163 Fabrycy, A., 549 Factor, R. E., 332 Fadeev, V. V., 10 Fahlen, T. S., 8 Fahrenbuch, A. L., 589 Fahrenholtz, S. R., 439 Faigle, J. F. G., 133 Fairbank, W. M., 38 Fairchild, E. H., 439 Faith, M. R., 90 Fajer, J., 585, 602 Fajszi, C., 598 Falconer, I. S., 14 Fales, C. L., 26 Falkenstein, W., 15, 52 Fallis, A. G., 241 Faltynek, R. A., 219 Fan, B., 12, 13, 44 Fang, J. M., 438 FaraDonova. G. P.. 150 Farenhorst, -G., 367 Farid, S., 88,306,381,387,532 Farmer. C. B.. 152 Farmer; M.,207 Farmilo, A., 99, 168, 206 Farrar, T. C., 145, 501 Fastie, W. G., 29 Faughaenel, E., 456 Faure, J., 49, 50, 94 Faust. W. L.. 15 Favaro, G., Sl,92, 94,99, 227, 322,472 Fazal, N. A., 230 Feast, W. J., 544 Febvay-Garot, N., 427 Fedorovich, I., 611 Feher, C., 603

Feher, F., 230 Feher, G., 585 Fehsenfeld, F. C., 149, 153, 157. 161 Feitelson,-J., 80, 451 Feld, W. A,, 474 Felder, W., 40 Feldman, P. D., 16 Feldmann, D., 142 Felix, G., 409, 500 Felt, G. R., 519 Felton, R. H., 225 Ferns. J. P., 148 Fender, J. S., 151 Fenna, R., 584,604 Fenske, R. F., 197 Ferard, J., 310 Ferderber, L. J., 154 Ferendeci, A. M., 19, 29 Ferguson, E. E., 149, 157, 161 Ferguson, J., 104,406 Fernandez-Alonso, J. I., 271 Ferrar, C. M., 44 Ferrario, A,, 14 Ferraro, J. R., 20 Ferraudi, G. J., 184, 185, 187 Ferreira, J. A., 54, 61 Ferris, J. P., 95, 106, 466, 467 Ferrone, F. A., 23 Fesenko, E. E., 226 Fes'kova, T. M., 551 Fessenden, R. W., 228 Fetizon, M., 246 Fetter, J., 475 Feve, M., 195 Fichtner, W., 26 Field, R. J., 227 Field, R. W., 22 Fields, E. K., 191, 359, 526 Fields, R., 321 Fields, T. R., 320 Figuera J. M 128, 509 Filby, W. G.,'i64, 478 Filimoshkin, A. G., 550 Filipescu, N., 192, 389 Filippov, Y. V., 131 Fillard, J. P., 37 Filler, R., 277 Filseth, S. V., 58, 126 Fincher, C. L., 40, 151 Findeisen, A., 36 Fink, E. H., 57 Finlayson, B. J., 150 Finnigan, R. A., 502 Finzi, J., 163 Fischer, E., 66, 92, 102, 322, 392,412 Fischer, E. O., 220 Fischer, G., 66, 102, 322 Fischer, H., 7, 69, 417, 418 Fischer, I., 230 Fischer. 5. E.. 18 Fischer; M., 345 Fischer, S., 159 Fischler, I., 200, 208 Fish, R. W., 212 Fisher. C. H.. 9 Fitzsimmons,' W. A., 10 Flach, R., 14 Flanagan, T. M., 40 Flanders, D. C., 16 Flanigan, D., 151 Flavell, R. G., 45 Flechtner, T., 250, 416

620 Fleischhauer, J., 237 Fleischmann, E. K., 19 Fleming, G. R., 60, 111 Fleming, I., 245, 246 Fleming, R. H., 177 Fleming, R. N., 14 Fletcher, I. S., 29, 157 Flicker, W. M., 73,93,95, 132 Flint, C. D., 176 Flippen, J., 337 Flippen, J. L., 399, 462 Flohr, H., 214 Florida, D., 138 Flynn, C. M., 435 Flynn, G. W., 28, 164 Fokin, E. P., 193 Foley, H. M., 152 Folin, M., 192, 453 Foltz, G. W., 154 Fong, F., 597, 599 Fong, F. K., 146, 576, 584 Fong, K., 9 Fontaine, C., 217 Fontaine, J. C., 18 Fontaine, M. C., 108 Fontanella, J.-C., 21 Fontjin, A., 40 Foo, P. D., 46, 144 Foote,C. S. 100,101,122,435, 436,444,445,449,470 Ford, A. L., 155 Ford, M., 20 Ford, P. C., 183, 189 Foreman, P. B., 156 Forero, V., 271 Forman, A., 585, 602 Formosinho, S. J., 61, 86, 90, 109, 170 Fornier de Violet, P., 233 Forrest, J., 151 Forster, L., 593 Forster, L. S., 53, 56, 176 Forzatti. P.. 438 Foster, C. H., 441 Fotakis, C., 133 Fouassier, J.-P., 49, 50, 94 Fourrey, J.-L., 290, 497 Fournier, G. R., 139, 154 Fournier. J.. 134 Fowkes, F. ’M., 549 Fowler, B. O., 20 Fox. D. L.. 150. 151 Fox; J. R.,*120,‘543 Fox, W. B., 232 Fraites, J. L., 153, 154 Franceschetti, D. R., 226 Frank. C. W.. 546 Frank; G., 346,459 Frank, H., 603 Frank, S. N., 582 Franklin, F. R., 9 Frankowiak, D., 596, 607 Franks, J. K., 32 Fransen, M., 610 Franz, F. A., 144 Franzen. D. L., 5 , 144 130. Fraser, J. R., 130 Fraser-Reid, B., 269 FraseriReid, Frater, G. Y., 257, 281 Fratoni, S. S., 46, 94 Fravel, H. G., jun., 314, 320, 430 Fravenfelder, H., 46 Fray, G. I., 369

Author Index Fullam, B. W., 231 Frazier, G. F., 29 Funahashi, T., 405 Fred, M. S., 19 Funakura, M., 333 Freear, J., 486 Freed, K. F., 60, 62, 108, 111, Funfschillung, J., 598 113, 116, 138 Fung, D., 87, 374 Fung, K. H., 113 Freedman, A., 145 Funk, J. E., 574 Freedman, B., 553 Funke, C. W., 238,419 Freedman, P. A., 161 Freeman, C. G., 140, 148, 158 Furasawa, M., 590 Furata, N., 575 Frei, B., 285 Furrer, R., 104 Frej, R. W., 41, 78 Furth, B., 265 Freiser, B. S., 99, 115, 144 Furukawa, K., 306 Frenz, B. A., 177 Furuta, T., 476 Freudenstein, K., 157 Furutsuka, T., 53 Freund, K., 297 Furuyama, S., 156 Freund, R. S., 155 Fuss, I., 256 Freund, S. M., 24 Frey, H. M., 122, 124 Gab, S., 317 Frey, J. G., 122 Gaffney, J. S., 156 Frey, J. J., 149 Gafni, A., 54, 598 Frey, R., 84 Gagne, J. M., 6 Fried, S., 142 Gagnon, P., 106 Friedman, H. A., 196 Gagosian, R. B., 245 Friedman, H. W., 9 Gaidetzka, H. W., 370 Friedrich, D. M., 56, 111 Gaidyalis, V., 548 Friedrich, J., 92 Gailer, N. M., 16 Fr!edrichsen, W., 303, 421 Gaily, T. D., 41 Frieling, D. H., 572 Gak, Y. V., 184 Friichtenicht, J., 145 Gakis, N., 354 Frimer, A., 227 Gal, M., 18 Frimout, D., 151 Gal, P., 330 Frimston, J. M., 418 Galante, R., 84, 409 Frith, P. G., 418 Gale, R., 223, 225 Fritz, H., 348 Galiazzo, G., 192, 322, 453 Fritzler, U., 22 Galin, F. Z., 543 Froehlich, P., 38 Galitsin, Y. G., 229 Froix, M. F., 548 Gallei, E., 19 Frolov, A. N., 383 Galley, W. G., 98 Frolova, N. V., 550 Gallivan, J. B., 87 Frolow, A. N., 491 Gallo, C. F., 4 Fromm. D. C.. 5 Galloway, L., 584, 597 Galuszko, K., 527 Galvez, C., 524 Galy, J., 154 Gan, L. H., 412, 551 Gandolfi, M. T., 169 Gano, J. E., 249 Fudu&h,’R. L., 588 Gans,. F., 46 Fuerniss, S. J., 328 Ganter, C., 243,288 Fugate, R.. 593 Garbi, M., 14 Fuhr, H., 435 Gardner, J. L., 153 Fuhrhop, J.-H., 223 Garetz. B. A.. 161 Fuhrmann, J., 36 Garner, A., 416 Fujimoto, A., 192 Gamier, F., 18, 102, 200 Fujimoto, S., 452 Garnir, H., 148 Fujimura, H., 536 Garrett, P. E., 441 Fujisawa, S., 104 Garrilova, V., 594 Fujishima, A., 579 Garside, B. K., 21, 151 Fujita, K., 333, 335, 349 Gartner, E. M., 41, 161 Fujita, M., 92, 607 Gasa, S:, 296 . Fujita, S., 403, 536 Gasiot, J., 37 Fujita, Y., 192 Gaspar, P. P., 15 5 Fujiwara, K., 41 Gasparyan, Zh. M., 128 Fukasawa, Y., 500 Gassman, P. G., 348 Fukawa, I., 528 Gati. E.. 407 Fukaya, K., 383 Gattkrmann, H., 113 Fuke, K., 21 Gattinger, R. L., 148 Fukuda, K., 153 Gaudemer, A., 217 Fukui, K., 428,436,467 Gauglitz, G., 31, 61, 415 Fukui, S., 383 Gaultier, J., 408 Fukui, T., 452 Gauyacq, J. P., 153 Fukumoto, K., 480, 536 Gawley, R. E., 266 Fukushima, K., 219 Gawlowski, J., 25, 106 Fukuzawa, T., 193

Author Index Gayler, R. E., 308 Gaylord, N. G., 545 Gechnizdjani, H., 191 Geenevasen, J. A. J., 293 Gehrlein, L., 482 Geibel, J., 225 Geiger, M. W., 71, 90, 237 Geigert, J., 278 Geist, J., 30 Gelbart, W. M., 116 Gelernt, B., 36, 58, 126 Gelfand, J., 106 Gellerstedt, G., 447, 553 Gennari, G., 192, 453 Gentieu, E. P., 163 Geoffroy, G. L., 177,202,213, 217 George, E. V., 135 George, M. V., 529 George, T. A., 201 George, T. F., 132 George, T. V., 8 Georghiou, S., 53, 607 Geosling, C., 176 Gerace, M. J., 308, 526 Geraghty, M. B., 504 Gerardo, J. B., 6 Gerasimenko, T. I., 193 Gerasimov, G. N., 273 Gerber, G., 38, 144 Gerber, R. B., 153 Geresh, S., 398 Gerhartz, W., 64 Gerischer, H., 580 Gerko, V. I., 98 German, A., 233 German, K. R., 54, 161 Gerner, S., 19 Gessen, N. N., 515 Getfand, J., 22 Gethin, A., 155 Gethner, J. S., 28 Getoff, N., 68, 524 Getz, D., 600 Ghandi, R. P., 311 Ghatak, U. R., 218 Ghiggino. K. P., 45, 95, 553 Ghiron, C. A., 87 Ghosh, A. S., 83 Ghosh, P., 541, 542 Ghosh, S. K., 548 Ghosh, V. J., 85, 606 Giachardi, D. J., 155, 163 Giacobino, E., 153 Gianotti, C., 197, 202, 215 Gibbs, H. M., 38 Gibson, A. J., 152 Gibson, D. M., 357, 534 Giering, L., 94, 413 Giering, W. P., 212 Gieves, G., 29 Gifkins, K. B., 329 Gijzeman, 0. L. J., 60, 111 Gilbert, A., 311, 362, 368, 369, 372, 380, 389 Gilbert, J. R., 156 Gilchrist, T. L., 406, 530 Gilgen, P., 354,464 Gilles, A., 39, 126 Gillespie, D. W., 518 Gillespie, G. D., 121 Gillespie, H. M., 116, 157 Gjllespie, P. S., 151 Gillet, R., 598

621 Gillis, H. A., 26, 45, 105 Gillotay, D., 138 Gilman, L. B., 542 Gilmore, F. R., 149, 154 Gingell, A. C., 220 Ginley, D. S., 36, 196, 202, 203, 579, 580 Ginsburg, P., 423 Ginzburg, A. G., 220 Giori, F. A., 37 Girard, A., 21, 151 Girard, Y., 349, 481 Girardeau-Montaut, J. P., 6 , 7 Girnus, R., 7 Gisin, M., 238, 498 Gissler, W., 579 Giuffre, L., 553 Giuffrida, A., 455 Giuffrida, S., 455 Givens, R. S., 243, 253, 259, 525 Giziewicz, J., 289 Glaenzer, K., 108 Glanville, W. K., 231, 502 Glanzel, K., 127, 162 Glaspey, J., 15 Glass, G. P., 133, 155 Glass, R. S., 95 Glasson, W. A., 125 Glenn, W. H., 9 Gleria, M., 167, 183, 573 Giicker, S., 131 Gliemann, G., 191 Glockner, E., 104 Gloor, J., 31, 281, 415 Goasdoue, C., 443 Gobrecht, J., 580 Godard, B., 7 Goddard, T. P., 144 Goddard, W. A., 65 Godfrey, T. S., 416 Godfroid, M., 138 Goedde, A. O., 548 Gothe, R., 385, 433 Gogotov. I. N.. 585 Goher, hi. A. S., 191 Golan, D. E., 101 Golankiewicz, K., 289, 481 Gold, E. H., 244, 478 Gold, P., 249 Goldan. P. D.. 149 Goldberg, I. B., 95, 230 Goldberg, L. S., 12, 14, 15 Golden, D. M., 125 Gol’denberg, V. I., 553 Gol’dfel’d, M., 600 Golding, B. T., 214, 514 Goldman, A., 152 Goldman, A. I., 5 Goldman, M. A., 152 Goldschmidt, C., 610 Goldschmidt, C. R., 48, 193 Goldstejn, M. D., 416 Goldstein, R., 163 Goldwire, H. C., jun., 154 Gollnick, K., 381 Golomb, D., 162 Golovaneva, I. F., 191 Golovina, A. P., 193 Golubkov, G. E., 549 Gomes, W. P., 579 Gomez, J., 315, 438, 488 Goncharenko, L. V., 211 Good, R. E., 152

Goodman K., 157, 159 Goodman, L., 94 Goodman, L. S., 19 Goodman, R., 582 Goon, D. J. W., 412, 473 Gorbunova, S. P., 191 Gordiets, B. F., 146 Gordon, D., 554 Gordon, M. S., 68, 106, 107 Gordon, P., 19 Gordon, R. J., 159 Gordon, S., 29, 47 Gore, W. G., 26 Gorker, G., 595 Gorman, A. A., 98, 342,434 Gornall, A. G., 453 Gorokhov, Yu. A., 146, 147, 229 Gorsane, M., 395 Gorse, R. A., 120 Gorshkov, N. G., 135 Goryainova, I. F., 515 Gosney, I., 514 Gostisamihelcic, B., 164 Goswami, A., 590 Goto, S., 434, 498 Gotoh, N., 574 Gotlib, Yu. Ya., 545 Gottignies, M., 151 Gottlieb, 0. R., 273, 299 Gouterman, M., 43, 51, 223, 593 Gouverneur, P., 387 Govindarajan, N., 26 Govindjee, R., 602, 606, 608 Gowenlock, B. G., 130,489 Gower, J., I5 Grabner, G., 68 Grabowski, Z. R., 77 Gradyushko, A. T., 224, 597 Graedel, T. E., 149, 151 Griitzel, M., 79, 99, 170, 582 Graf, F., 49 Graf, R., 303, 331 Graham, E., 153 Graham, M. A., 198 Graham, R. E., 156 Gramain, J.-C., 449 Gramont, L., 21 Graneli, A., 19 Granneman, E. H. A., 142 Granoth, I., 92 Grant, E. R., 132 Grant, W. B., 11, 152 Grassie, N., 550 Grate, J. H., 215 Graves, R. E., 191 Gray, H. B., 177, 187, 197, 199, 202, 204, 217 Grazdev, P. F., 153 Graziani, M., 218 Green, A, E. S., 32 Green, B. S., 273,299,403,406 Green, G. L., 35 Green, J. G., 503 Green, M., 209, 220 Green, M. A., 589, 590 Green, R. G., 478 Greenberry, A., 609 Greene, A., 299 Greenwood, R., 551 Grenfell, T. C., 19 Greve, K. S., 51 Grevels, F. W., 212

Author Index

622 Grevesse, N., 148 Grice, R., 125, 156 Griffin, G. W., 254, 256, 306, 324, 357: 358, 390, 393, 526, 534 Griffin, P. M., 133 Griffith, B. G., 190 Griffith, O., 598 Grigo, U., 470 Grimbert, D., 338 Grimley, E., 19 Grimsrud, E. P., 152 Grinwald, A., 54 Grishchenko, A. E., 549 Gritsan, V. I., 138 Gritscov, A. M., 172 Grob, R. L., 152 Grobe, J., 220 Groberman, L., 37 Grobowski, Z. R., 75 Grodkowski, J., 45 Groff, R. P., 546 Gromer, J., 104 Gromoglasov, Y. A., 489 Gromova, M. I., 193 Gronkiewicz, M., 92 Grose, W. L., 149 Gross, S., 454 Grossweiner. L. I.. 76. 101. 452,453 Groth, W., 161 Grovenstein, E., jun,,, 327 Groves, J. T., 179 Gruber, G. W., 333, 345, 396 Gruenter. K.. 430 Grum, F:, l7,40 Gruneis, F., 36 Grunwald, E., 23 Grupe, K. H., 144 Gruzdev, P. F., 154 Gruzdev, V. P., 194 Grynberg, G., 153 Grynkewich, G. W., 221 Gryvnak, D. A., 21 Grzybowski, J. M., 20 Gudkov, N., 594 Guegen, H., 163 Guenard, D., 247, 315 Guenther, K., 30, 478 Guiraud, H. J., 100, 436 Gunther, W. H. H., 501 Gueskens, G., 546, 547 Guglielmetti, R., 102 Guilett, J. E., 69, 545 Guillory, W. A., 25, 107 Gullivan, J. B., 78 Gundermann, K.-D., 101,102, 161,434 Gunning, H. E., 107, 155, 370 Gunthard, H. H., 49 Gupta, A., 97, 177, 321 Gupta, D., 83, 86 Gupta, R., 142 Gupta, R. K., 446 Gupta, R. N., 149 Gurd, R. C., 418 Gureev, N. G., 552 Gurinovich, G., 594, 597 Guseva, L. N., 550, 553 Gusinov, M. A., 45 Gustafson, T. K., 10, 12, 13 Gustafsson, K., 458 Gusten, H., 35, 94, 152, 158 Gutcheck, R. A., 6

Guth, T. D., 180 Gutman, D., 156 Guyer, M. F., 152 Guzzi, R., 150 Haagenson, P., 151 Haas, A., 606 Haas, Y., 24, 57, 147, 162 Haberman, J. A., 139 Hacker, W., 26 Hackett, P. A., 55 547 Haddad, G. N., 26 Haddon, W. F., 477 Hadley, M., 311, 594 Hadley, S. G., 145 Hadicke, E., 509 Haehnil, W., 600 Haensel, R., 39, 456 Hafner, F., 84 Hagedoorn, H. L., 19, 144 Hagens, R., 239 Hager, G., 145, 180 Hagiwara, T., 512 Hahn, E. L., 100 Hahn, U., 39 Haink, H. J., 73 Hains, C., 44 Hair, M. L., 546 Haire, M. J., 67, 325 Hakala, D. F., 162 Hakanson, R., 77 Hake, R. D., jun., 151, 152 Hales, B., 603 Hales, J. M., 151 Hall, D. N. B., 26, 151 Hall, D. O., 583, 585 Hall, D. R., 154, 514 Hall, R. A,, 200 Hall, S. S., 22 Hall, T. D., 4, 38, 43 Hallock, S. A., 204 Halpern, A. M., 73, 128, 426 Halpern, J. B., 58, 126, 127 Halton, B., 527 Ham, W. T., 5 Hamai, S., 102 Hamanaka, N., 296 Hamer, N. K., 301,498 Hamilton, D. E., 79 Hamilton, E. J., jun., 161 Hamity, M., 543 Hamm, F. M., 29 Hammond, G. S., 97, 177, 182, 197, 199, 202, 217, 321, 411 Hammond, J., 606 Hammond, P. R., 11 Hammond. T. J.. 4 Hanaya, K.,487‘ Hanchett, A., 149 Hancock, G., 58, 126, 127 Handeli, D. I., 198 Handv. B. J.. 156. 159 Hanh: ‘N. H.: 155’ Hann; R. A.,, 590 Hanna, D. C., 14 Hansch, T. W., 10, 18, 38 Hansen, D. A., 71, 120, 158, 159 Hansen, H.-J., 354, 464 Hansen, J. F., 402, 536 Hansen, K. B., 43, 53 Hanvey, J. A., 159 Happer, W., 142, 144, 153 Hara, H., 403, 536

Hara, K., 81 Hara, Y., 191 Harbaugh, K. F., 42 Harding, D. R. K., 492 Harding, J. R., 364 Hardt, H., 598 Hardt, H. D., 191 Harel, Y., 428, 599 Harel, Z., 264 Hari Rao, C. V., 586 Harker, A. B., 18, 140, 150, 156,226 Harmon, C. A., 210 Harosi, F., 611 Harries, J. E., 151 Harrigan, R. W., 197 Harrington, D. C., 37 Harrington, J. S., 190 Harris, D. O., 145 Harris, E. W., 31, 435 Harris, F. I., 23 Harris, G. W., 39, 155, 163 Harris, H. H., 40, 155 Harris, J. A., 179 Harris, J. M., 12 Harris, L., 606 Harris, M. S., 101 Harris, N. W., 7 Harris, P. J., 221 Harris, R., 145 Harris, S. E., 13 Harrison, G. M., 364 Harrison, H., 150 Harrison, J. F., 123 Harrison, R. M., 151 Harrison, T. G., 191 Harrit, N., 104, 481, 494, 530 Harrop, W. J., 149 Hart, E. J., 53, 228 Hart, G. A., 7, 153 Hart, H., 262, 283, 292, 348, Ad6

Haiy L. P., 34 Hart, R. J., 103, 397 Hart, R. L., 452 Hartan, H.-G., 307 Hartford. A.. 196 Hartke, K.,276 Hartley, F. R., 19 Hartman, S. E., 88, 306, 381 Hartmann, W., 369,416 Hartwick, T. S., 151 Harvey, A. B., 44 Harvey, R. B., 150 Hasa, J., 549 Hase. W. L.. 123 Hasegawa, H., 361 Hasegawa, M., 336, 544, 553 Hasegawa, T., 273, 274,491 Haselton. H. H.. 133 Hashimoto, S., 310, 423 Hashimoto. T.. 271 Haslanger, -M.; 257 Hassell, J. A., 40 Hasselmann, C., 373, 453 Hassner. A.. 356 Hasson,’V.,’7 Hastie. D. R., 158 Hastings, J. S:, 397 Haszeldine, R. N., 230, 321, 363 364, 365 Hata,”., 73, 277, 388, 472, 49 1

623

Author Index Hatanaka, Y., 303, 304, 399, 426,462, 479,485,494

Hatch, C. E., 253, 480 Hathorn, F. G. M., 133 Hattori, K., 548 Hattori, S., 28 Haugen, G. R., 34 Hauser, M., 55, 69, 84 Hautala, R. H., 415 Hauw, C., 408 Havel, J. J., 156 Haveman, A., 602 Haveman, J., 601 Haveman, M., 601 Havens, W. H., 26 Haverty, J. L., 200 Havey, M. D., 142 Havinga, E., 341, 382 Hawke, J. G., 41 Hawke, R. S., 44 Hawkins, D. W., 527 Hawks, G. H., 11 Hawley, J. G., 11, 151 Hawranek, J. P., 20 Hayakawa, K., 257, 281, 379, 428,467

Hayashi, H., 96, 299, 312 Hayashi, J., 84 Hayashi, K., 322,543,544,550 Hayday, K., 420 Haydon, S. C., 11, 71 Haynes, R. K.. 441 Hayon, E.. 76, 95,227 Hays, A. K., 7, 8, 135 Hays, P. B., 148, 152, 157 Hazan, G., 54 Hazeldine, R. N., 190 Head, C. E., 131, 153 Head, R. A., 201 Healey, P. G., 27 Heaney, E. K., 206 Heaps, W., 14 Heath, G. A., 201 Hecker, L. H., 25 Heckscher, H., 6 Hefner, R. D., 57 He&e7mann, D. G., 345, 402, JLI

Heicklen, J., 125, 127, 128,

138, 139, 150, 151, 156, 157, 162,449,453 Heidner, R. F., 161 Hejdt, G., 55, 84 Heidt. L. E.. 149 Heimgartner, H., 341 354, 463, 464 Heine, H. G., 369, 416 Heinrich, G., 35, 152 Heinzelmann, W., 354, 463 Heiss. A.. 54 Heizer, K. W., 586 Helder, R., 381 Hellner, C., 63 Heller, H. G., 103, 397 Hellner, L., 39 Hellwig, K., 144 Helmstreit, W., 58, 81, 416 Hemmerich. J.. 19 Hemmerich; P., 485 Hempel, K., 188 Hemperly, J. J., 275. Henderson, W. A., jun., 553 Henderson, W. R., 149, 155 Henesian, M. A., 144 9

1Henne, A., 69,417,418 1Henne, W., 346 IHennecke, M., 36 1Hennig, H., 188 1Henrich, V. E., 579 1Henrichs, P. M., 454 1Henriksen, K., 157, 161 1Henri, J. P. L., 159 1Henry, B. R., 42, 89 1Henry, M., 598 1Henry, M. S., 174 1Henry, R. A., 11, 77 1Henson, R. M. C., 75 1Hentzschel. P.. 89 Herber, B.,- 220 Herberhold, M., 204, 219, 220 Herbrechtsmeier, P., 156 Herbstroeter, W. G., 96, 98, 455 Herbst, E., 127 Herbst, R. L., 14, 144 Hercules, D. M., 75, 411 Herget, W. F., 21 Herkstroeter, W. G., 206 Herlem, D., 447 Herm, R. R., 139, 140, 145 Herman, J. A., 106 Heroux, L., 148 Herring, F. G., 132 Herring, J. W., 449 Herron, J. T., 454 Herron, N., 195 Hertel, I. V., 144 Hervo, G., 80, 606 Herz, M. L., 534 Hesse, G., 370 Hesse, R. H., 488, 530 Hester, N. E., 149 Heumann, E., 49 Heusinger, H., 369 Hevesi, L., 232, 454 Hickok, N., 582 Hida, M., 390 Hielscher, F. H., 549 Higginson, B. R., 205 Higuchi, T., 225 Hikida, T., 40, 112, 162 Hilberg, R. P., 44 Hilgeman, T., 30 Hilpern, J. W., 416 Hill. C. G., 44 Hill, D. W., 20 Hill, R., 217 Hill, R. M., 6, 135 Hiller C. A 77 Hills, 'G. W:: 39 Himel, C. M., 75 Hinkley, E. D., 21 Hino, T., 47, 88, 450, 451 Hinohara, J., 71 Hinojosa, O., 179 Hioki, Y., 588 Hips, K. W., 180 Hirabayashi, Y., 274 Hirai, H., 92, 315. 429, 485 HIrai, M., 264, 459 Hrai, T., 299 Hirai, Y., 480 Hirakawa, K., 530 Hirano, S., 403, 536 Hirao. K., 404, 536 Hiraoka, H., 550 Hirayama, S., 403 Hirobe, M., 411,481

Hirota, K., 291 Hirsch, M. D., 15 Hirsch. R. H., 350 Hirschmann. R.. 222 Hisada, H., 531 Hisatsune, 1. C., 139 Hitchcock, P. B., 201 Hitzel, E., 218 Hixson, S. H., 521 Hixson, S. S., 319, 321, 331, 332,521

Hiyama, T., 403, 536 Ho, K., 587 Ho. K. K., 77 Ho; P., 605 Ho, S. Y., 120 Hobbs, L. M., 148 Hochstrasser, R. M., 24, 49, 51. 104

H ocken, R., 19 H odes, G., 580, 581 H odgson, B. W., 58, 81, 416 H odgson, R. T., 13 H oell, J. M., jun., 151 H off, A., 596, 601,603 H offman, J. M., 7, 8, 135 H offman, M. Z., 174, 189 H offmann, R., 197 H ofmann, D. J., 150 H ofman, H., 144 H ogen, P., 56, 161 H ogenkamp, G., 21, 151 H ogenkamp, H. P. C., 215 H ogeveen, R., 347 H oggard, P. E., 51,170,176 Hohlfeld, J., 456 Holbrook, K. A., 122 H olcomb, W. D., 207 H olcombe, J. A., 15 H olland, L. M., 5 H ollinger, F. W., 7 Hollis, J. M., 148 H olm, A., 104,494, 530, 531 H olmes, J. R., 150 H olmes, N., 6 H olt, P. M., 127 H olt, R. A., 41 H olten, D., 593 Holteng, J. A., 226 Holton, D., 51 Holzapfel, C., 54 Holzapfel, W. B., 19, 44 Holzinger, W., 202 Holzwarth, G., 22 Homer. J., 547 1Hon, N.-S., 553 1Honda, H., 104 1Honda, K., 264, 579 1gong. H.-K.. 113 1Hang; N. H.; 231 1ionig, B., 608, 609 1iook, W. R., 44 1iopf, F. R., 221 Ijopfield, J. J., 23. 226 1iopkins, A. G., 25, 164 Iioppe, B., 461 Iiopper, W., 142 I-Ioranska, V., 102 1iori, M., 536 Ilorie, K., 547 I-Iorie, O., 155 tjoriguchi, H., 140 I-Iorinaka, A., 438 I
624 Hornig, J. F., 58 Hornyak, I., 104 Horspool, W. M., 254, 392 Horton, P., 593 Horvath, E., 206 Horwitz, H., 58, 142 Hoshi, T., 93 Hoshino, M., 408, 495 Hosoyama, K., 433 Hospital, M., 409 Houghton, J. T., 152 Houk, K. N., 281 Houston, P. L., 24, 57, 147, 162 Houston, W. R., 57 Hovel, H. J., 588 Howard, C. H., 157 Howard, C. J., 58, 159, 161 Howard, J. A., 446 Howard, J. A. K., 221 Howard, K. A., 267,483 Howard, P. H., 149 Howard, W. E., 18, 113 Howell, G. V., 554 Hoyle, C. E., 351, 378 Hoyt, S. D., 27 Hrdlovic, P., 542, 551 Hrncir, D. C., 197 Hsieh, J. C., 115 Hsu, S. S., 438 Huang, C.-S., 115 Huang, C.-T., 439 Huang, W.-L., 190 Hubbard, R., 364 Huber, B., 299, 524 Huber, J. R., 35, 60, 73, 78, 117

11 I

Huber, P., 5 Hubert-Brierre, Y., 447 Hudec, J., 262 Hudson, A., 204 Hudson. B. S.. 44 Hudson; J. W:,24, 146,229 Huebner, R. H., 163 Huestis, D. L., 6, 135, 145 Hug, W. F., 44 Hughes, R. P., 209 Hughes, V., 19 Hughes, W. M., 25, 153 Hughey J. L., 202 Hugo, D., 210 Hui, K.-K., 159 Hui, M. H., 42 Huie, R. E., 454 Huis, R., 218 Hull, D. R., 6 Human, H. C. G., 145 Hume, D. N., 179 Hummel, R., 17 Hundeshagen, G., 303,304 Hunt, C. J., 156 Hunt, R. G., 269,481 Hunter, F., 599 Hunter, P. W. W., 468 Hunter, R., 25, 153 Hunziker, H. E., 121 Huppert,-D., 51Hurst, G. S., 153 Husain, D., 29, 46, 133, 144, 157 Husar. R. B.. 150 Hussain, W .,’ 201 Husson, H. P., 405, 447,461 Hutchby, J. A., 588

Author Index Hutchinson, D. A., 422 Isaacson, R., 603 Hutchinson, M. H. R., 6, 154 Isaacson, R. A., 585 Hutzinger, O., 385, 433 Isaken, I. S. A., 149 Isakov, V. A., 146 Hyatt, J. A., 249 Isayan, G. A., 128 Hyer, R., 606, 607 Isbrandt, L., 211 Hyer,.R. C., 12, 52 Isherwood, B., 15 Hylarides, M., 446 Ishibashi, H., 264, 342, 459, Ibaraki, T., 40 460 Ishibashi, N., 138 Ichijyo, T., 543 Ishibe, N., 78 Ichimura, T., 40, 112, 162 Iddon, B., 518 Ishibitsu, K., 145, 501 Ishikawa, K., 357, 359, 390, Igier, C., 518 Ido. Y.. 225 534 Ieiri, S.; 14 Ishikawa, M., 472, 501 Ishikawa, S., 259, 491 Ihara, M., 536 Ishii, S., 9 I’hara, Y. J., 104 Ishii, T., 546 Ihda, S., 249, 531 Ishii, Y., 408 Iida, H., 462 Iske, S. D. A., 201 Ikawa, T., 531, 542 Ikeda, M., 264, 342, 459, 460 Isoe, S., 237 Ikegami, S., 588 Isolani, P. C., 133 Itahara, T., 295 Ikeshoji, T., 81 Itani, J., 8 Ilan, Y., 63, 368, 446 Itaya, A,, 548 Ilani, A., 583 Itkan, I., 48 Ilavsky, M., 549 Ito, H., 548 Ilenda, C. S., 318 Ito, J., 93 Imai, H., 172 Ito, M., 20, 130 Imai, N., 141 Ito, S., 405 Imamura, A., 489 Ito, T., 201 Imamura, T., 171, 278, 453 Ito, Y., 468, 521 Imhof, R. E., 27, 58 Itoh, K., 93, 399, 446, 462 Imura, J., 594 Itoh, M., 86, 88,254, 358,411, Imura, T., 53 48 1 Imuta, M., 101, 450 Itsubo, H., 542 In, 0. A., 553 Itzkan, I., 147 Ina, K., 438 Iuchi, K., 202 Inaba, H., 10,435 Iurkevich, I. P., 156 Inaba, T., 141 Ivanov, V. B., 447, 553 Inagaki, S., 436 Ivanov, V. S., 138 Inaguki, F., 609 Ivleva, I. N., 184 Inaki, Y., 179, 541 Ivnitskaya, I., 594 Inamoto, N., 491, 528 Iwabuchi, S., 206, 550 Incorvia, M. J., 184 Iwakura, Y., 544 Ingersoll, K. A., 56 Iwamura, H., 327 Tngham, F. A. A., 553 Iwanami, M., 220 Ingle, J. D., 17, 27, 29 Iwase, E., 192 Inglis, T., 206 Iwashima, S., 104 Ingold, K. U., 229, 529 Izatt, J. R., 9 Inn, E. C. Y., 39 Izawa, Y., 387, 531 Innen, E. P., 78 Izumi, G., 171,278 Innes, K. K., 24, 130 Inokuchi, H., 36, 93 Jackels, C. F., 162 Inoue, A., 97 Jackson, G. E., 124 Inoue, H., 390, 428 Jackson, J . O., 25 Inoue, K., 541 Inoue. Y.. 108. 294. 316. 317. Jackson, R. J., 95, 248, 429, 479 Jackson, W. M., 56, 148 Jacob, J. H., 7 Jacob, T. A., 530 Jacobs, H. J. C., 341 Jacobs, P., 240 Jacobs, R. R., 145, 193 Jacobsen, J. S., 150 Jacquier, R., 477 Jager, E. G., 170 Jaeschke, W., 152 Jahnke, J. A., 21 Irving, R. J:, 19 Jajn, D. C., 126 Irwin, D. J. G., 144 Jam, K., 6, 608 Irwin, R. S., 156 Jain, R. K., 10 Irwin, W. J., 405, 461 Janes, D. C., 19 Isaac, S. R., 473 Janes, G. S., 147 Isaacson, D., 603

Author Index Janse-Van Vuuren, P., 59 James, B. W., 9 James, L. W., 589 Jameson, D. G., 56 Jamieson, A., 551 Jan de Vries, G., 28 Jansen, P., 610 Jansen, T., 598 Janson, M. L., 142 Janssen, E., 454 Jaouen, G., 199 Japar, S., 159 Jaraudias, J., 52, 77 Jarnot, R. F., 152 Jasny, J., 33 Jaworska-Augustyniak, A., 206, 321 Jayanty, R. K. M., 127, 138, 157, 453 Jedrzejewski, J., 548 Jeffery, J., 213 Jefford, C. W., 445, 454 Jeffrey, G. H., 151 Jeffs, P. W., 402, 536 Jeger, O., 246, 247, 278, 287, 338, 340,420 Jegoudez, G., 30 Jelinski, L. W., 305 Jennings, B. M., 308 Jennings, D. A., 58, 157 Jennings, D. E., 16 Jensen, F. R., 216 Jensen, K,, 15 Jepsen, J., 15 Jeser, J. P., 58 Jesson, J. P., 149 Jessup, P. J., 322 Jewett, D. N., 586 Jeyes, S. R., 137 Jezek, B., 548 Jiang, J. B.-C., 446 Jimenez, M., 506 Jin, K., 187 Jinguji, M., 416 Job, B. E., 369 Joergensen, C. K., 196 Johann-Gilles, A., 139 John, P., 35 Johnsen, R., 153 Johnson, A. W., 6 Johnson, B. F. G., 221 Johnson, B. W., 153 Johnson, D., 507 Johnson, D. A,, 151 Johnson, D. E., 312, 351,421 Johnson, D. R., 148 Johnson, G. E., 548 Johnson, M., 553 Johnson, M. G., 3 11 Johnson, N. P., 206 Johnson, P. A., 155 Johnson, P. Y., 253,260, 261, 480, 527 Johnson, T., 333 Johnson, T. H., 348 Johnson, W. D., 138, 588 Johnston, H. S., 40, 149, 159, 163 Jomita, G., 75 Jonah, C., 29 Jonah, C. D., 53 Jonas, H., 594 Jones, D. P., 28 Jones, E. D., 9

625 Jones, G., 273; 458, 572 Jones, G., jun., 260, 262, 343 Jones, M. W., 547 Jones, O., 604 Jones, P., 597 Jones, R. N., 20 Jones, R. P., 51 Jones, W. E., 132 Jones, W. M., 523 Jori, G., 192, 453, 491 Jorns, M. S., 485 Jortner, J., 39, 45, 51, 108 Joshi, B. D., 193 Joshi, G. C., 196 Joshua, C. P., 396,461 Joullie, M. M., 256 Joussot-Dubien, J., 11, 95, 104, 233, 400,434, 435, 494 Joyce, R., 26 Judd, O., 8 Judeikis, H. S., 40 Judish. J. P.. 153 Jiirges; P., 292 Judge, D. L., 54, 116, 131, 163 Jugelt, W., 401, 463 Juillet. F.. 579 Julienne, P. S., 148, 157, 158 Julliard, M., 252 Jung, I. N., 501 Jura, M., 148 Jurdeczka, K., 188 Juris, A., 169 Jursich, M., 163 Jutz, C., 461 Juvet, M., 330, 499 Kaase, H., 30 Kabanov, V. A., 545 Kabassanov, K., 172 Kachan, A. A.. 545, 550 Kacher, A., 104 Kachura, T. F., 223 Kadoma, Y., 544 Kafalas, J. A., 579 Kagan, H., 395 Kaganovich, V. S., 200 Kagawa, K., 8 Kagiya, T., 139, 453 Kagramanov, N., 219 Kai, M., 493 Kaiser, J. K., 438 Kaiser, M. A., 152 Kaiser, S. W.. 579, 581 Kaiser, W., 50, 52 Kaizu, Y., 225 Kajimoto, O., 19, 157 Kajiwara, M., 480 Kakehi, A., 405 Kakisawa, H., 294 Kakuta, A., 189 Kalder, A., 159 Kalicky, P., 250, 416 Kalinina, G. S., 203 Kalmus, C. E., 411 Kalontarov, I. Ya., 553 Kalra, B. L., 142 Kalvoda, J., 534 Kamens, R., 150 Kametani, T., 480, 536 Kamiya, M., 227 Kamiya, N., 574 Kamm, K. S., 67, 321 Kamogawa, A., 452 Kamogawa, H., 544

Kamogawa, K., 130 Kamra, D., 16, 132, 526 Kan, K., 597 Kan, R. O., 411 Kanamaru, N., 83, 232, 407 Kanamori, K., 202 Kanaoka, K., 303, 304 Kanaoka, Y., 304, 399, 426, 462,479,485,494 Kanazawa, H., 452 Kanazawa, R., 442 Kandel, M., 453 Kandel, S. I., 453 Kandelaki, M. D., 582 Kaneko, C., 472 Kaneko, T., 450 Kane-Maguire, N. A. P., 167, 174, 188, 190 Kanematsu, K., 379 Kaneto, K., 225 Kang. K.. 214 Kanno, M.,187 Kanno, S., 383 Kano, K., 226, 423, 427, 582 Kanzig, H., 18 Kapjl, R. S., 243 Kamnus. E. I.. 222. 594 Kaplan, ‘F., 306 ’ Kaplan, L., 368, 370 Kaplanova, M., 598 Kaptein, R., 218 Karapetyan, N., 603 Karasev, V. E., 193 Karaseva, E. T., 193 Karchikhina, V. V., 489 Karl, R. R., jun., 24, 130 Karle, I. L., 399. 462 Karlov, N. V., 146, 147 Karlova, N. V., 229 Karmazin, V. P., 360. 532 Karny, Z., 38, 41, 156 Karol, P. J., 29 Karplus, M., 338 Karpov, G. V., 553 Karpukhin, 0. N., 550, 551 Karras, T. W., 8 Karwick, D., 597 Kashiwagi, H., 48 Kashnikov, G. N., 131 Kasper, B., 225,438 Kasper, H. M., 588, 589 Kassal, T., 152 Kastening, B., 164 Kastner. M.. 582 Katayama, M., 193 Katayama, N., 41 Kato, H., 137, 466, 530 Kato. K.. 13. 14. 545 Kato; M.’, 333 Kato, S., 390, 452 Kato, T., 255, 536 Kato, Y., 142 Katritzky, A. R., 411, 478 Katsushima, T., 348 Kattenhorn, D., 149 Katulin, V. A., 135 Katz. B.. 38. 41. 156 Katz; J.; 595, 596, 597, 598, 599, 603 Katz, J. J., 584 Katz, R. N., 215 Kaufman, D., 17 Kaufman. F.. 158. 159. 1.61 Kawabe, K.,*53 ’

626 Kaufman, K. J., 50, 146, 585, 602,603 Kaufmann, H., 534 Kaupp, G., 359, 379, 533 Kauski. A.. 85 Kawabe, k.,594 Kawahara, A., 493 Kawakami, H., 93 Kawamori, M., 404, 536 Kawanisi, M., 314, 348 Kawasaki, M., 138 Kawashima, K., 266 Kawski, A., 548 Kayama, R., 520 Kayser, D. C., 152 Kayushin, A., 610 Kayushin, L., 608, 609 Kazakov, V. P., 195 Kazandjian, A., 21, 151 Kazansky, V. B., 172 Ke, B., 584, 604 Kearns, D. R., 192 Keck, G. E., 101, 295, 327, 439,506 Keene, J. P., 58, 81, 416 Keller, Ph., 22 Keller, R. A., 23, 24 Kellogg, R. M., 103, 261, 305, 365,438 Kelley, P. L.,148 Kelley, P. M., 123 Kelly, F. M., 142 Kelly, J. R., 97 Kelly, P. J., 108 Kelm, H., 19, 176 Kelsey, M. S., 56 Kemp, D. R., 17,400,494 Kemp,T. J., 47, 179, 194, 195, 202,214,454 Kemp-Jones, A. V., 245 Kempe, U. M., 214 Kemper, P. R., 115 Kenkre, V., 606 Kenley, R. A., 120 Kenner, R. D., 41,45, 91, 548 Kenney, J. W., 43 Kenny, E., 38 Kent, G. C., 28 Keravec, M., 310 Kerber, R. C., 209 Kerimov, 0. M., 8 Kernahan, J. A., 144 Keroulas, H., 411 Kerr, D. E., 29 Kerr, M., 604 Kershaw, M. J., 363, 364, 365 Kertscher, P., 188 Keszthelyi, C. P., 40, 61 Keto, J., 10 Kevan, L., 77 Khalil, M. H., 512 Khalil, 0. S., 73, 74 Khan, A. R., 490 Khan, A. U., 41, 45, 91, 548 Khan, K. A., 130 Khandelwal, S. C., 225 Khangulov, S., 600 Khardin, A. P., 552 Kharlamov, B. M., 42 Khatri, H. N., 504 Khe, P. V., 45 Khidekel, M. L., 579 Khokhlov, R. V., 10 Khomutov, R. M., 77

Author Index Khristenko, S. V., 154 Khuong-Huu, F., 447 Khudyakov, I. V., 296 Khuong-Huu, Q., 515 Khvostach, 0. M., 447 Kibayashi, C., 462 Kiefer, E. F., 305, 508 Kielkopf, J. F., 142 Kiemeneij, A. M., 25, 219 Kienzle, F., 525 Kiguchi, T., 399,485 Kikuchi, K., 68, 206 Kikuchi, M., 206 Kikuchi, O., 342 Kikuchi, R., 576 Killesreiter H 89 Kilner, M.,' 206 Kim, B., 64, 87, 377,378 Kim, C.-K 296,402, 536 Kim, V., 534 Kimbell, G. H., 105 Kimel, J. D., 7, 8 Kimura, H., 488 Kimura, K., 89, 100,232,271, 306, 430,451 Kimura, M., 304 Kimura, Y., 8 King, A. A., 22 King, D. B., 153 King, D. S., 24, 104 King, G. C., 27 King, J. F., 492 King, R. B., 210 King, S R., 151 King, T. A., 44, 153 King, T. E., 226 Kinoshita A., 545 Kinoshita: M., 104 Kinsey, J. L., 57 Kinstle, J. F., 543, 545 Kinugasa, T., 202 Kipperman, A. H. M., 586 Kirchner, R. F., 207 Kirk, A. D., 51, 170, 173 Kirk, A. W., 138 Kirk, D. I., 450 Kirkbright, G. F., 22 Kirmse, W., 523 Kirpichnikov, M. P., 77 Kirsch, A. D., 310 Kirsch, P. P., 167, 573 Kirsh, Yu. E., 545 Kirushin, Yu. A., 125 Kiryukhin, Y. I., 532 Kisch, H., 210 Kiselov, B., 594, 595 Kishimoto, T., 97, 194 Kiskis, R. C., 216 Kister, J., 102 Kita, Y., 264, 459 Kitagawa, Y., 255 Kitahara, Y., 521 Kitajima, E., 466 Kitamura, M., 172 Kitao, K., 442 Kite, K., 590 Kito, Y., 61 1 Kitterer, B. D., 40 Kiwi, J., 545 Kiyosawa, T., 466, 530 Klar, R., 370 Klassen, N. V., 26 Klauminzer, G. K., 34,44 Kleibeuker, J., 598

Klein, F., 146 K!ein, L., 108 Klein, M. V., 6 Klein, R., 75 Klein, U. F., 19 Klein, U. K. A., 69 Kleiner, B., 150 Kleinschmidt, E., 220 Klemm, R. B., 131 Kleo, J., 598, 603, 604 Klewer, M., 142 Kley, D., 161 Kliger, D., 608 Kliger, D. S., 177, 182 Klimov, V., 599, 603 Klobucher, R. L., 29 Klochkov, V. P., 114 Kloosterboer, J. G., 25, 219 Klopffer, W., 72 Klug, H.-H., 446 Klump, K. N., 227 Klunder, A. J. H., 512 Klunklin, G., 380 Klusmann, W., 113 Knappe, W.-R.,524 Knecht, D. A., 121 Knight, A. E. W., 130 Knight, A. R., 131, 132, 142, 527 Knight, P. L., 139 Knittel, D., 90 Knoche, K. F., 574 Knof, J., 57 Knox, R. S., 85, 606 Knox, S. A. R., 217,221 Knudtson, J. T., 47 Knyazev, B. A,, 193 Knyazev, I. N., 152, 155 Knyazhanskii, M. I., 360, 532 Knyukshto, V. N., 224 Kobayashi, H., 225 Kobayashi, M., 531 Kobayashi, S., 317, 393 Kobayashi, S. O., 575 Kobayashi, T., 21, 299 Kobayashi, Y., 493 Kobrina, N. S., 276 Koch, T. H., 267,429,483 Kochevar, I. E., 85 Kochi, J. K., 204 Kodera, K., 40 Koehler, F. H., 202 Koehler, H. A., 154 Koehn, W. P., 428, 485 Kolbl, H., 330, 331 Kolle, U., 415 Koenig, H. S., 551 Koerner von Gustorf, E. A., 200, 208, 209, 212 Koesler, V. J., 584, 597 Koga, J., 79 Kogan, J. L., 579 Kohayakawa, K., 579 Kohler, B. E., 39, 65 Kohler, G., 68 Koitabashi, T., 510 Koizumi, H., 193 Koiima. K.. 206. 550 Kdima; M.', 493 Kok, B., 593, 601 Kokubun, H., 68, 102, 206 Kolb, C. E.,, 40, 132 Kolc, J., 65, 303, 513 Kolditz, L., 144

627

Author Index Kolesar, D. F., 579 Kolind-Anderson, H., 530 Kolle, U., 31 Kollmann, V., 52,605,606,607 Kolln, W. S., 19, 156 Kolobkov, V. P., 194 Kolobova, N. E., 211 Koltzenburg, G., 371 Komarov, V., 608, 609 Komm, D. S., 20 Kommandeur, J., 117 Kompa, K. L., 6, 24, 146, 229 Kondakova, V. P., 194 Kondo, K., 441,442,443,444, 523 Kondo, T. 387 Kondo, Y., 433 Konenstein, P., 92 Kongslie, K., 598 Koning, R. E., 23 Konno, S., 152 Kono, A., 28 Kononenko, A., 603, 604 Kononenko, L. I., 193, 194 Konoshita, M., 93 Konschin. H.. 546 Koo, P. J: S.,-291 Koob, R. D., 68,106,145,229 Kooi, J.,.239, 261, 515, 534 Kopeickina, E. K., 142 Korchev. 0. I.. 543 Koren, i,597 ’ Koreneva, L. G., 192 Korenstein, R., 102, 392 Korte, F., 317 Korzhak, A. V., 171, 179 Kosele, V., 84 Koshiyama, Y., 206, 550 Kosower, E. M., 72, 74, 75 Kossanyi, J., 240, 265 Koster, A., 14 Koster, D. F., 146 Koster, R. J. C., 312 Koster, S. K., 276 Kostromina, N. A., 179 Kostyshin, M. T., 231 Kosugi, H., 267 Kotin, E. B., 273 Kotlo, V. N., 224 Koussini, R., 397 Kovalenko, N. P., 322 Kovitch, G. H., 219 Kovrikov, A. B., 191 Koyano, I., 164 Kozankiewicz, B., 92 Koziar, J. C., 92, 352, 353 Kozlov, Yu., 594 KOZO,I., 120 Kozub, G. I., 184 Kraft, K., 371 Krainyukov, V. I., 549 Krakhmaleva, I., 603 Krakovyak, M. G., 545 Krampitz D., 276 Krantz, A., 104, 307, 461 Krasnovskii, A. A., 585, 594, 597,599 603 Kraulinya,’E. K., 140, 142 Krause, G., 103 Krause, L., 140, 142, 144 Krauss, H. J., 268 Krauss, M., 156, 157, 158 Krausz, P., 200 Krebs, P., 18

Krebs, T., 214 Krecek, V., 454 Kreiter, C. G., 220 Kremen, J. C., 34 Kressel, H., 586 Krestonosich, S., 372 Kresze, G., 130 Krey, P. W., 149 Krief, A,, 232, 454 Krinsky, N. I., 436 Krishnamachari, S. L. N. G., 141,440 Krivonogov, V. P., 497 Krivykh, V. V., 200 Krogh-Jespersen, K., 299 Kroll, M., 39, 126 Kroll, N. M., 148 Krongauz, V. A., 102,440,611 Kropf, H., 225, 438 Kropp, P. J., 268,314,320,430 Kroth, H.-J., 220 Krowerzynski, A,, 75 Krupa, Z., 596 Krupke, W. F., 135 Kryszewski, M., 549 Kryukov, A. I., 171, 179 Ksenofontova, N. M., 191 Ku, A., 357,465,467 Ku, R. T., 21 Kubacki, R., 17 Kubokawa, Y., 237 Kubota, H., 542, 545 Kubota, S., 153 Kucera, H. W., 240 Kucher, A. A., 193 Kucherov, W. F., 266 Kuchitsu, K., 40 Kuchmii, S. Y., 171, 179 Kudryashov, P. I., 194 Kuehn, I., 54 Kuemmerle, E. W., jun., 280 Kuendig, E. P., 199 Kugel, R., 232 Kuhhirt, W., 543 Kuhlmann, R., 543 Kuhn, H. J., 25, 381 Kuhn, P. M., 150 Kuhnle, W., 75 Kuin, N. P. J., 395 Kuis, S., 157 Kuivila, H. G., 231, 260 Kulakov, V. N., 226 Kulbitskaya. 0. V., 383, 491 Kulig, M. J., 435 Kulkarni, R. N., 152 Kulomzina, S. D., 266 Kumada, M., 501 Kumadaki, I., 321, 493 Kumagai, A., 510 Kumagai, T., 326 Kumamoto, S., 482 Kumano, Y., 88 Kumar, D., 117 Kumar, Y., 56 Kume, Y., 84 Kung, R. T. V., 163 Kunai, A,, 271, 306, 430 Kung, M., 603, 604 Kunkely, H., 180,186,221,222 Kupchan, S. M., 296,402,536 Kuppermann, A., 73, 93, 95, 132 Kuramachi, M., 104 Kurata, S., 294

Kurganova, M. N., 552 Kurita, J., 474 Kurkchi, G. A., 489 Kuroki, N., 79 Kuromiya, N., 387, 531 Kursanov, D. N., 220 Kurucsev, T., 546 Kurutsuka, T., 594 Kurylo, M. J., 133, 159 Kusabayashi, S., 543, 548 Kushnick, A., 608 Kusumi, T., 294 Kusumoto, K., 553 Kusunoki, I., 40 Kutal, C., 172, 190 Kuwabara, T., 5 Kuwana, T., 27 Kuyatt, C. E., 163 Kuz’rnin, G. P., 229 Kuz’min, V. A., 296 Kuznetsova, K. E., 489 Kuznetsova, M. N., 553 Kuznetsova, V. V., 193 Kuzuya, M., 262, 348,476 Kwan, C. C. Y., 21, 151 Kwan, C. L., 204 Kwan, M. H., 495 Kwok, H.-S., 9 Kwong, S., 22 Laarhoven, W. H., 66, 345, 359, 391, 392, 395, 396, 39b, 533 Laarz, W., 200 Labana, S. S., 541 Labbe, G., 516 Lablache-Combier, A., 381, 387, 393, 427,428, 432, 485 Labonov, A. N., 8 Labovitz, J., 441 Lachambre, J.-L., 9 Lachish, U., 47, 180 Ladd, M. F. C., 527 Lafferty, J., 594 Lagomarsino, R. J., 149 Lahaniatis, E. S., 317 Lahav, M., 273,407 Lahmani, F., 117 Lahmann, W., 6 Lakshmi, P., 299 Lalancette, B. D., 158 Lalo, C., 39 Lam, C., 284 Lam, F. L., 473 Lam, L. K., 453 Lamb, D., 164 Lambert, E. C., 20 Lambert, G., 151 Lambropoulos, M., 11 Lamm, W., 401,463 Lamola, A., 593, 608 Lamola, A. A., 439 Lamotte, M., 23, 24, 104 Lampert, K., 475 Land, E., 594, 606, 607, 608, 609 Land, E. J., 58, 67, 72, 79, 94, 416 Landa, I., 34 Landais, J., 141 Landsberg, P. T., 586 Lane, E. J., 81 Lane, L., 609 Lane, N. F., 153

628 Lange, G. L., 293 Lange, W., 127 Langelaar, J., 113 Langendam, J. C., 398 Langford, C. H., 167,174,177, 178, 188, 190 Langlet, J., 608 Laniepce, B., 140 Lankin, D. C., 324 Laporte, P., 19 Laposa, J. D., 63.93 Lapouyade, R., 84, 397, 400, 408,409,494, 500 Lappert, M. F., 204 Larichev, M. N., 133 Laroff, G. P., 69, 417 Larsson, L M . , 97, 238, 239 Larsson, L. I., 77 Laser, N., 80, 451 Lassettre, E. N., 227 Lassila, J. D., 268 Laterza, M. E., 93 Latimer, C. J., 154 Laub, F., 407 Lauberreau, A., 50 Laubert, R., 133 Laufer, A. H., 124, 125 Lauher, J. W., 197 Laureni, L., 104 Laustriat, G., 373, 453 Lauterbach, R. T., 416 Lavigne, P., 9 Lavorel, J., 602 Lavrushin, V. F., 276 Lawburgh, C. M., 130 Lawesson, S. O., 498, 530 Lawler, J. E., 10 Lawn, J. W., 460 Lawrence, A. H., 497 Lawrence, G. M., 15 Lawton, R. G., 257 Lazar, M., 543 Lazareva, M. P., 489 Leasure, E. L., 153 Lebedev, N., 597 Lebedev, S., 594 Lebron, F., 150 Lechtken, P., 101, 370 Leclaux, R., 45 Lecler, D., 140 Leclerc, G., 441 Lecluijze, R. E. L. J., 312 Ledbury, K. J., 554 Ledger, M. B., 46 Ledwith, A., 421, 541, 543 Lee, A,, 595, 604 Lee, C., 300 Lee. C. G.. 85 Lee; C . H.; 15 Lee, C. K., 94 Lee, E. K. C., 36, 40, 71, 104, 112, 120, 122, 162 Lee, G. A., 254, 317, 430 Lee, L. C., 54, 116, 131, 163 Lee. S. J.. 138 Lee; S. pi, 18 Lee, W., 139, 154 Leeming, W. B. H., 550 Lees, A. B., 156 Leftwich, R. F., 26 LeFur, P., 13 Legay, F., 20 Legg. K. D., 53, 69 LeGoff, M. T., 383,447

Author Index Lehmann, J. C., 57, 137, 147 Lehnig, M., 219, 231 Leichner, P. K., 153, 154 Leickmann, J., 598 Leigh, G. J., 201 Leigh, J., 602 Leight, J. W., 9 Leipunski, I. O., 133 Leiserowitz, L., 407 Leismann, H., 370 Leite, R. C. C., 43 Lekveishvili, E. G., 381 Lema, R. H., 99,231 Lemaire, J., 306 Lemal, D. M., 308, 526 Lemine, A,, 94 Lengel, R. K., 39, 160, 161 Lennuier, R., 139 Le Noble, W. J., 19 Lensi, P. L., 579 Lenz, G. R., 270 Lenzi, M., 58, 126, 127 Leonard, D. A., 48, 151 Leone, S. R., 133, 134, 147, 163 Leplyanin, G. V., 543 Lepeltier, J. P., 30 Lerdal, D., 444 Le ROUX,J. P., 442, 443, 516 Le Saint, L., 303,455 Leshcheva, I. F., 211 Lessing, H. E., 50 Lester, W. A., jun., 132 Letokhov, V. S., 57, 146, 147, 148, 155, 229 Leung, K. H., 266 Levanon, H., 224, 428, 451, 599 Levashova, L. A., 489 Levatter, J. I., 7 Levek, J. T., 508 Levenson, R. A., 204, 219 Leventhal, J. J., 40, 155 Levey, G., 228 Levi, N., 503 Levin, L., 147 Levin, L. A., 144 Levina, 0. V., 489 Levoir, P., 18 Levy, D. H., 40, 137, 162 Levy, M., 542 Levy, M. R., 45 Levy, O., 398 Levy, R. H., 147 Lew, H., 159 Lewars, E. G., 492 Lewis. A.. 609 Lewis; B.; 209 Lewis, C., 86 Lewis, E. L., 139 Lewis, F. D., 350, 351, 378, 416 Lewis, G. E., 102 Lewis, J., 221 Lewis, R. S., 122 Leyshon, L. J., 63 Leyshon, W. M., 528 Li, W. Y., 292 Li, Y.H., 75 Liao, C. C., 497 Liao, P. F., 10 Liberman, I., 8 Libert, V., 395 Liberti, A., 151

Libman, J., 87, 89, 375, 376, 377, 378,427, 431, 524, 525 Lichtenberger, D. L., 197 Lichtin, N. N., 179, 575 Liddy, J. P., 148 Liebmann, K., 610 Lien, J. C., 586, 590 Liepa, S. Y., 140 Lightner, D. A., 450 Lilie, J., 190 Lillian, D., 149 Lim, E. C., 83, 115, 121,407 Lin, C., 10, 12, 47, 108 Lin, C. K., 90 Lin, C. T., 92, 147, 182, 575 Lin, C. Y., 307 Lin, H. P., 90 Lin, M. C., 21, 124, 131, 156, 157, 159 Lin, S.-C., 7 Lin, S. H., 52, 60, 90, 111,112 Lin, W. C., 225 Linck, R. G., 173 Lind, M. A., 26, 30 Lindauer, R. F., 337 Lindenau, D., 545 Lindinger, W., 153 Lindner, W., 41, 157, 161 Lindqvist, L., 46, 81, 93, 251 Lindsay, J. M., 29 Lindstrom, C. G., 446 Lineberger, W. C., 57, 117 Ling, J. H., 137 Lintner, M. A., 408 Liou, K.-N., 150 Limitskii. I. V.. 191 Liptay, W., 69 . Lisitsyn, V. N., 153 Lissi, E. A., 69, 94, 98, 120, 125. 244 Liston, E. M., 11 Litsov, N. I., 545 Littlehailes, J. D., 369 Littlewood, I. M., 139, 153 Litvin, F., 597 Litynski, D. M., 8 Liu, B., 154 Liu, C. S., 8 Liu, K., 285, 287, 295, 414 Liu. M.-K.. 150 Liu; R., 606, 608 Liu, S., 250, 416 Liu, S. C., 149, 150 Livingston, A. E., 144, 148 Livshits, R. M., 550 Llovd. A. C.. 159 Lloid; D. R.; 205 Lo, c. c., 44 LO, S.-F., 527 Loach, P., 603 Locker. D. R.. 589 Lockhart, H. B., 179 Lockhart, R. W., 489 Lockwood, G., 5 5 , 60 Lodder, G., 281 Lobering, H.-G., 461 Loew, G. H., 207 Loewenstein, M., 152 Logan, S. R., 206, 233 Loginov, A. V., 154 Loh, L. C.-H., 139, 140 Lohman, T., 46 Lohse, C., 299, 492, 494 Lokshin, B. V., 211

Author Index Lombardi, J. R., 111 Lompre, L. A., 29, 153 London, A. G., 138 Long, W. E., 245, 246 Longo, F. R., 79, 224 Longuet-Higgins, H. C., 362 Lopasova, T. A., 31 Lopatko, A. D., 548 Loper, G. L., 112 Lopez-Delgado, R., 27,54,162 Lorents, D. C., 6, 58 Lory, E. R., 24 Los, J., 155 Losev, A., 594, 597 Lotem, H., 22 Loth, K., 49 Lotnik, S. V., 195 Lotz, S., 509 Loucks, L. F., 164 Loughin, S., 18 Loughnot, D.-J., 49, 50, 94 Louisnard, N., 21 Loutfy, R. O., 238, 239 Lovas, F. J., 148 Love, G. M., 262 Love, L. J. C., 54 Loveland, P., 443 Lovelock, J. E., 151 Lovering, G., 434 Low, H. C., 125 Low, L. H., 130 Low, L. K., 450 Lowe, J. T., 22 Lown, J. W., 339, 507 Lozac’h, N., 299, 492 Lozhkin, B., 595 Lozier, R. H., 583 Lubkin, G. B., 149 Lucas, G., 489 Lucci, R. D., 279 Luckhurst, G. R., 204 Ludmer, Z., 83, 104, 407 Lueb, R., 149 Luerman, S. J., 153 Luganskaya, A., 597 Lugtenburg, J., 335, 610 Lui, Y. H., 73 Luini, F., 297 Luk, C. K., 192 Lukac, I., 551 Lukovnikov, A. F., 553 Lum, R. M., 137 Lund, A., 153 Lundquist, G. L., 153 Luria, M., 63, 368, 446 Lussier, F. M., 146 Lustig, R. S., 332 Lutar, K., 233 Luther, F. M., 149 Luttringer, J. P., 343, 458 Lutz, H., 49, 251 Lutz, M., 603, 604 Luu, s., 108 Luvten. W.. 610 L’VOV, K.,611 L‘vov, M., 610 Lwowski, W., 518, 519 Lykhmus, A. E., 158 Lvman. J. L.. 147 LGon, R. K.,‘127 Lyons, A. L., 426 Lyons, L. E., 582 Lyons, T. J., 150 Lytle, F. E., 12, 32, 56

629 McQuitty, 5. J., 204 McRae, J. E., 150 McRobbie, I. M., 518, 519 Maas, G. E., 484 McVey, J. K., 64, 87, 377, 378 Maas, J. G., 155 Macarovici, R., 477 M[adan, P. B., 478 M/adhusudanan, P. M., 191 McAfree, K. B., jun., 137 McAlpine, R. F., 24 M .adigan, D. M., 333, 335 M .aeda, K., 4-48 Macauley, L. J., 125 M aeda, M., 10,493 McAuliffe, C. A., 79, 221 McBride, J. A. H., 365 M ’aestri, M., 170, 174, 574 M‘agalhaes, E. G., 299 McBride, R. P., 31 McCaffery, A. J., 137,177,223 M agde, D., 51 McCarty, R., 596 M agnus, P. D., 297, 298, 441 McCausland, J. H., 339 M agnuson, V. E., 217 McClain, W. M., 21, 108, 111 M ah, S. Q., 6 McClenny, W. A., 21 M ahaney, M., 78 McClusky, F. K., 24 M ahaney, M. A., 35, 60 M aher, J. P., 20 MacColl, A., 138 McCullough, J. J., 67, 87,268, M ahoney, C., 56 344 M ahr, H., 15 McCurnin, T. W., 26 M ahuteau-Corvest, J., 340 McCusker, M. V., 6, 135 M aier, M., 53 McDaniel, R. S., 510 M aier, G., 307 McDermid, I. S., 39, 161 M aier, J. P., 4, 81, 115 McDonald, J. R., 34, 58, 128, M aier, W. B., 153 145 M aillard, B., 529 MacDonald, R. G., 133, 135, M ainfray, G., 29, 153 147 M akano, T., 381 McDonnell, J. A., 125 M akarenko, S. N.. 137 McDonnell, L. P., 260 Makarov, G. N., 146, 229 McDowell, C. A., 132 Makhijani, A., 571 McDuffy, J. R., 43 Maki, Y., 476, 497 McElroy, M. B., 148, 149, 151 Makushenko, A. M., 130 McEwan, M. J., 140, 148, 158 Malament. D. S.. 503 McFadden, D. L., 145 Mlalashke&h, G: E., 193 McFarlane, R. A., 46,133,135 M[aleki, L., 131 McGarvey, J. J., 47 Mjal’gusheva, T., 597 McGee, T. J., 128 M alievski, A. D., 158 McGeoch, M. W., 6, 139 Mjalinovski, A., 172 McGhie, J. F., 527 M alkes, Ya. L., 322 McGlynn, S. P., 73 Mlalkin, R., 593, 599, 601 McGrath, J. M., 415 M alkin, S., 606 McGrath, W. D., 159 M allet, J. J.-B., 351 McGregor, K. G., 582 M alley, M. M., 49, 50 McGuire, A., 600 M allik, B., 608 Machin, P. J., 452 M almstadt, H. V., 37 McIntosh, A., 599, 601 M aloney, P. J., 10 McIntosh, C. L., 307 M aloy, J. T., 84 McKee, H. C., 150, 152 M alpas, R. E., 590 McKellar, J. F., 417, 546, 547, M alrieu, J., 608 <
w

630 Maravigna, P., 551 Marcandalli, B., 277, 398 Marcondes, M. E. R., 231 Marconi, E. C., 93 Marconi, W., 551 Marcus, M., 609 Marek, M., 197, 541 Marette, G., 30 Margano, J. A., 7 Margaretha, P., 31, 268, 295, 339,415

Margitan, J. J., 159 Margulies, L., 60, 114 Mariano, P. S., 321, 324 Marinero, E. E., 5, 22 Maritz, B. S., 191 Markham, J. L., 451 Markin, E. P., 146 Marko, I., 381, 432 Markov, P., 303 Markovits, Y., 398 Marks, T. J., 221 Marling, J. R., 11, 24, 146 Marotta, A., 44 Marowsky, G., 6, 10 Marquard, J., 38 Marsh, D. G., 501 Marshall, E. J., 168 Marten, D., 212 Martens, C. J., 138 Martens, J., 387, 529 Martin, B., 149 Martin, C. L., 542 Martin, D. H., 151 Martin, D. R., 232 Martin, J. R., 579, 580 Martin, M. M., 81 Martin, R. H., 395 Martin, R. M., 139, 154 Martin, T. Z., 148 Martinuzzi, S., 588 Martz, P., 191, 471 Maruyama, K., 309, 312,453 Marynick, D., 229 Masamune, T., 449 Masenet, J., 39, 106, 126 Masetti, F., 77, 81, 322 Maslakiewicz, J. R., 365, 469 Masoud. N. A.. 472 Massey,.G. A.,-14 Massiff, G., 125 Masuhara, H., 47, 86, 88, 98, 424

Maraia, N., 47, 51, 52, 84, 86, 98,424

Mataga, W., 88 Mateo, Y. J. L., 361, 533 Matheson, M. S., 53 Mathews, C. W., 118 Mathey, F., 220 Mathis, P., 601, 609 Mathur, M. S., 142 Matisovarychla, L., 543 Matson, J. A., 502 Matsuda, T., 434, 498 Matsugashita, S., 342, 459 Matsugo, S., 101, 450 Matsui, €I., 56 Matsui, K., 71, 81, 333, 335, 349, 510, 520

Matsui, T., 241 Matsuka, Y.,428 Matsuma, T., 101 Matsumoto, A., 171

Author Index Matsumoto, H., 500, 536,588, 609

Matsumoto, M., 41, 441, 442, 443,444

Matsumoto, T., 267, 296, 403 Matsumura, M., 580, 590 Matsunaga. S.. 296. 546 Matsuo, T.; 226,403,423,427, 537, 582

Matsushima, R., 97, 189, 194, 195

M[atsushita, T., 589 M[atsuura, T., 295, 373, 438,

447,450,468,478,487 548 M latsuzaki, A., 71, 116 Mlatsuzaki, K., 542 M[atsuzaki, L., 92 M [atsuzewski, B., 243,253,525 M [atthews, A. P., 176 M [atthews, B., 604 M atthews, B. W., 584 M 'atthews, T., 72 M attox, D. M., 571 M atusch, R., 276 M ,atwiyoff, N. A., 215 M lau, A., 598 M au, A. W.-H., 80 Mlaudinas, B., 94, 606 M laugh, T. H., 22 M aumy, M., 442 M auzerall, D., 594, 605 M 'avrogenes, G. S., 29 M avroides, J. G., 579 M awby, R. J., 213 M axfield, P. L., 23 1, 260 M axia, V., 23, 37 M ay, R., 96 M ay, R. P., 226 M aya, J., 140 M aya, Mrs., 328 M aykut, G. A., 19 M aylotte, D. H., 545 M aytal, M., 54 M azur, L. E., 179 M azur, S., 101, 444 M azur, Y., 158,412, 454 M azzocchi, P. H., 332, 345, 360, 396, 420, 534, 551 M azzu, A., 357, 465 M azzucato, U., 77, 81 M eagher, J., 125, 128, 449 M easures, R. M., 57 M eath, W. J., 155 M eckley, J., 5 M edary, R. T., 85 M ee, J. M. L., 443 M eese, C. O., 492 M eese, J. M., 589 M egarity, E. D., 93, 322 M egit, R. M., 103, 397 M ehnert, R., 58, 81, 416 M eier, H., 493, 513, 531 Meinwald, J., 345, 396 M elaugh, R. A., 455 Melhuish, W. H., 33, 34 Mellor, J. M., 284 Mellors, A., 594 M elnick, B., 454 M elskens, J., 15 M elton, L. A., 53, 88, 144 Memming, R., 183 Mende, S. B., 27 M ende, U., 256

M[atsuyama, Y.,

M endelhall, G. D., 40, 446 M endes, G. F., 43 M enendez, V., 128 Menger, E., 608 M engersen, C., 223 M enon, U. K., 380 M entall, J. E., 163 M enzel, E., 598 M enzel, G., 552 M enzies, I. D., 441 M enzinger, M., 145 M eredith, R. S., 312, 421 M erkelo, H., 32, 606 M erle, A. M., 104 M erle, G., 202, 215, 217 M errigan, J. A,, 586 M erris, J. P., 25 M erritt, V. Y., 534 M eshkova, S. B., 193, 193 M etcalfe, J., 121 M etcalfe, M. P., 124 M eth-Cohn, O., 518, 519 M ettler, S. C., 37 M etts, L., 93 M etz, F., 108 M etzger, J., 102 M eyer, A., 199 M eyer, J. W., 411 M eyer, T. J., 168, 183, 184, 202. 574

M eyerson, S., 191, 526 M eyling, J. H., 34 M ialocq, J. C., 52, 77 M ichael, B. D., 56 M ichael. J. V.. 29 M ichaelh, P., 292 M ichaelson, R. C., 164 M icheli, K. P., 317, 430 M ichel-Villas, M., 606 M ichl, J., 64, 65, 83 M iddleton, R., 365, 469 M iehe, J. A., 26, 27 M ielczavek, S. R., 163 M ielenz, K. D., 3, 33, 36 M igita, T., 435, 438, 439, 512 M igazawa, T., 609 M ihai, G. G., 389 M ihara, M., 387 M ihara, S., 403, 537 M ihichuk, L., 220 M ikami, T., 361 M ikawa, H., 90, 543, 548 M ikhailov, Y. I., 229 M ikhailovskaya, E. V., 231 M jkheev, A. M., 229 M Ikheev, Yu. A., 550, 553 M ikheeva, L. E., 553 M jkova, 0. B., 273 M ilano, M. J., 18 M ilano, R. A., 590 M ile, B., 126 M ilinchuk, U. K., 551 M jlkey, R. W., 148 M iller, D. F.. 150 M iller, J., 152 M iller, J. A., 18 M iller, J. N., 42 M iller, J. R., 53 M iller, J. S., 213 M iller, L. J., 546 M iller, R. C., 87, 374 M iller, R. D., 534 M iller, S. E. H., 406 M iller, T. A,, 155

Author Index Miller, T. L., 42 Millet, P., 154 Millington, D., 232, 498 Mills, J. L., 220 Milonni, P. W., 139 Mimura, T., 86, 88 Minato, H., 531 Minisci, F., 421 Minot, M. J., 17 Minoura, Y., 542 Mirels, H., 7 Mirza, M. Y., 142 Miskowski, V. M., 177, 187 Misra, S. C., 527 Mistelberger, K., 104 Misumi, S., 84, 86, 527 Mita, I., 547 Mitewa, M., 172 Mitina, V. G., 276 Mitchell, G. H., 309 Mitchell, M. J., 438 Mitra, P. S., 542 Mitra, S. K., 29 Miwa, T., 333 Miyake, A., 485 Miyake, K., 384,432 Miyamoto, T., 219, 308 Miyano, K., 296,402, 536 Miyashi, T., 292 Miyashita, K., 521 Miyata, T., 271 Miyata, Y., 521 Miyazaki, K., 153 Miyazawa, T., 289, 480 Miyazoe, Y., 10 Mizoguchi, T., 426,479,494 Mizuno, H., 510 Mizuno, K.,267,373,374,375, 383,431, 500 Mizuno, T., 81 Mizutani, J., 528 Mizutani, Y., 553 Mlavsky, A. I., 586 Moan, J., 77 Mobius. D., 36 Mochalkin, A., 604, 607 Mochizuki, H., 493 Mock, W. L., 339 Modlin, R., 54 Moe, G., 142, 153 Mprller, J., 475 Moffett, R. J., 152 Moggi, L., 167, 170, 174, 573, 574 Mohan, M., 147 Mohlmann, G. R., 164 Mohr, W. B., 11 Moin, F. B., 156 Moiseev, V. G., 150 Mojelsky, T., 486 Moldover, M. R., 19 Moljna, L. T., 36, 104, 122 Molina, M. J., 149 Molin, Y.N., 168 Mollenauer, L. F., 18 Mollenkopf, H. C., 586 Molyneux, R. J., 443 Momicchioli, F., 321, 322 Monaco, W. J., 312,421 Monahan, K. M., 163 Monchalin, J. P., 6 Monger, T., 604 Moniz. W. B., 239 Monkhouse, P. B., 156

63 1 Monnerie, L., 545 Monroe, B. M., 85, 300 Monson, P. R., 50 Montaudo, G., 551 Montenay-Garestier, T., 85 Montgomery, F. L., 101 Moody, C. J., 406,530 Moody, R. T., 75 Moon, R. L.,589 Moore, C. A., 12, 14 Moore C. B., 23, 24, 57 133, 135,’146, 147, 162, 1 6 j Moore, D., 36 Moore, D. E., 41 Moore, D. S., 19, 45, 141, 142 Moore, D. W., 77 Moore, H. W., 517 Moore, P. D., 150 Moore, T., 608 Moorhouse, S., 21 1 Moorthy, P. N., 95 Moortgat, G. K.. 157 Moos, H. W., 153 Mora, F., 271 Moradpour, A., 395 Moralev, V. M., 193 Morand. J. P.. 11. 409 Morawski, J.-C., 545 MorC, C., 552 Morell, J., 3 15, 438 Morey, W. W., 9 Morgan, D. R., 21 Mornan. F. J.. 226 Morgan; J. P.; 79 Morgan, W. T., 452 Mori, A., 31 1 Mori, K., 195 Mori, Y.,40, 112, 162 Moriarty, A., 14 Moriarty, R. M., 212, 337, 454,493 Morikawa, A., 97 Morimoto, A., 536 Morita, T., 65, 408 Moritani, I., 510 Moritsugu, K., 3 17 Moriwaki, S., 548 Morizur, J. P., 240 Morley, J. O., 389 Moron, J., 290, 497 Morozov, I. I., 133 Morozov, Y. M., 77 Morrocchi, S., 421 Morren G., 395 Morris,’D., 596 Morris, D. A. N., 582 Morris, J. V., 35, 60 Morrison, H., 85, 90, 91, 249, 277, 285, 315, 321, 368 Morrison, V. J., 63 Morrow, T., 44 Morse, D. L., 36, 196, 198, 199,204, 579, 580 Morse, J. G., 231, 502 Morse, K. W., 231, 502 Morse, R. I., 10, 11 Morshev, V. G., 155 Morton, D. R.. 239 Morton; R. G.; 9 Moseley, J. T., 157, 163 Moses, E. I., 12 Mosher. 0. A,. 73. 93. 95. 132 Moskalenko, A., 403 ’ ’ Moskowitz, J. W., 196

Mostashari, A. J., 468 Mosterd, A., 276, 312 Moulton, G. C., 7, 8 Mourou, G., 50 Mousa, J. J., 42 Mowat, J. R., 133 Moxon, P. D., 366, 465 Mruzek, M. N., 138 Mueller, C. R., 153 Muller, E. P., 246, 247, 287, 420 Mueller, H. A., 5 Mueller, J., 202 Mueller-Westerhoff, U. T., 207 Muenter, J. S., 42 Muggleton, B., 419 Muir, M. M., 190 Mukai, T., 263, 326, 327, 349, 468, 504 Mukaiyama, T., 454 Mukamel, S., 108 Mukhergi, S. M., 311 Mukherjee, D., 218 Mukherjee, N. R., 149 Mukherjee, R., 493 Mulac, W. A., 47 Mulik, J., 152 Muller, C., 151 Muller, H., 104 Muller, J., 164 Muller-Eberhard, U., 452 Mullik, S. U., 171, 204, 218, 54 1 Mulvihill, J. N., 126 Mumma, M. J., 131 Munasinghe, V. R. N., 478 Munchausen, L., 486 Mungall, W. S., 542 Munn, R. W., 79,221 Munro, 1. H.. 54 Muntoni, C., 37 Murachi, T., 452 Murahashi, S.-I., 5 10 Murai, A., 449 Murai, H., 94, 416 Murai, S., 267, 500 Murakami, H., 428 Murakawa, M., 52 Murata, R., 303, 485 Murcray, D. G., 152 Murphy, D. P., 402 Murphy, J. A., 152 Murray, E. R., 151 Murray, J. R., 7, 8, 135 Murray, N. G., 412 Murray, R. W., 18, 435 Murton, R.. 20 Musgrave, W. K. R., 365 Uusienko, N. G., 229 Musselman, R. L.,20 Muszkat, K. A., 66, 92, 299, 338, 391, 392, 403 Mutch, G. W., 125 Muth, E., 19 Mykytka, J. P., 523 Vaberukhin, Y. I., 168 Vachtmann, F., 78 Vadolski, B., 549 Vaegele, D., 550 Vaegelen, V., 547 Vagahiro, I., 306 Vagai, Y., 500 Slagakura, S., 21, 71, 116, 322

632 Nagamatsu, T., 463 Nagano, T., 411, 481 Nagarajan, K., 536 Nagasawa, C.. 304, 479 Naaata. C.. 489 Nah-, 6. GI R., 191 Naito, I., 545 Naito, T., 399, 461, 462 Najbar, J., 92, 547 Nakadate, M., 489 Nakacrawa. M.. 450. 451 Nakagawa; N.,‘541’ Nakahira, T., 206, 550 Nakai, H., 304, 426, 479, 494 Nakajima, S., 40, 162 Nakamura, H., 452 Nakamura, K., 187, 390, 428 Nakamura, T., 333 Nakane, A., 448 Nakanishi, F., 237, 544 Nakanishi, H., 237, 544 Nakanishi, K., 609 Nakanishi, L., 609 Nakanishi, S., 446 Nakano, H. H., 6 Nakano, K., 349, 504 Nakano, M., 435 Nakano, T., 306, 481 Nakashjma, N., 51, 52, 84, 86 Nakashima, R., 438 Nakashima, Y., 176 Nakata, R., 153 Nakata, T., 289, 480 Nakatani, N., 41 Nakato, Y., 80, 582 Nakayama, N., 588 Nakazawa, S., 530 Namba, K., 147 Namioka, T., 15 Nanasawa, M.. 544 Nang, T. T., 589 Napier, G. D. R., 192 Narain, N. K., 532 Narisawa, T., 104 Naruta, Y., 312 Naruto. S., 404, 536 Nash, C. P., 20 Nasielski, J., 198, 200 Nathan, R. A., 40, 572 Natarajan, P., 99 Naundoff, G., 72 Naiis, J., 597 Nay, B., 517 Nayasawa, N., 61 1 Navaratnam, S., 433 Naya, K., 442 Nayfeh, M. H., 153 Nazarov, V. B., 98 Nazarova. I., 595 Neckers, D. C., 434 Nedelec, O., 141 Needles, H. L., 545 Neelakantan, P , 20 Neely, W. C.. 4, 38, 43 Nefedov, 0. M., 219 Neidig, P., 77 Neill, D. C., 470 Neilson, J. D., 192 Neilson, R. D., 548 Nekhoroshev, N. S., 273 Nelander, R., 25 Nelson, L. Y., 9 Nelson, M., 600 Nelson, P. J., 268

Author Index Nemanich, R. J., 43 Nemeth, K., 18 Nemirovskaya, I. B., 220 Nemzek, T. L., 69 Neporent, B. S., 114 Neporent, I. B., 176 Nepras, M., 65 Nesrneyanov, A. N., 200,211 Netzel. T. L., 51, 602 Neuberger, K. R., 93 Neudert, B., 220 Neumann, D., 156. 158 Neumann, G., 11 Neumann, G. M., 5 Neumann, H., 220 Neumann, W. P., 219,230 Neumann-Spallart, M., 524 Neumuller, 0. A., 387 Neunhoeffer, H., 473 Neusser, H. J., 21, 57, 111 Neuwald, K., 454 Nevedomskaya, R. N., 550 Nevskii, L. V., 552 New, G. H. C.. 52 Newbound, K. B., 17 Newell, R. E., 150 ATewman, B. E., 5 Newman, L., 151 Newton, J. W., 585 Newton, M. D., 307 Newton, R. P., 389 Neywick, C. V., 253, 371 Nguyen, V. T., 14, 545 Nguyen, Y. V., 151 Nibler, J. W., 41 Nicholls, C. H., 45. 95, 553 Nicholson, B. K., 204 Niclause, M., 418 Nicol, M., 93 Nicolet, M., 149 Nieboer, H., 150 Niedert, E., 293 Niedzielski, J., 25 Nielsen, U., 39 Nielson, D., 207 Nier, A. O., 152 Niewitecka, R., 142, 144 Niguchi, J., 93 Nikandrov, V. V., 585 Nikitm, E. E., 142 Niland, R. A., 14 Ning, R. Y.. 475 Ninomiya, I., 399, 461, 462, 48 5 Nishi, N., 93, 96, 104 Nishigaki, M., 553 Nishihara, K., 306 Nishii, M., 544 Nishijima, C., 232 Nishiiima, Y., 88. 351, 375 Nishikawa, S., 106 Nishikubo, T., 543 Nishimura. H., 528 Nishimura, M., 604 Nishimura, T., 259, 491 Nishinaga, A., 295 Nishio, T., 274, 279, 353, 480, 49 1 Nishiyama. T., 466 Nitadori, R.. 536 Nltta, S., 468 Nivard, R. J. F., 391, 392 Nobbs, J. H., 546 Nobukta, T., 611

Nocchi, E., 214 Noda, H., 15, 274 Nodov, E., 22 Nogales, A., 244 Nojima, K., 383 Noltes, J. G., 231 Nomura, Y.. 580 Norden, B.. 22, 25, 176 Nordine, P. C., 140 Nordstrom, R. J., 20 Noreen, A. L., 254 Norlund, T. M., 46 Norman, C., 149 Norris, J., 593, 595, 596, 597, 599, 603 Norris, J. R., 584 Norris, R. D., 450 North, A. M., 546 Norton, R. H., 152 Nortstrom, R. J., 107 Nosach, V. Yu., 135 Nosonovich, A. A., 179 Noth, H., 24, 146, 229 Novikov, S. S., 552 Novikov, V. A., 542 Nowak, D., 41 Nowakowska, M., 547 Noxon, J. F., 151, 152 Noyes, R. M., 227 Noyes. W. A., jun., 120 Nozaki, H., 403, 536 Nozawa, T., 226 Nozik, A. J., 577, 578 Nozoe, T., 299 Nurmikko, A. V., 9 Nurmukhametov. R. N., 108 Nusse, B. J., 347 Nygaard, K. J., 142 Nylund, T., 91, 321, 368 Oba, K., 551 Obara, H., 488 Obata, N., 510 Oberhansli, W. E., 354, 464 Obi, K., 94. 106, 416 Obremski, R. J.. 20 O’Brien, R. J., 150 Obyknovennaya, I. E., 407 O’Connor, G. G., 152 Oda. M.. 521 Oda; N.,- 548 Odaira, Y., 271, 306, 308, 430

O’Donnell, C. M., 33, 42 Oe, K., 484 Oesterhelt, D., 583 Oettinger, P. E., 10, 12 Oettmeier, W., 595 Ofenberg, H., 381, 393, 432 Offenberger, A. A,, 15 Ogata, N., 542 Ogata, Y., 249, 301, 531 Ogawa, K., 474 Ogawa, M., 54, 131, 158, 163 Ogawa, O., 453 Ogawa, S., 158 Ogawa, T., 138, 152 Oginets, V. Ya., 423 Olziwara. 13.. 479 Oiiwara; Y . ; 542, 545, 552 Ogo, Y., 542 Ogryzlo, E. A., 158,438 Ohara, A., 452 Ohara, M., 411

Author Index Ohashi, M., 89, 369, 384, 385, 432,433 O’Haver, T. C., 35 Ohfune. Y..267 Ohinashi. Hi..549 Ohine, TI, 468 Ohloff, G., 246, 287, 338, 434, 440 Ohnishi, T., 582 Ohnishi, Y.,34 Ohno, K., 36,93 Ohsawa, A., 493 Ohsawa, N., 500 Ohsawa, T., 587 Ohta, T., 574 Ohta, H., 88, 483, 491 Ohta, T., 86 Ohtsuka, R., 388,491 Oine, T., 254 Ojanpera, S., 425 Ojima, I., 523 Ojo. I. A. O., 220 Oka, T., 131 Oka, Y., 485 Okabe, H., 39,40 Okada, J., 86 Okada, M., 14 Okai, T 90 Okajima: H., 451 Okajima, S., 115 Okarnoto, H., 267, 373, 431, 500

Okamoto, K., 548 Okamoto, T., 86, 88, 411, 481 Okamura, M., 603 Okamura, M. Y.,585 Okamura, S., 191, 543, 545 Okamura, T., 81, 115 Okawara, M., 548 O’Keefe K., 37 Okhrimlnko, B. A., 229 Okimoto, T., 541 Okita, T., 152 Okuda, M., 589 Okumura, K., 452 Okuno, T., 296 Okuno. Y., 404, 536 Olander, C. R., 328 Olbregts J 108 138 Olbricht: f.’,52; Oldman R. J., 137 Oleinik,’A. V., 515, 550 Olekhnovich, E. P., 360, 532 Oleson, J. A., 429 Olier, R., 579, 580 Oliver, B. G., 578 Olmsted, J., 35, 60, 472 Olson, C., 608 Olson, D. H., 18 Olson, G. L., 51 Olson, J 604, 605 Olson, J:’M., 584 Olson, N. T., 8, 25 Olson, R. E., 142 Olson, T. N., 153 Olszyna, K. J., 23 Olten, D. E., 32 Omar, M. H., 586 Omarini. S.. 277. 398 Omenetto, N., 34 Omori, T., 271, 430 Omote, Y., 273, 274, 278, 353, 480.491 Onak; T., 229

633 Onan, K. D., 328 O”ea1, H. E., 158 O’Neill, P., 96 On& K. G.. 140 On& I., 73,’277 Ono, Y., 81 Onodera, J., 488 Onohara, M., 423 Oparaeche, N. N., 478 Op Het Veld, P. H. G., 398 Ophir, Z., 45 Opila, R. L., 117 Oraevsky, A. N., 131, 146 Orahovats, A., 354, 463 Oran, E. S., 148, 152 Orbach, N., 86, 224 Orchard, A. F., 205 Orchin, M., 218 Ordukhanyan, K. A., 552 Oref, I., 128 Orekhov, V. D., 228 Orger, €3. H., 368 Orio, A. A., 202 Orisheva, R. M., 191 Oritz de Landaluce, J., 152 Orlandi, G., 65, 93, 322 Orner, G. C., 57, 99 Ornstein, M. H., 10, 39, 137 Ortmann, W., 456 Osada, H., 225 Osawa, Z., 552 Oschchopkov, V. P., 585 Oseroff, A., 609 O’Shea, D. C., 225 Osherovich, A. L., 139 Osif, T. L., 157 Ostermayer, F. W., 17 Ostrern, D., 268 Dstroumov, S., 608 Dstrovskii, A., 611 Dstrovskii, M., 610, 611 Dta, Y., 405 Dtagawa, T., 574 Dtsubo, T., 527 Dtsuka, K., 97 Dtsuki, T., 312 3tt. W. R., 29, 30 Dttesen, D. K., 204 3ttinger. Ch., 105, 144 Dtto, H., 191, 222 Dttolenghi, M., 86, 610 Duannes, C., 411 h c h i , A., 177 herend, J., 20 herend, R., 157, 159 Dverend, R. P., 159 3wens. C. M.. 157 3wens; D. K.;545 Dyiwara, T., 65 Dysuki, T., 309 3zeki. M.. 548 3zin,-G. A., 199 3zolina, I., 604, 607 Paaren, H., 337 Pac, C., 267, 373, 374, 375, 383. 500 Pacansky, J., 104, 259, 307 Pace, P. W., 38, 142 ?ack, D., 152 Packard, R. E., 17 Packer, L., 585 Pacifici, J. G., 249 Padhye, M. R., 547

Padwa, A,, 254,278, 354, 355, 356, 357, 428, 463,464, 465, 467. 482.485 Paech; F.,-37, 162 Page, M. A., 455 Pagsberg, P., 43, 53 Pai, B. R., 536 Pailliton, G., 80, 606 Pailthorpe, M. T., 42, 45, 95, 553 Pajak, J., 321 Pakkanen, T. A., 66, 391 Palensky, F., 85, 91, 321, 368 Palibroda, N., 477 Palmer, H. B., 159 Palmer, K. F., 154 Palmer, M. A., 9 Pancrazi, A., 5 15 Panda, S. P., 545 Panfilov, V. N., 138, 229 Pankratov, A. V., 146 Pansevich, V. V., 189 Pantell, R. H., 45 Pao, Y . - H . , 22 Papacosta, P., 164 Papirer, E., 545 Pappas, S. P., 543 Paquette, L. A., 329 348 Paranski, C. F., jun.: 239 Paraskevopoulos, G., 157, 159 Pardue, H. L., 18 Paredes, R., 466 Paresce, F., 32 Parham, J. C., 473 Pariiskii, G. B 179, 550 Parini, C., 277” Parish, R. V., 190 Park, C., 138 Park, S. M., 90, 101 Park W., 153 Park, Y.-T 450 Parkanya, %., 381, 432 Parker, D. R., 230, 520 Parker, J. G., 22 Parkes, D. A., 125 Parkes, J., 610 Parks, J. H., 53 69 Parks, W. M., 35 Parlar, H., 317 Parmar, S. S., 532 Parmenter, C. S., 130 Parola, A. H., 423 Parr, T. P., 145 Parravano, C., 144 Parrish, D. D 140 Parshin, G. S.: 195 Parson, W., 593, 602, 604 Parsons, G. H., 189 Parthasarathy, P. C., 536 Parvizi, B., 309 Pascale, J., 142 Paschenko, V., 605 Pascoe, H. K., 152 Pasechnik, V. I., 226 Pasquier, J. L.. 151 Pasquon, I., 438 Pass, S., 158, 454 Passenheim, B. C., 40 Pasternak, M., 283 Paszyc, S., 289, 484 Patchornik, A., 477 Patel, D., 608 Paton. R. L., 536 Patrick, T. B., 219

634 Pattenden, G., 322, 323 Patterson, H. H., 191 Patty, R. R., 21 Patumtevapibal, S., 50 Pauson, P. L., 200 Pavlik, P. W., 364 Pavlova, N. R., 545 Payne, M. G., 138, 153 Payne, W. A., 38, 155 Paynter, R. A., 41 Payzant, J. D.. 127 Peacher, J., 105 Peacock, R. D., 225 Pearce, R., 240 Pearman, A. J., 201 Pearson. R. K.. 145, 193 Pecher, J., 395 . Pechet, M. M., 488, 530 Pecora R., 60 Pedersin. C. T.. 299. 492, 494 Peebles, W. A.,'9 . . Peet, N. P., 268 Pegg, D. J., 133 Peiffer, R., 321 Pelvas, A., 606 Penzhorn, R.-D., 94, 158, 164 Penzkofer, A,, 15, 52 Pepin, H., 9 Perchalski, R. J., 38 Pereyre, J., 95, 434 Perez, C., 306, 381,481 Perez, J. M., 128, 509 Perichet, G., 100 Perkampus, H.-H., 292 Perkins, M. J., 248 Pernei, D., 161 Perol, N., 343,458 Peron, A., 218 Perona, M. J., 125 Perone, S. P., 46, 94 Perora, R., 37 Perry, B. E., 145 Perry, R., 151 Perry, A. R., 159 Personov, R. I., 42 Persson, K. B., 5, 144 Perutz, R. N., 198, 199 Pesa, F., 218 Peshkin, A., 594 Pete, J.-P., 259, 274, 433, 456, 457

Peterman, K. E., 129 Peters, G. M., jun., 67 Peters, J. W., 435 Petersen, A. B., 47, 134 Petersen, H., 401, 494 Petersen, J. L., 197 Peterson, A. P., 163 Peterson, J. D., 189 Peterson, J. R., 148, 157, 163 Peterson, R. S., 133 Petkov, I., 608 Petrosyan, R. A., 552 Petrov. A. K.. 229 Petrov; A. L.,'135 Petrov, B. I., 203 Petrova, G. L., 177 Petruzzi, J., 59 Pettersson, E. L., 447, 553 Pettifer, R. E. W., 27, 45 Pettiiohn. R. R.. 125 Petti, K.; 603 . Petzke, F., 69 Peynircioglu, N. B., 248

Author Index Pfab, J., 130, 449, 489 Pfau, M., 252 Pfeiffer, G., 130 Pfohler, P., 218 Pfoertner, K.-H., 270, 417 Pfordte, K., 360 Phabre, M., 611 Phelps, F. M., 17 Phifer, J. E., 174 Philen, D. L., 39 Philipson, N. A,, 168 Phillips, C. R., 151 Phillips, D., 111, 112, 121,543, 547

Phillips, D. L., 42 Phillips G. 0 417 433 546 Phillips: L. F.',' 126: 134, 140, 157

Phillips, R. P., 221 Philpott, M. R., 545 Phipps, J. R., 273 Photos, E., 54, 117 Picard-Bersellini A., 163 Pickering, M. W',, 5 18 Pickford, M. E. L., 206 Pichon, R., 303,455 Pierce, E. L., 13 Pierce, J. B., 240, 500 Pierre, A., 191 Piers, E., 504 Piestrup, M. A., 45 Pietra, S., 473 Pike, C. T., 147 Pilar, J., 197, 541 Pilcher, C. B., 148 Pilipovich, D., 135 Pillai, V. N. R., 396, 461 Pillay, K. S., 486, 487 Pilling, M. J., 124, 168 Pimenov, Y. D 229 Pimentel, G. C:: 126 Pine, A. S., 18 Pinnick, R. G., 150 Pinnington, E. H., 144 Pinto Coelho, F., 170 Pionteck, S., 133 Piper, G. F.. 28 Piper, J. A., 8, 139 Piper, L. G., 153 Pipes, J. G., 20 Piret, W., 547 Pirkle, R. J., jun., 135 Pisanias, M. N., 66 Pistara, S., 455 Piszkiewicz. L. W.. 125 Pitts, J. N.,*jun., 40, 150, 156, 158, 159,435

Pitts, R. B., 202 Pivovarov, A. P., 184 Pizzini, S., 579 Plaistowe, J., 155 Plankey, F. W., 34 Plantenkamp, R., 598 Platt, E., 231 Pleasance, L. D., 9 Plekhanov, V. G., 231 Plinke, G., 346 Plotkin, J. S., 228, 502 Plotnikov, V. G., 108 Plummer, B. F., 25 Plus, R., 598 Plyusnin, V. F., 178 Podo, F., 595 Podolske, J., 46

Podzorova, E. A., 228 Poe, R. T., 153 Poehler, T. O., 9 Poffenberger, C. A., 21 I Pokoneshchikova, N. K., 497 Polansky, O., 608 Polanyi, J. C., 133 Poliakoff, M., 104, 198, 494 Polichnowski, S. W., 202 Poljkarpov, S. S., 135 Poling, B., 510 Pollack, E., 153 Pollack, J. B., 150 Pollak, A., 317, 393 Polles, J., 597, 598 Polles. M.. 601 Pollock, E;, 608 Pollock, W., 149 Poluektov, N. S., 193, 194 Poluektov, V. A., 125 Pomerantz. M.. 333. 345. 396 Pomeshchenko; A. A., 152 Pommier, J., 153 Poncet, H., 30 Ponomarev, Yu. N., 152 Ponpon, J. P., 586 Pool, C. R., 230 Poole, A. D., 571 Poole, J. A., 36 Poole, P. R., 126 Popescu, D., 142 Popescu, I., 142 Popov, B. M., 131 Popov, V. G., 191 Popovich, M. P., 131 Poppe, W., 453 Port, H., 104 Portabella, I., 273, 481 Portella, C., 259, 433 Porter, G., 50, 52, 416, 571, 583, 596, 605, 606

Porter, G. B., 51, 170 Porter, N. A., 503 Post, H., 103 Post, K. J., 144 Postlethwaite, D., 227 Potier, P., 405, 461 Potter, C. J., 269, 481 Potter, W. E., 152 Pottier, R., 435 Pospisil, J., 548 Postnikov, L. M., 552 Poulsen, L. L., 161 Poulsen, O., 139, 144 Poupko, R., 454 Pouyet, B., 100 Povey, D. C., 527 Povey, M., 15 Powell, R. C., 548 Powell, F. X., 128 Powell, H. T., 8 Powell, R. A., 45 Powell, R. C., 85 Power, T., 611 Pownall H. J., 94 Poznyai, A. L., 188, 189 Pozzi, V., 553 Prabhu, K. V., 95,295,467 Praefcke. K.. 387. 406. 527, 529, 530

'

.

Pragst, F., 401, 463 Prasad, S. S., 148 Pratesi, R., 9 Pratt, A. C., 79, 221

.

Author Index Pratt, G. W., 9 Pratt, L., 607 Pravednikov, A. N., 551 Premila, M. S., 536 Prescher, G., 292 Preston, P. N., 478, 490 Preussler, D., 7 Previtali, C. M., 94, 421 Prezant, D., 94, 250, 416 Pribush, R., 176 Price, D., 145, 230 Priestley, E. B., 48 Prilezhaeva, E. N., 440 Prince, R. C., 585, 604, 605 Prince, R. H., 226 Prins, W. L., 305 Prinzbach, H., 347, 348 Prischepov, A., 597 Pritchard, D. E., 19 Pritchard, G. O., 503 Pritt, A. T., jun., 135 Proch, D., 6 Prochorow, J., 92 Prock, A., 103 Proshenkova, A. D., 489 Pross, L., 19 Protasov, S., 605 Pruett, J. G., 57 Pryor, W. A., 155 Puaux, J., 195 Puddephatt, R. J., 218, 541 Pudov, V. S., 549 Pueschel, R. F., 150 Pukhal’skaya, G. V., 158 Pulfrey, D. L., 589 Pulfrey, R. E., 4 Pulles, M., 601 Pullukat, T. J., 542 Pumrner, H., 6 Puplett, E., 151 Puretzki, A. A., 146 Puri, S., 448 Puric, J., 11 Purvin, V., 608 Purzvchi. K. L.. 242 Pushkina, E., 594 Pusset, J., 68, 336, 312, 383 Putman de Lavareille, N., 546 Putney, S. D., 42 Quaisar, S., 194 Quast, H.. 508, 515 Quested, P. N., 23 Quimby, D. J.. 79, 224 Quina, F. H., 62, 63 Quinkert, G., 240,292, 509 Quistad, G. B., 276 Quon, H. H., 481 Rabache, P., 151 Rabek, J. F., 552 Rabinowitz, I., 118 Rabinsohn, Y., 273 Rahy, B. A., 34 Radchenko, S. S., 552 Radeva, E., 303 Radhakrishnan, S., 590 Radlein, D. St. A. G., 156 Radmer, R., 593 Radner, R., 601 Radtke, R., 30 Radziernski, L. J., 29 Rae, I. D., 269 Rafikov, S. R., 543

635 Rahn, K. A., 149 Remuson, R., 449 Rains, T. C., 4 Renger, C., 601 Raith, W., 142 Renner, G., 53 Raju, N. R. K., 448 Rennert, J., 423 Rakhmankulov, D. L., 360 Rentzepis. P. M., 50, 51, 585, 602, 603, 608 Rakouski, S. K., 158 Ram, N., 438 Repolles, J., 490 Ramachandran, B. R., 262, Report, R., 310 268, 572 Repoux, S., 153 Ramakrishnan, V., 380 Resler, E. L., 56 Ramamurthy, V., 497,606,608 Resnick, B. M., 280 Ramanujam, P. S., 139 Rest, A. J., 198, 199, 220 Ramaprasad, K. R., 145,230 Retey, J., 214 Ramdas, P. K., 396 Rettig, T. A., 280 Ranalder, U. B., 18 Rettschnick, R. P. H., 113 Ranby, B., 552 Reucroft, P. J., 548, 590 Rao, B., 20 Revelli, M. A., 22, 145 Rao, K. K., 585 Reverdy, G., 509 Rao, N. V. S., 299 Reynolds, I). W., 150 Rao, T. N., 438 Reynolds, G. A., 11 Rao, V. R., 380 Reynoldson, T., 206 Rao, Y.S., 277 Reznikova, I. I., 130 Raper, 0. F., 152 Rheault, F., 9 Rasmussen, J. K., 354, 355 Rhodes, C. K 8, 108, 148 Rasmussen, R., 150, 152 Ricard, D., 50’ Ratajczak, E., 364 Riccieri, P., 172 Ratcliffe, M., 361, 485 Rice, J. K., 6 Rath, H. P., 542 Rice, R. O., 29 Ratner, M. A., 299 Rice, S., 608 Rau, R., 220 Rice, S. A., 138 Raucher, D., 542 Richards, J. R., 151 Rausch, M. D., 197 Richards, J. T., 58, 92 Ravi, K. V., 586 Richards, K. E., 328 Rawlins, W. T., 161 Richards, R. L., 201 Ray, G., 598 Richardson, D. E., 174 Ray, J. K., 218 Richardson, D. M., 574 Rayex, J. C., 397 Richardson, F. S., 192 Rayl, G. J., 17 Richardson, J. H., 81 123 Rayner, D. M., 55 Richardson, M. C., 13 Raz, B., 39, 45 Richardson, W. H., 101 Razin, V. V., 507 Riche, C., 217, 405, 461 Razi Naqvi, K., 64 Richter, K., 220 Razumovskii, S. D., 158 Richter, P., 7, 8 Readhead, D. L., 154 Rickwood K. R., 9 Reay, N. K., 16 Ridley, B.’A., 135 Rebane, L. A., 158 Ried, W., 310 Rebbert, R. E., 138 Riesner, D., 223 Rebentrost, F., 132 Rigaudy, J., 442, 518 Reber, E. E., 151 Rigdon, L. P., 34 Rebours, B., 151 Riley, S. J., 137 Recca, A., 551 Rimbault, C. G., 445 Reed, G. H., 585 Rimerman, R., 306 Rees, C. W., 406,471,509,530 Rinck, R., 24, 146, 229 Reeves, E. M., 148 Ring, J., 16 Reeves, R. R., 162 Rio, G., 451 Regitz, M., 507, 511 Ritter, J. J., 23, 24 Rehak, V., 46 Rivas, C., 306, 381, 481 Rehder, D., 219 Riveros, J. M., 133 Reich, H. J., 338 Rivers, G. T., 257, 534 Reich; I. L.,.338 Rivett, D. E., 553 Reich, S., 11 Roantree, M. L., 269, 481 Reid, A. A., 344, 458 Robb, J. C., 541 Reid, E. S., 50, 52 Robbins, D. E., 149 Reid. S. T.. 269. 481. 489 Robbins, D. J., 58 441 ’ Reid; W. Roberge, P. C., 93 Redley, C. N., 18 Roberts, B. P., 230 Reisch, J., 491 Roberts, B. W., 306 Reiser, A., 63 Roberts, C. W.. 547, 551 Reisfeld, R., 196 Roberts, T. D., 486 Reiss, J. A., 322 Robertson, D. F., 150 Reist, P. C., 151 Robertson, E. E., 206 Rejto, M., 273 Robertson, J. A., 124 Reminnikov, S., 604 Robinson, D. A,, 331,362,370 Remnev, A. A., 579 Robinson, D. W., 163

w.,

636 Robinson, G. A., 45 Robinson, G. W., 50 Robinson, J. W., 41 Robinson, P. H., 586 Roble, R. G., 148, 152 Robrish, P., 28, 57, 162 Roche, A. E.. 16 Rockley, J., 610 Rockley, M. G., 51, 170, 593, 602 Rockwood, S. D., 24, 40, 145, 146.229 Rodehorst, R. M., 267,483 Rodgers, J. E., 317, 430 Rodgers, M. A. J., 98, 434 Rodriguez, H. J., 121 Rodriguez, O., 277 Rodriguez-Hahn, L., 506 Roebber, J. L., 225 Roker, K.-D., 101 Roesch, L., 220 Rogov, V. A., 168 Rohmer, M.-M., 207 Rokach, J., 349, 481 Rol, P. K., 156 Romanenko, V. I., 146 Romanov, V. F., 179 Romijn, J., 604 Rondelez, D., 112, 231, 362 Ronn, A. M., 146, 164 Roobeek, C. F., 218 Roof, A. A. M., 338 Root, J. W., 125, 132 Rosa, L., 585 Roscoe, J. M., 157 Rose. A. W.. 423 Rosebush, W. J.. 173 Rosen, D. I., 159 Rosen, H., 28, 57, 162 Rosen, J. M., 150 Rosenblum. M.. 212 Rosencwaig, A.; 22 Rosenfeld, T., 610 Rosenquist, N. R., 303, 360, 513 Rosenthal, A., 361 Rosenthal, I., 100, 227, 435, 454 Rosenwaks, S., 145 Rosner, S. D., 41 Ross, J., 146, 162 Ross, J. E., 148 Rosseinsky, D. R., 590 Rossetti, C., 163 Rossi, J. A., 6 Rossi, R., 222 Rossler, K., 19 ROSSO,P. D., 333, 335 Roth, H. D., 424, 509 Roth, H. J., 238, 303,304,426, 479,485 Roth, R., 307 Roth, R. W., 232 Roth, W., 593 Rothbart, G. B., 45 Rothman, L. D., 17 Rothman, W., 35 Rothwell, H. L., jun., 154 Rotkiewicz, K., 75 Rotolo. A.. 597 Roulet; R.’R., 19 Rousseau, A. D., 93, 265, 322 Rousseau, D. L., 56 Roussi, G., 254, 383

Author Index ROUX,J. A., 20 Rowe, M. D., 137 Rowland F. S 149 Rowley, A. G.;’491 Roy, C. R., 161 Royt, T. R., 15 Rozantsev, E. G., 447 Rozenthal, A., 485 Rtishchev, N. I., 383, 390 Rubaszewska, W., 77 Rubin, A., 604, 605 Rubin, L., 605 Rubinov, A. N., 15 Rubstova, T. A,, 553 Rudakova, 1. P., 215 Ruderman, M. A,, 152 Rudnyi, R. I., 191 Ruge, B., 326 Ruhl, F., 7 Rulliere, C., 11, 93 Rumfeldt, R., 190 Rumin R 68 218 340 Rundei, R.’ D.: 149: 153, 154 Rusch, D. W., 152, 157, 161 Rushworth, P. M., 12 Russell, G. J., 587 Russell, G. R., 8 Russell, P. G., 36 Russell, R. K., 348 Russwurm, G. M., 21 Ruzicka, Z., 20 RUZO,L. O., 385,433 Ryabov, E. A., 57, 146,229 Ryan, J. A., 149 Ryang, H.-S., 239, 352, 424 Rybinskaya, M. I., 200 Rychla Matisova, L., 102 Rychly, J., 101, 543 Rynbrandt, R. H., 454 SaB, J. M., 438 Saatzer, P. M., 68, 106 Sacchi, C. A., 57, 196 Sackett, P. B., 19, 133 Sacks, R. D., 15, 17, 45 Sadafule, D. S., 545 Sadowski, C. M., 5 8 , 157 Saeki, M., 138, 315 Safarik, I., 155 Safe, S., 385, 433 Sagun, E. I., 224 Sahar, E., 47 Sahay, B. K., 230 Saher, E., 10 Saibil, H., 611 Saiki, T., 438 Sailer, K.-H., 476 Saito, I., 101, 373, 388, 447, 450.478.487 Saito,’K., 327, 428, 467 Sakaguchi, U., 176 Sakai, K., 303, 485 Sakai, Y., 588 Sakata, T., 90 Sakata, Y., 84, 86, 527 Sako, M., 497 Sakota, N., 306 Sakuraba, S., 189 Sakuragi, H., 73,259,277,491 Sakuragi, M., 336, 553 Sakurai, A., 192 Sakurai. H., 38, 108, 239, 267, 316, 317, 318, 352, 383, 424, 431, 500

Sakurai, K., 144 Sakurai, T., 153, 428 Sala, K., 15 Salaun, J. Y., 339 Salazar, I. 297 Salem, L.,’357, 610 Salet, C., 180 Salisbury, K., 111, 321, 397 Salmon, G. A., 46 Saloman, E. B., 30 Salomon, D., 31 Salomon, R. G., 218, 337 Saltiel, J., 87, 93, 96, 322, 377, 378 Salzmann, H., 8 Sam, T. W., 439 Samat, A., 102 Sammes, P. G., 251, 253, 400, 446, 451, 452, 471 Samoilenko, A. A., 489 Samori, B., 225 Sample, J. O., 21 Samson, J. A. R., 28, 153 Samuel, C., 364 Samuel, C. J., 333 Samuel, E., 197 Sander, R. K., 48, 114, 137 Sanders, V., 604 Sandford, W., 594 Sandle, W. J., 139 Sandorfy, C., 149 Sandroni, S., 19 Sandstrom, D. R., 4 Sanhueza, E., 138, 139, 156, 453 Sanitra, R., 280, 324 Sano, M., 104 Santa Cruz, T. D., 101 Santhanam, M., 448 Santi, W., 41 Santus. R., 76, 101, 452 Sanz, F., 209 Sapir, M., 65 Sapunov, V. V., 223 Sari, S. O., 52 Sarzhevskaya, M., 594 Sasaki, Y.,219 Sasaki, T., 379 Sasamori, H., 449 Sasamori, T., 150 Sasse, W. H. F., 376, 410 Sasson, I., 441 Sasson, S., 292 Sastre, R., 361, 533 Satake, T., 361 Satani, S., 527 Satatin, J. Y.. 68 Sato, C., 449. Sato, M., 255,403 Sato, S., 141 Sato. T.. 171. 237. 278. 428. 453,467, 551 ’ ‘ Sato, Y ., 426, 479, 494 Satra, S. K., 359, 390 Satsburg, H. M., 546 Satyukov, B. N., 549 Sauer, K., 225, 226, 600, 601, 602 Sauers, R. R., 265, 300 Saunders, D. S., 94 Saunders, R. D., 30 3aunders, W. H., jun., 249 $,us, A., 542 Sautereau, H., 195 *

637

Author Index SauvC, J.-P., 299, 492 Savage, H., 152 Savin, F. A., 77 Savina, M. V., 177 Savy, M., 576 Sawa, Y., 536 Sawada, M., 442, 533 Sawada, T., 104 Sawai, M., 500 Sawai, T., 220 Sawyer, D. T., 585 Saxena, N. K., 328 Saxon, R. P., 154 Sayer, P., 43 Sayrac, T., 307 Scaddan, R. J., 16 Scaiano, J. C., 94, 98, 99, 120, 229,231, 244,421, 543 Scala, A. A., 31 Scamporrino, E., 55 1 Scandola, F., 173, 187, 222 Scandola, M. A., 187 Schaack, G., 22 Schaafsma, T. J., 224 Schaag, E. W., 21 Schaap, A. P., 434 Schack, C. J., 233 Schadow, E., 19 Schaefer, F. P., 10 Schaefer R., 30 Schafer-kidder, M., 446 Schaeffer, R. C., 152 Schafer, F. P., 145 Schaffer A. M., 83 Schaffnir, K., 31,97,237,238, 281,415 Schaffsma, T., 598 Schagen, P., 32 Schanda, J., 231 Scharf, G., 283 Scharf, H. D., 237, 370 Schawlow, A. L., 38 Schechter, H., 486 Scheer, H., 596, 597, 598 Scheffer, J. R., 308 Schelly, Z. A., 11 Schenck, G. O., 387 Scherer, K. V., jun., 262 Scherz, A,, 224 Schexnayder, M. A., 283,284 Schiff. H. I.,~58, 127, -149,. 157,.. 158. Schiff, R., 159 Schildbach, K., 13 Schilling, J. S., 19 Schimitschek, E. J., 11 Schindler, J. F., 221 Schipper, P. E., 406 Schlag, E. W., 18, 57, 111, 113 Schlagheck, W., 142 Schlessinger, F., 598 Schleyer, P. von R., 326 Schlott, R., 19 Schmelling, S. G., 144 Schmeltekopf, A. L., 58 149, 9

157

Schmid, H., 257,341,354,463, 464 Schmidkte, H. H., 176 Schmidt, A. J., 12 Schmidt, E. M., 5 Schmidt, G. M. J., 273 Schmidt, K. H., 29,47 Schmidt, M., 45, 105, 150

Schmidt, P., 602 Schmidt, R. V., 16 Schmiedl, R., 57, 162 Schmiegel, W. W., 407 Schmitz, H., 507 Schmocker, U., 157 Schmuff, N. R., 253,480 Schnabel, W., 543, 545 Schnatterly, S. E., 23 Schneck, L. J., 148 Schneider, M., 504 Schneider, R. E., 150 Schneider, S., 36 Schoch, J. P., 412, 473 Schollnhammer, G., 485 Schoemaker, W. H., 224 Schonberg, A., 387 Schonholzer, I?., 354, 464 Scholler, D., 259, 433 Schoof, S., 35 Schooley, D. A., 276 Schrader, G. L., 44 Schrauth, T., 238, 426 Schrauzer, G. N., 180, 215 Schreiber B., 576 Schreiber: U., 37 Schroeder, B., 230,492 Schroder, G., 346, 459 Schroeder, M. A., 196, 207, 430 Schrubovich, V. A., 550 Schuchmann, H.-P., 454 Schuetz, R. D., 433 Schuler, P., 369 Schulman, S. G., 80 Schulte-Frohlinde. D., 66.292. 322, 524 Schultz, A. G., 232, 279, 303, 399,499 Schumacher, K.. D., 38 Schumaker, C. D., 223 Schumann. H.. 220 Schumann; H.'J., 69 Schumann, W. C., 345 Schurath, U., 39, 94, 158 Schurter, J. J., 395 Schuster, D. I., 69, 280, 295, 299,414, 416 Schuster, G., 85, 237 Schuster, G. B., 101 Schutt, J. B., 30 Schutten, E., 117 Schutyser, J. A., 479 Schwab, A. J., 7 Schwartz, S. E., 55 Schwarz, D., 304, 479, 485 Schwarz, F. P., 39, 40 Schweizer, W. B., 246,287,420 Schwendiman, D. P., 191 Schwoerer, M., 104 Scorer, R. S., 149 Scotney, A., 550 Scott, G., 552, 553 Scott, G. W., 49 Scott, P. M., 83 Scott, R. A., 122 Scott, R. L., 18 Scriven. E. F. V., 517 Scullv. F.. 315 Seale, R., 408 Sealy, R. C., 422 Searles. S. K., 7, 8, 135, 153 Sears, G. N., 226 Seela, F., 521 ,

_

,

Seely, G., 596 Seely, G. R., 452 Seery, V. L., 452 Segal, G., 338 Segal, J. A., 200 Seger, G., 66, 391 Seiber, R. P., 545 Seilrneier, A,, 50 Seinfeld, J. H., 150 Seka, W., 27 Sekiguchi, S., 267, 510, 520 Sekine, Y., 493 Sekita, R., 267 Sele, G., 157 Selinger, K., 54 Sellan, J., 190 Selle, J. M., 32 Sellin, 1. A., 133 Selling, H. A., 492 Sellmann, D., 220 Selvarajan, N., 380 Selwyn, G., 163 Selzer, P. M., 23 Selzle, H. L., 18 Semeluk, G. P., 56 Semenov, Y. S., 150 Semichshen, V. A., 147 Senda, S., 291 Senum, G. I., 55 Sepucha, R. C . , 47 Serdyak, V. V., 587 Serebryakov, E. P., 266, 276, 360 Serkiz, R., 451 Serlin, R., 584 Serov, A. P., 176, 194 Serra-Errante, G., 446, 471 Serve, .M. P., 474 Servedio, F. M., 503 Servera, F., 273, 481, 490 Sethuram, B., 448 Setkina, V. N., 220 Setser. D. W.. 6. 153 Sevast'yanov,*V: I., 225, 576 Seya, M., 15 Seybold, P. G., 42, 474 Shablva. A. V.. 193 Shaef&,'H. F.,'tert., 123 Shafer, Y. G., 150 Shaffer, G. W., 242 Shafferman, A., 47, 180 Shagisultanova, G. A., 191 Shah, P., 587 Shahin, I. S., 14 Shai, C. M., 30 Shakhverdov, T. A., 45, 192 Shani, A., 398 Shank,C.V., 10,12,16,50,51, 52, 78 Shannon, P., 87, 377 Shannon, P. T., 93, 322 Shapiro, A. B., 447 Shapiro, M., 214 Shapiro, S. L., 49,52,605,606, 607 Sharafi-Ozeri, S., 66, 338, 391 Sharipov, G. L., 195 Sharovol'skaya, L. N., 550 Sharp, D. W. A., 220, 221 Sharp, J. T., 343, 344, 458 Sharp, K. G., 145, 501 Sharp, L. E., 9 Sharp, W. E., 152. 157, 161 Sharpe, L. A., 338

Author Index

638 Sharpless, R. L., 58, 116, 131, 157, 163 Sharyi, V. M., 231 Shatokhina, E. I., 158 Shatwell, R. A., 177, 223 Shaver, L. A., 54, 60, 61 Shaw, B., 596 Shaw. J. F.. 44 Shaw; M. J:, 78, 104 Shaw, R. W., 93 Shay, J. L., 587, 588, 589 Shea, K. J., 101 Sheinson, R. S., 108, 122, 438 Shekhtman. R. I .. 440 Shekk, Yu.~B., 322 Sheldrick, G. E., 551 Shelley, E. G., 27 Shelly, J., 210 Shelnutt, J. A., 225 Shemansky, D. E., 161 Shen, C.-H., 151 Shepherd, A., 597 Shepherd, R. A., 330 SheDherd, T. M.. 192 She;, A., 2 6 . Sheridan, J. B., 342 Sherstyuk, V. P., 179 Shetlar. M. D.. 291. 484 Shevandin, V. S., 194 Shewchun, J., 21, 151, 589 Shiba, T., 466 Shibata, T., 226, 327 Shibuya, K., 106 Shichi, H., 608 Shifrina, R., 611 Shigemitsu, Y., 308, 314 Shillcock, M., 597 Shimabukuro, F. I., 151 Shimazaki, T., 152 Shimazu, H., 447 Shimiza, F., 147 Shimizu, M., 543 Shimizu, S., 14 Shimizu, Y., 435 Shimoda, K., 147 Shimou, T., 482 Shindo, Y., 42, 96 Shinohara, A., 399, 461 Shiokawa, J., 171 Shipman, L. L.,584, 598, 599 Shipp, W. S., 16 Shira, C., 599 Shirahama. H., 267 Shirai, M.,‘ 3 10 Shirane, K., 610 Shirasaki, H., 240, 241 Shirota, Y., 543, 548 Shivanandan, K., 16 Shizuka, H., 65, 81, 408, 411, 520 Shkirman, S. F., 224 Shlyapintokh, V. Ya., 447, 551. 553 Shobin, V., 597 Shold, D. M., 87, 377, 378 Sholeen, C. M., 140 Shon, R. S., 407 Shono, T., 333, 335, 349 Shoppee, C. W., 195,294,295, 403, 536 Shortridge, R., 125, 128, 449 Shortridge, R. G., 21, 131, 156, 157 Shreeve, J. M.,129, 232

Shub, D. M., 579 Shubnyakov, V. F., 553 Shugar, D., 289 Shuker, R., 45 Shulalov. V., 603 Shul’ga, A. M., 224 Shul’ga, S. M., 288 Shul’ga, Y. M., 184 Shur, J., 552 Shurcliff, W. A., 571 Shuvalov, A., 599 Shvets, V. A., 172 Siara, I. N., 140 Sibbett, W., 29 Sichel, K., 223 Sidel’nikov, V. N., 229 Sidhu K. S., 448 Sidis, 153 Sie, B. K. T., 159 Sieber, W., 354, 464 Siebert, M., 605 Siebrand, W., 65, 322 Siegfried, R., 521 Siegman, A. E., 6 Siegoczynski, R. M., 548 Siegrist, M. R., 59 Siffert, P., 586 Signorelli, A. J., 145, 230 Silberg, I. A., 477 Sil’dos, I. R., 158 Silk, J., 148 Silva, D. M., 152 Silver, B., 600 Silver, D. M., 338 Silver, J. A., 57, 155 Silvers, A. E., 553 Silvers, S. J., 128 Silverstein, B., 606 Silvfast. W. T., 10 Sim, S.-K., 507 Simamura, O., 575 Sime, M. E., 112 Simic, M. G., 190 Simmons, E. L., 25 Simmons, J. P., 45, 137 Simon, M., 107 Simonaitis, R., 127, 139, 157, 159, 162, 453 Simonis, J., 105 Simonneaux, G., 199 Simons. J. P.. 39. 380 Simpson, G. A., ‘101 Simpson, R., 469 Sindler-Kulyk, M., 345, 396 Sinelkikova, G. E., 149 Slneshchekov, V., 597 Singer, L. A., 53, 69 Singh, H., 93 Singh, H. B., 149 Singh, J., 56 Singh, P., 436 Singh, R., 589 Singh, S. N., 529 Singh, S. P., 532 Singleton, D. L., 156 Sinitsyna, Z. A,, 532 Sinton, W. M., 148 Sipp, B., 26, 27 Sisson, W. B., 16, 150 Sivonen. M.. 447 Skalski,’B., 289, 484 Sket, B., 317, 393 Skidmore, D. R., 158 Skipka, G., 349, 534

k.,

Skokan, E. V., 131 Skolnik, E. G., 132 Skorianetz, W., 440 Skotnicki, P. A., 25, 164 Skouboe, N. J., 144 Skryabm, N. G., 150 Skudra, A. Ya., 140 Skulachev. V. P.. 583 Skurat, V., E., 553 Slagle, I. R., 156 Slanger, T. G., 58, 116, 131, 134, 156, 157, 161, 163 Slifkin, M. A., 18, 89, 92, 392 Slinev. D. H.. 32 Slivnik, J., 233 Sloan, J. P., 125 Slobodetskaya, E. M., 550,551 Slovetskii, V. I., 490 Slusser, P., 101 Smalc, A., 233 Small, R. D., jun., 101, 435 Smalley, R. E., 40, 137, 162 Small-Warren, N. E., 153 Smardzewski, R. R., 232 Smerchanski, R. G., 389 Smets, G., 549 Smilanski, I., 144 Smillie, R. D., 504 Smirnov, B. M., 150 Smirnov, E. V., 383 Smirnov, V. A., 98 Smith, A. B., tert., 242, 265 Smith, A. V., 142 Smith, A. M., 20 Smith, B. V., 248 Smith, B. W., 34 Smith, C. S., 132, 527 Smith, F. B., 151 Smith, F. X., 404, 536 Smith, G., 297 Smith, G. H., 553 Smith, G. P.. 57 Smith, G. W., 26 Smith, I. W. M., 133, 135, 159 Smjth, L., 381 Smith, L. L., 435 Smith, M. J., 164 Smith, P., 542 Smith, P. L., 151 Smith, P. P., 151 Smith, P. W., 10 Smith, R., 151 Smith, R. A., 124 Smith, R. G., 25 Smith, R. L., 256, 393, 526 Smith, S. D., 6 Smith. W.. 606 Smith; W.‘€i., 22, 106, 160 Smith, Vv. L., 22 dFSmolanc, J.. 357. 464 Smoreonskaia. G : L.. 177 Smv.P. R.. 15’ Snively, B.‘ B., 196 Sneddon, L. G., 228, 502 Snellgrove, T. R., 23 Snelling, D. R., 105 Snider, D. E., 151 Snow, R. A., 329 Snyder, F. F., 268 Snyder, J. J., 71, 90, 237, 239, 415, 426 Snyder, L. E., 148 Snyder, R. W., 489 Soag, P.-S., 80

Author Index Sobieralska, M., 231 Sodeau, J. R., 199 Soep, B., 47, 114 Sogonova, S. V., 179 Soini, E., 29 Sojka, S. A., 239 Sokolov, L. F., 542 Sokolov, S. V., 542 Sokolov, V. D., 150 Solarz, R. W., 196 Solie, T. N., 33 Solin, S. A., 43 Soller, B., 72 Solmon, G. A., 96 Solomentseva, L. I., 191 Solomon, J. E., 152 Solomon, Z., 596 Solov’ev, E. A., 177 Solov’ev, K. N., 224, 597 Soma, M., 224 Somanathan, R., 471 Somehawa, K., 482 Somersall, A. C., 238, 545 Sommer, L. H., 230, 520 Song, P.-S., 92, 593, 607 Sonoda, N., 267, 500 Sonoda, T., 317, 393 Sood, H. R., 344,458 Sood, S. K., 490 Sorokin, A. R., 153 Sorokin, P. P., 13 Sorokin, Y. A., 203 Sorrain, O., 608 Sostero, S., 206, 218 Soucy, M., 504 Soukup, R. J., 587 Soulignac, J. C., 45 Soumillion, J. P., 387 Soutar, I., 35, 548 Sowersby, G., 23 Spaulding, L. D., 585, 602 Specht, D. P., 387, 532 Specht, L. T., 37, 545 Speiser, S., 10, 31 Spencer, D. J., 21, 151 Spencer, G. L., 102 Spencer, J. E., 155 Spicer, C. W., 150 Spicer, L. D., 151 Spielman, J. R., 229 Spinner, M. L., 93 Spiro, T. G., 43 Spitzer, D., 36 Spitzy, H., 78 Sprague, E. D., 524 Spratley, R. D., 232 Springer, G. S., 151 Springer, L. W., 8 Sprintschnik, G., 167, 573 Sprintschnik, H. W., 167, 573 Squire, P. G., 90 Sridharan, U. C., 145 Srimannarayana, G., 299 Srinivasachar, K., 87, 371, 378, 379 Srinivasan, M., 26 Srinivasan, R., 335, 370 Srivastava, K. C., 365, 469 Srivastava, T. S., 223, 225 SrniC,. T., 218, 453, 488 Stadzinskii, 0. P., 390 Staerk, H., 34 Staiger, L. E., 276 Staley, R. H., 144

639 Stallard, R. F., 150 Stamos, I. K., 220 Standage, M. C., 139 Standley, R. D., 16 Stanney, G., 83 Starr, W. L., 39 Starratt, A. N., 488 Starukhin, A. S., 224 Stary, F. E., 435 Staudacher, L., 220 Staudner, E., 101 Stauff, J., 152 Stebbings, R. F., 153. 154 Stedman, D. H., 25, 149, 150 Steedman, W., 548 Steel, C., 94, 237, 413 Steele, R. E., 145 Steen, H. B., 36, 226 Steenken, S., 524 Steer, R. P., 111, 131, 132, 527 Stefaniuk, E., 343, 458 Steichen, J. C., 41 Steidl. H., 41 Stein,.A.,-36 Stein, G.,47,63, 179, 180, 193, 368.446. 571. 575 Steinberg, I.-Zy,’ 54, 60 Steinbruegge, K. B.. 12 Steiner, B., 20 Steinert, W., 46, 159 Steinfatt, M., 434 Steinfeld, J. I., 146, 161 Steinhorn, G., 574 Steinmetzer, H.-C., 101 Stella, G., 22, 106, 160 Stenberg, V. I., 532 Stephens, E. R., 149, 150 Stephenson, D. G., 57 Stephenson, L. M., 81, 123 Steppel, R. N., 11, 474 Stermitz, F. R., 278 Sternfels, R. J., 69, 244 Stevens, A. L. N., 4 Stevens, B., 101, 139, 435 Stevens, R. D., 542 Stevens, W., 157 Stevenson, J. M., 39, 65 Stevenson, K. L., 176 Stevenson, R., 240, 241 Stevenson, W. H., 37 Stewart, A. I., 152 Stewart, K., 20 Stewart, R. P., 211 Stewart, S., 611 Stewart, T. B., 40 Steyer, B., 145 Stief, L. J., 38, 131, 155 Stienberg, I., 598 Still, I. W. J., 495 Stille, J. K., 280 Stiller, K., 477 Stillman, J. S., 226 Stillman, M. J., 226 Stingelin-Schmid, R. S., 243, 288 Stirn, R. J., 590 Stobie, R. W., 20 Stock, M., 191 Stockburger, L., tert., 151 Stockburger, M., 113 Stoecken ius, W., 583 Stohrer, G., 101 Stohrer, W.-D., 240 Stoicheff, B. P., 142

Stokr, J., 20 Stolarski, R. S., 148, 149 Stolbova, 0. V., 130 Stolen, R. H., 47 Stolovitskili, Yu., 594 Stolzberg, R. J., 179 Ston, M., 85 Stone, E. J., 15 Stone, F. G. A., 220, 221 Stonehill, H. I., 389 Storey, R. N., 13 Storr, R. C., 471, 509 Stothers, J. B., 294 Straatmann, R., 25 Strambini, G. B., 98 Strandberg, M., 19 Stranks, D. R., 19 Strathdee, R. S., 343, 458 Strausz, 0. P., 107, 155, 370 Streets, W. J., 153 Streit, G. E., 40, 58, 157, 159 Streith, J., 191, 343, 458, 471 Strekowski, L., 259 Strelfsova, A. A., 489 Strickland, D. J., 152 Strobel, D. F., 148, 152 Strohacker, H., 504 Strohmeier, W., 196, 218, 430 Strohwald, H., 8 Stromberg, E., 30 Strong, R. L., 95, 467 Strouse, C. E., 584, 596 Struchkov, Y. T., 21 1 Struve, W. S., 50, 51 Strydom, P. J., 397 Stryer, L., 609, 611 Stubberfield, J. F., 26 Stuber, F. A., 67 Stuckey, J. E., 553 Stucky, G. D., 176, 193 Stuhl, F., 126 Stults, B. R., 177 Su, H. Y., 155 Su, S. R., 211 Suau, R., 42 Subrahmanyam, G., 370 Subramanian, J., 223 Subudhi, P. C., 83, 407 Suchard, S. N., 7, 161 Suchlov, A. F., 8 Suemitsu, M., 153 Suess, G., 219, 204 Suetaka, W., 20 Sugaya, T., 501 Sugimori, A., 202 Sugimota, H., 78 Sugioka, T., 375 Sugiyama, N., 274 Sugowdz, G., 410 Suguna, H., 536 Suivko, M., 597 Sukawa, H., 263 Sultana, Q., 47, 195, 202 Summers, A., 150 Suna, A., 546 Sundberg, R. J., 404, 518, 536 Sundholm, F., 546 Sundholm, G., 546 Sundler, F., 77 Sung, T. V., 241 Sunitani, M., 322 Sunyaev, R. A,, 148 Suppan, P., 238 Surek, T., 586

Author Index

640 Surkov. V. T.. 549 Surles, T., 17 Suschitzky, H., 517, 518, 519 Sutcliffe, L. H., 478 Sutherland, J. C., 22, 42 Sutherland, J. K., 439 S u m . N.. 169. 182. 575 suto;, P . ~ A . i45, , 501 Sutphin, H. D., 12, 52, 607 Sutton, D. G., 7, 161 Sutton, J., 52, 77 Suwa, S., 369 Suzuki, A., 254, 358, 589 Suzuki, M., 97, 194, 195, 292, 476,488 Suzuk-i, O., 271 Suzuki, S., 531 Suzuki, T., 84, 317, 383, 550, 610, 611 Svelto, O., 57, 196 Sveshnikova. E. B.. 176. 194 Swallow, A. J., 58,'81, 416 Swaminathan, S., 253 Swann, D. A., 398 Swanson, N., 159 Swanson, W. W., 179 Swart, D. J., 408 Swartz, J. C., 586 Swenton, J. S., 280, 289, 290, 324, 333, 335, 482, 483, 484 Swibalus, R. A., 148 Swigert, J. L., 473 Swingle, J. C., 7, 135 Swofford, R. L., 21, 108 Swords, M. D., 111 Syassen, K., 44 Svdenham. P. H.. 59 Sykes, B. D., 48 . Syldark, A., 292 Symons, M. C. R., 204,231 Synowiec, J., 606 Szabad. J.. 598 Szabo, A. G., 55, 608 Szalai, G., 475 Sze, N. D., 148, 149 Szewczyk, M., 103, 397 Szilagyi, S., 455 Szoke, A., 38, 45, 156 Szychlinslti, J., 23 1 '

Tabusa, F., 342,459 Tabushi, I., 347 Tachikawa, E., 138 Tachin, V. S., 192 Tada, K., 543 Tadasa, K., 141 Tahan, M., 552 Taieb. G.. 20 Tait, B. S;, 501 Tait, J. C., 132 Takabatake, E., 192 Takada, Y., 423, 582 Takagi, K., 249,301, 531 Takagi, M., 434,498 Takahashi, F., 576 Takahashi, K., 220, 536, 548, 588, 589, 590 Takahashi. M.. 179. 291. 54 1 Takahashi; T.,'240, '241, ,' 480 Takamatsu, K., 352 Takami, M., 373, 388, 447, 478,487 Takamiya, A., 593 Takamiya, K., 604

Takamuku, S., 108, 316, 317, 318 Takashima, H., 310, 423 Takasugi, H., 399, 462 Takaya, T., 387, 531 Takayama, K., 435 Takayama, M., 515 Takeda. K.. 544 Takeda; N.; 296 Takeda, T., 454 Takeda, Y., 191, 543 Takekawa, M., 104 Takemoto, K., 139, 179, 453, 54 1 Takemoto, T., 433 Takemura; J . , ~84 Takemura, M., 536 Takemura, T., 42, 93, 95, 96 Takeshita, H., 3 11 Takeuchi. N.. 48 Takeuchi; T.,' 177 Taki, M., 533 Takimoto, H. H., 21 Takizawa, K., 14 Takizawa, T., 510 Talbert, S. G., 572 Talekar, R. R., 284, 525 Talmi, Y., 32 Tal'roze, V. I., 133 Tam, A., 153 Tam, A. C., 142, 144 Tam, J. C. L., 239,240 Tamai, H., 551 Tamaki, T., 388, 537 Tamamura, T., 548 Tamm, H., 369 Tamura, K., 453 Tamura, Y., 264,342,459,460 Tanabe, H., 387 Tanaka, F., 64,428 Tanaka, H., 264 Tanaka, I., 81, 106, 115, 146, 41 1 Tanaka, K., 141,482 Tanaka, L., 81 Tanaka, M., 310 Tang, C. L., 12, 14 Tang, C. W., 53, 590 Tang, S. P., 145 Taninaka, T., 542 ranigaki, T., 541, 542 raniguchi, H., 317, 393 rarakanov, 0. G., 552 rarassoff, P. G., 192, 389 rasaka, S., 385, 433 Tashiro, M., 484 rasumi, M., 289, 480, 609 ratarczyk, T., 39, 57 ratemitsu, H., 527 ratischeff, I., 75 raube, H., 232 rausta, J. C., 319 ravss. E.. 300 raylor, D. R., 443 raylor, D. S., 518 raylor, E. C., 411, 525 raylor, G. N., 268 ravlor. K. G.. 473 -ailor; 0. C.,' 149 'aylor, T. G., 120 'azake, S., 548 'azawa, A., 187 'azuke, S., 191, 543, 548 'chir, P. O., 232

Teather, G. G., 26 Tedder, J. M., 125 Tedder, S. H., 104 Teitei, T., 376, 410 Tejwani, G. D. T., 162 Tell, B., 589 Telle, J. M., 14 Templer, H., 77 Ten Bosch, J. J., 36 Tennent, N. H., 211 Terada, Y., 263 Terasawa, H., 536 Termonia, M., 120, 543 Teschke, O., 12 Testa, A. C.. 77. 87. 426 Teuchner, K., 54 . Texier, F., 507 Tezuka, T., 263, 342 Tchen, H., 155 Tfibel, F., 46, 93 Thal. C.. 405. 461 Thayer, A. L:, 434 Thayer, C. A., 56 Thebault, J., 29, 153 Thebtaranonth, C., 309 Thiel, W., 436, 437 Thieneman, M., 154 Thiers. J.. 606 Thiery, J.; 80 Thiess, F.-J., 57 Thistlethwaite, P. J., 77 Thoe, R. S., 133 Thoen, J., 549 Thomas. B.. 93. 96. 322 Thomas; E.'J., 245' Thomas, G. E., 152 Thomas, J., 596 Thomas, J. J., 551 Thomas, J. K., 58, 79, 92, 582 Thomas, J. M., 104, 106 Thomas, L., 152 Thomas, M. J., 244 Thomas, M. T., 495 Thomas, P., 188 Thomas, R. L., 153 Thomas, T. F., 243 Thomas, T. M., 121 Thomaz, M. F., 54, 61 Thomson, A. J., 167 Thompson, J., 152 Thompson, K. H., 584, 604 Thompson, M., 518 Thompson, R. T., jun., 151 Thompson, T. L., 149 Thornber, J., 597, 599 Thrush, B. A., 41, 157, 161 Thulstrup, E. W., 65 Thurnauer, M., 603 Tibulski, K., 6 rice, J., 47 Tiede, D. M., 585 Tiefenau, H., 150 Tikhonov, G. P,, 177 Tilford, S. G., 161 Tillman, R. W., 328 Timberlake, J. W., 455 Timofeev, K., 605 Timofeev, N. T., 45 Timofeev, V. V., 131 Tinnemans, A. H. A., 66, 391 Tipping, A. E., 230, 486 Tjsdale, V., 90, 285 Tishchenko, M. A., 193 risone, G. C., 7, 8, 135

641

Author Index Title, A. M., 16 Tittel, F. K., 10, 39 Titus, J. A., 59 Tkachenko, Z. A., 171, 179 Toby, F. S., 108, 158 Toby, S., 108, 158 Toda, F., 301, 352 Toda, T., 349, 504 Todo, E., 301 Tokel-Takvoryan, N. E., 40 Tokousbalides, P., 193 Tokuda, M., 254, 358 Tokue, I., 40 Tokumaru, K., 73, 259, 277, 483,491 Tokunaga, F., 609 Tolbert. L. M.. 324 Tolkachev, V. A., 114 Tollin, G., 593 Toman, L., 197, 541 Tomashov, V. N., 131 Tomasi, C., 150 Tomeno. T.. 545 Tomioka, H., 303, 513 Tomita, Y., 153 Tomkiewicz, M., 582, 595 Tomlinson, W. J., 25 Tompkins, H. G., 20 Tondello, G., 45 Toney, C. G., 174 Tong, D. A., 49 Toon, 0. B., 150 Topouzkhanian, A., 161 Topp, M. R., 57, 58, 99, 416 Toptygin, D. Ya., 179,550,553 Torchinskii, I. A., 545 Torikai. A.. 550 Torr, D. G:, 152, 161 Torr, M. R., 152 Toru, M., 536 Toscano, V. G., 231 Toshima, N., 92, 315,429,485 Toubro. N. H.. 494. 530 Tourbet, H., 32 ' Townsend, D. E., 87, 377, 378 Townsend, L. W., 19, 156 Toyoda, M., 138 Trahanovsky, W. S., 200 Train, R. E., 151 Trainor, D. W., 46, 144, 159 Tramer, A., 54, 117, 162 Traverso. 0.. 206. 218 Traylor, T. G., 225 Treacy, E. B., 52 Tredwell, C. J., 50, 52, 605, 606. Treinin, A., 227 Tremont, S. J., 264 Tremper, A., 354, 357, 464 Treshchalov, A. B., 158 Trethewey, K. R., 101, 447 Treu, J. I., 226 Treushnikov, V. M., 515, 550 Treves, D., 10 Trjas, J. A., 11 Tnc, C., 113, 117 Trickes, G., 531 Triebel, W., 49 Trifiro, F., 438 Trimmer, R. W., 466, 467 Triplett, K., 203, 213 Tripp, G. R., 29 Trivedi, H. S., 255 Troe, J., 108, 127, 138, 162

Troin, Y., 449 Trotman-Dickenson, A. F., 138 Trout, G. J., 41 Troyanowsky, C., 46 Truscott, A., 609 Truscott, T., 594, 606 Truscott, T. G., 72 Tsai, A. I., 220 Tsai, K.-H., 231, 260 Tsai, M. D., 438 Tsaryuk, V. I., 192 Tscharnuter, W., 18 Tseng. S.-S.. 251 Tsepijov,. V: F., 554 Tshibangila, B., 120, 543 Tsikora, I. L., 137 Tsubomura, H., 80, 90, 98, 424, 448, 580, 582, 590 Tsuchiya, S., 140 Tsuchiya, T., 474, 528 Tsuda, M., 23, 531 Tsuge, O., 484 Tsuji, M., 333 Tsuji, Y., 435 Tsujimoto, K., 89, 369, 384, 385, 432,433 Tsunashima, S., 107, 141, 370 Tsunetsugu, J., 299 Tsunoda, T., 515 Tsuyuki, T., 240, 241 Tsvirko, M. P., 223, 224 Tuan, V. D., 104 Tuccio, S. A., 196 Tucker, A. W., 40, 151 Tugbaev, V. A., 114 Tukada, H., 327 Tully, C. R., 232 Tul'skii, S. V., 226 Turaeva, Z. N., 192 Turbini, L. J., 343, 458 Turchi, I. J., 437 Turco, R. P., 149 Turk, M. I., 14 Turkova, A., 597 Turnbull, J. H., 95, 398 Turner, C. E., 7, 135 Turner, D. W., 56 Turner, J. J., 12, 104, 198, 199, 494 Turner, R. F., 198 Turon, P., 148 Turro, N. J., 85, 90, 101, 122, 237, 239, 245, 300,415 Tustin, C., 606 Tuttle, M., 284 Tweddle, N. J., 501 Twieg, R. J., 257, 534 Twigg, M. V., 221 Tyer, L., 75 Tyrrell, H. M., 368, 369 Tyte, R. N., 43 Tyutyulkov, N., 608 Uchida, K., 130 Uchiyama, T., 206, 550 Uda, H., 267 Uebelhart, P., 354, 464 Ueda, J., 347 Ueda, K., 352 Ueno, A., 544 Uji, K., 611 Ulich, B. L., 148 Ullal, H., 590

Ullman, E. F., 73, 251, 277, 436 Ulrich, I L , 67 Umbrasas, B. N.. 269 Umemoto, T., 527 Umreiko, D. S.. 191 Umrikhina, A.. 594 Umstead, M. E., 156 Undzenas, A., 548 Unger, I., 56 Uno, K., 544 Untterback, N. G., 145 Uohashi, H., 590 Uriante, A. K., 93, 322 Urisu, T., 40 Uselnian, W. M., 40, 162 Uyama, Y., 139, 453 Uzikova, V. N., 360 Vacek, K., 598 Vahrenkamp, H., 220 Vaidyanathan, S., 380 Vaish, S. P., 418 Valades, L., 506 Valat, P., 46 Valenti, P. C., 300 Valeur, B., 545 Valkovich, P. B., 499 Vallancejones, A., 148 Vallouy, C., 580 Valot, H., 552 Valovoi, V. A., 489 Van Assett, N. P. F. B., 155 van Bergen, T. J., 365 Van Breugel, P., 610 Van Brunt, R. J., 131 Van Camp, W., 150 Van den Berg, J. A., 191 Vanden-Heuvel, W. J. A., 530 Van der Bent, S., 224 Vander Donckt, E., 65 Vanderhoff, J. A., 157 Vanderkooi, J. M., 521 van der Laan, J. E., 151 Vander!inden, D., 68 Vanderlinden, P., 335 Van der Merwe, S. W. J., 509 Van der Ploeg, J. P. M. 418 van der Waals, J. H., 103, 223, 224, 601 van der Werf, R., 54, 117 van der Werf, R. P., 34 Van der Wiel, M. J., 142 Van Dorp, W. G., 223, 224 Vanest, J. M., 395 Van Gorkom, H., 601 van Ham, J., 150 Van Houten, J.: 574 Van Ttallie, F. J., 139 van Koeveringe, J. A., 335 Van Leeuwen, P. W. N. M., 218 van Meurs, B., 54 Vanni, U., 9 Vannier, M., 7 van Raan, A. F. J., 142 Vanthujl, J., 610 van Wageningen, A., 293, 338 van Woorst, J. D. W., 113 Van Zyl, B., 154 Vapshinskaite, I., 548 Varani, G., 173 Varfolomeev, S. D., 189 Varghese, A. J., 290, 480

642 Varisova, E. G., 543 Varkony, T. H., 158, 454 Varma, C. A. G. O., 102 Varma, R., 145, 163, 230 Varma, S. P , 17 Vasil'eva, T. E., 189 Vasilevskii, D. L., 587 Vedejs, E., 330 Veenuliet, H., 94 Veierov, D., 412 Veillard, A., 207 Veje, E., 15 Velazco, J. E., 6 Vellturo, A. F., 306 Veloso, D. P., 273 Velthuys, B., 602 Vember, T. M., 407 Venable, W. H., 30 Venediktov, P., 604 Venikouas, G. E., 85, 548 Venkataramani, P. S., 253,328 Venkatesan, T. N. C., 38 Venkitachalam, T. V., 141,440 Venn, M. A,, 27 Verbeek, J., 486 Verbeyst, J., 138 Verborgt, J., 549 Vereshchinskii, I. V., 228 Vergamini, P. J., 215 Vergragt, P. J., 103 Vermeersch, G., 427 Vermeglio, A., 601 Vermeil, C., 39, 63, 106, 126 Vernazza, J. E., 148 Verolainen, Ya. F., 139 Verschoor, C., 601 Veselova, T. V., 130, 407 Veselovskii, V. I., 579 Vetchinkin, S. I., 154 Veysey, S. W., 132 Viaene, L., 188 Vial, C., 246, 287, 338 Vichutinskaya, E. V., 552 Vickery, B., 380 Vjckery, L., 226 Vickrey, T. M., 215 Vidaud, P. H., 101, 158 Vidaver, W., 37 Vidyarthi, S. K., 129, 504 Viehmann. W.. 28 Vigny, ~ . , ' 3 7 . Vigue, J., 57, 137 Vikis, A. C., 141 Vilarrasa, J., 524 Vilesov, F. I., 126, 138, 161 Villa. A,. 125 Villem, Y . Y., 123 Vincent, S. E., 20 Vinogradov, I. P., 126, 161 Vinson, J. W., 347 Visser, J., 602 Vitt, G., 509 Vittimberga, B. M., 421 Vittori, O., 150 Vitz, E., 196 Vliek, R. M. E., 23 Vo-Dihn Tuan, 34 Vogel, D., 103 Vogel, E., 446 Vogel, J., 92 Vogl, O., 551 Vogler, A., 180, 186, 221, 222 Vojgt, B., 270, 276 Voigt, E., 513

Author Index Volk, C., 144 Vol'kenshtein, M. V., 226 Volkotrub, M. N., 553 Volkov, A. G., 582 Volkova, L. S., 184 Vollhardt, K. P. C., 217 Vollmer, H.-P., 102 Voltmer, F. W., 586 yon Bergmann, H. M., 7 Vonder Haar, T. H., 150 von Engel, A,, 156, 158 Von Fragstein, C., 229 yon Rosenberg, C. W., jun., 46, 159 Von Sonntag, C., 292,454 von Thuna, P. C., 20 von Wartburg, B. R., 278 Vorburger, T. V., 4 Vorob'ev, M. G., 550, 551 Vorob'eva, E. P., 549 Voss, A. J. R., 331, 362 Vossough, A,, 14 Voznyak, V., 594 Vselolodov, N., 610 Vuik, C. P. J., 188 Vyas, H. M., 422 Vystrcil, A., 454 Wachtmeister, C. A,, 385,433 Waclawski, B. J., 4 Wada, Y., 548 Waddell, W., 608 Waddell, W. H., 122 Wade, C. W., 16, 18 Wade, W. R., 151 Wagener, H., 446 Wagestian, F., 176 Wagner, E. B., 153 Wagner, H. G., 156 Wagner, P. J., 90, 196, 244, 245, 251,285, 287,415 Wagner, R., 208 Wagner, S., 587, 588, 589 Waiss, A. C., 477 Wakabayashi, M., 312 Wakefield, G. F., 586 Walborsky, H. M., 504 Wald, F., 586 Walgren, U. I., 207 Waligora, B., 547 Walker, A. C., 9 Walker, G. A. H., 15 Walker, J., 144 Walker, J. C. G., 149, 150, 152 Walker, M. L., 204, 220 Walker, R. E., 6, 9 Walker, R. W., 155, 530 Wallace, J. D., 40 Wallace, S. C., 6 Wallace, T. W., 253, 400 Wallenstein, R., 10, 18 Walmsley, S. H., 406 Walrafen, G. E., 52 Walrant, P., 101, 452 Walsh, E. J., 396 Walsh, R., 124, 372 Walter, H., jun., 151 Walter, W., 492 Walters, R. T., 173 Walther, D., 170 Walton, J. C., 125 Wampler, F. B., 58 Wamser, C. C., 85,449 Wan, J. K. S., 421,422

Wang, C. C., 152, 159 Wang, C. P., 7 Wang, C. W., 142 Wang, Y., 176,294, 295, 536 Wang, Y. S., 403 Wangemann, R. T., 32 Wanner, J., 6 Ward, I. M., 546 Ward, J. F., 142 Ward, R. R., 366, 465 Warden, J., 599, 600, 602 Warden, J. T., 48 Ware, W. R., 35, 42, 86 Warneck, P., 157 Warner, J., 144 Warner, P. O., 25 Warren, R. G., 229 Warren, S., 321 Warrington, D. M., 139 Warshel, A., 610 Warwel, S., 200, 229 Wasacz, J. P., 256 Washburn, W., 250,416 Washida, N., 40, 161, 162 Wasson, J., 123 Wasson, J. R., 167 Watanabe, H., 500 Watanabe, K., 41 1, 439 Watanabe, S., 153 Watanabe. T.. 106, 579

Watsonj'B. D., 87, 377 Watson, D. G., 324 Watson, R. T., 159 Watson, S. L., jun., 543, 545 Watson, W. D., 148 Watson, W. M., 176, 193 Watson, W. P., 214 Watt, D. S., 259, 454, 534 Watts, R. J., 180, 189, 190,574 Wauchotx T. S.. 164 Wayne, R.P., 39, 58, 101,l 39. 153, 155, 158, 163 Weaver, J., 125, 128, 449 Weaver, L. A., 8, 9 Webb, C. E., 139 Webb. T. R.. 177 Webber, P. M.,9 Webber, S. E., 104 Weber, J., 57 Weber, J. H., 217 Weber, M. J., 145, 193 Weber, W. P., 499, 501 Weeke, F., 454 Weeks, L. H., 152 Weichsel, C., 406, 527, 530 Wejdner, V. R., 30 Wed, C. M., 19 Weill, G., 545 Weinberg. R. B.. 338 Weir, N.-A., 130, 551 Weisberg, C. V., 43 Weisberg, K. V., 53 Weisbuch, C., 6 Weiser, C., 15 Weiss, B., 507 Weiss, C., 598 Weiss, D. S., 251, 258, 344 Weiss, R., 210, 330, 331 Weiss, R. G., 231, 253, 532 Weisstuch, A., 77

643

Author Index Welber, B., 19 Welch, A. J., 220 Welge, K. H., 58, 126, 127, 157, 164 Wellons. S. L.. 41 Welling.' H.. 6' Wells, D., 376, 410 Welter, W., 507 Wendlandt, W. W., 25 Wendling, L. A., 368 Wendt. H. R.. 121 Wenski, G , 289, 484 Wentworth, W. E., 571 Werkhoven, C. J., 113 Werner, F., 219 Werner, H., 220 Werner, T. C., 72 Wernette, D. A., 123 Werthemann, D. P., 67, 321 Wessel, J. E., 51 Wcssell. J., 601 West, A. D., 4 West, M. A., 17, 34, 54 West, P. W., 153 West. R.. 276 West: W: P.. 154 Westenberg,'A. A., 156 Westlake, J. F., 553 Westlin, U. E., 385, 433 Wetmore S. I., jun., 357, 465 Wexler. A.. 289. 290.482.483 Weyler; W.', 517 ' Weysenfeld, C. M., 54 Wharton, L., 40, 137, 162 Wheeler, C. S., 139 Wheeler, D. M. S., 328 Whipple, M., 225 Whitby, K. T., 151 White, J. D., 441 White, J. M., 16, 132, 155, 526 White, K. O., 151 White, T.P., 190 White, W., 42 White, W. H., 150 Whitehead, J. C., 125, 156 Whitehead, R., 145, 230 Whitesides, T. H., 205, 210 Whittaker, J. K., 39 Whitten, D. G., 167, 168, 183, 221, 573,574 Whitten, G., 149 Whitten, R. C., 149, 152 Whittle, E., 122, 525 Whittle. P. J.. 509 Whyte,'D. A , 46 Wicke, B. G., 145 Wicks, 2. W., jun., 543 Wiczer, J. J., 32 Widing, K. G., 148 Widmer, U., 341 Wiebe, H. A., 157 Wieder, I., 10 Wieder, J., 47 Wiersma. D. A.. 34. 94 Wiesenfeld, J. R.,45, 46, 133, 144 Wiesner, K., 270 Wife, R. L., 250, 416 Wight, D. R., 58 Wightman, R. M., 18 Wijnen, M. H. J., 138 Wilbrandt, R., 43, 53 Wilcomb, B. E., 139 Wilcox, D. M., 105 *

Wilcox, E. J., 489 Wild, U. P.. 18, 34, 64, 104 Wilde, H., 411, 478 Wilde, R. E., 518 Wilder, B. J., 38 Wiles, D. M., 412, 547, 551 Wilhelmi, B., 49 Wilkerson, T. D., 29 Wilkinson, F., 99, 168, 206, 416 Wilkinson, G., 211 Willett, C. S., 8 Williams, D., 13, 598 Williams, D. H., 339 Williams, D. J., 548 Williams, F., 231 Williams, F. W., 122, 438 Williams, G. A., 17 Williams, J. F., 239 Williams, J. L. R., 88, 306, 381, 387, 532 Williams, J. O., 78, 104, 406 Williams, P., 54 Williams, R. H., 571 Williams, R. K., 19 Williams, R. O., 262 Williams, T., 61 1 Williams, W. J., 152 Wjllis, C., 24, 129, 504 Wilshire, C., 530 Wilson C. W., jun., 161 Wilson: D., 231 Wilson, D. A., 528 Wilson, D. F., 521 Wilson, D. G., 102 Wilson, G. S., 95 Wilson, J., 47 Wilson, K. R., 48, 137 Wilson, N. H., 514 Wilson, R. D., 230, 233 Wilson, R. M., 42, 317 Wilson, T., 101 Wilson, W. J., 151 Wilzbach, K. E., 368, 370 Winde, W., 103 Windhager, W., 92 Windsor, M. W., 51, 170, 593, 602 Wine, P. H., 53, 88, 144 Winefordner, J. D., 34, 38,41, 42 Winer, A. M., 159, 435 Winicur, D. H., 153, 154 Winkler, P., 150 Winograd, N., 576, 597 Winn, K., 606 Winn, K. R., 12 Winnik, M. A., 94 Winter, B., 281 Winter, N., 502 Winter, R. E. K., 337 Winter, W., 354 Winterle, J. S., 182 Winters, B. H., 11 Wintle, H. J., 549 Wire, R. L., 94 Wirz, J., 238, 498 Wissner, A., 306 Withbroe, G. L., 148 Witkop, B., 399,462 Witman, M. W., 217 Witt, H. T., 593, 600 Wittig, C., 47, 134, 163 Wittig, G., 349, 534

Wodarnyk, F. J., 19, 133, 135 Wofsy, S., 149, 151 Wojcicki, A., 204, 211 Woitczak. J.. 206. 321 Woibarst; M.. L.,'32 Wolczanski, P. T., 579 Wolf, A. P., 128, 155, 509 Wolf. F. J.. 530 Wolfj H. C., 103, 104 Wolf, H. R., 246, 278, 279, 285,287, 338, 340, 420 Wolf, M., 571 Wolf, M. A., 151 Wolf, M. W., 53, 69 Wolff, Ch., 601 Wolff, S., 242, 275 Wolfrum, J., 146 Wolfson, J. M., 192 Wolga, G. J., 133 Wolleben, J., 87 Wong, D., 228 Wong, D. K., 73 Wong, J. L., 103, 456 Wong, K.-P., 22 Wong, M., 79 Wong, M. M., 11 Wong, S. K., 422 Wood, B. E., 20 Wood, D. G. M., 551 Wood, J. H., 196 Wood, M. H., 310 Wood, 0. R., 10 Wood, P., 600 Woodall, J. M., 588 Woodruff, W. H., 43 Woods, J., 587 Woodward, D. R., 364 Woodward, P., 221 Woodworth, J. R., 153 Woolhouse, A. D., 471 Woolsey, N. F., 512 Worcester, D., 611 Wosten, W. J., 20 Wostradowski, R. A., 308 Wotherspoon. N., 56 Wozniak, W. T., 6 Wraight, C., 604 Wren, D. J., 145 Wright, A., 611 Wright, A. N., 545 Wright, G. J., 328 Wright, J. J., 142 Wright, R., 176 Wrighton, M. S., 36, 93, 196, 198, 199,200,202,203 204, 207,430, 579, 580, 5 8 i Wrobel, D., 596 Wrona, M., 289 Wronski. C. R.. 587 Wu, C. H., 152; 159 Wu, W. S., 87, 374 Wubbels, G. G., 312,421 Wuebbles, D. J., 149 Wuerzberg. E.. 193 Wulfman,-b. S., 510 Wunsch, L., 21, 111 Wyatt, P. A. H., 75 Wyatt, R., 14 Wydeven, T., 17 Wydrzynski, T., 602 Wyman, G. M., 310 Wynberg, H., 261, 38'1 Wynne, J. J., 13 Wypych, J., 552

644 Xuan, C. N., 159 Yabe, A., 264 Yablonovitch, E., 9 Yagi, M., 96 Yagi, S., 40, 162 Yakhot, V 83, 104, 153 Yakushyi, k,593 Yamada, F., 466 Yamada, S., 472 Yamada, T., 41, 533 Yamada, Y., 519, 544 Yamae, A., 349, 504 Yamagishi, A., 10 Yamaguchi, R., 348 Yamaguchi, M., 574 Yamaguchi, R., 314 Yamamoto, A., 201 Yamamoto, H., 450 Yamamoto, K., 95, 428, 467, 580 Yamamoto, M., 88, 189, 351, 375 Yamamoto, N., 590 Yamamoto, O., 485 Yamamoto, S., 68, 141, 308 Yamamoto, T., 603 Yamamoto, Y., 189, 210, 322, 510, 543, 550 Yamamura, K., 347 Yamamura, S., 263 Yamane, M., 447 Yamaoka, H., 545 Yamaoka, T., 83, 515 Yamasaki, K., 384 Yamase, T., 531, 542 Yamashita, M., 29, 48 Yamashita, S., 75 Yamashita, Y., 263 Yamato, M., 289, 480 Yamauchi, S., 92, 399 Yamazaki, H., 210 Yamazaki, I., 130 Yan, C., 606 Yananihara. K. H., 443 YangrC. Y:, 18 . Yang, E. S., 589 Yang, N. C., 64, 87, 294, 371, 377, 378, 379 Yang, S. F., 452 Yarchak, M. L., 249 Yardley, J. T., 37, 46, 56, 117, 176, 193, 545 Yaron, M., 101, 156, 158 Yarter, C. P., 42 Yarunin, V. S., 163 Yarwood, A. J., 67, 344 Yasa, Z. A., 12 Yasina, L. L., 549 Yasunaga, T., 545 Yates, D. A., 27, 586 Yates, P., 239, 240 Yavari, I., 468 Yeh, H. J. C., 212 Yeh, Y. C. M., 590 Yelvington, M. B., 10 Yen, H.-H. B., 510 Yen, W. M., 14, 23 Yeoman, M. L., 11

Author Index Yeung, C. K. K., 151 Yeung, E. S., 38, 43, 162 Yersin, H., 191 Yip, K. L., 226 Yip, R. W., 294 Yogev, A., 24, 60, 114, 146, 228,412 Yokawa, M., 542 Yokota, S., 589 Yokota, T., 155 Yokoyama, M., 90,548 Yoneda, F., 463 Yoneda, H., 176 Yoneda, S., 304 Yonemitsu, O., 404, 536 Yonetani, T., 223 Yonezawa, T., 384 Yoshida, M., 75,253,259,491, 532 Yoshida, Z., 304 Yoshihara, K., 322 Yoshiie, S., 428, 467 Yoshikawa, A., 588 Yoshikawa, K., 450, 451 Yoshikawa, M., 438 Yoshimura, A., 452 Yoshimura, K., 327 Yoshimura, M., 548 Yoshino, A., 384 Yoshino, I., 225 Yoshizawa, T., 609 Young. A. N.. 29 Young; A. T.,‘27 Young, D., 11 Young, J. F., 13 Young, J. J., 152 Young, J. P.. 153 Young, J. T.: 32. 396 Young, R. A., 161 Young, R. C., 168, 1883, 574 Young, R. P., 20 Yu, c.-A., 226 Yu, L., 226 Yu. N.. -T.. 225 Yu; S. L.,’232 Yu, W., 605, 606 Yudd, A., 608 Yuen, M. J., 144 Yung, Y. L., 149, 151 Yur’ev, M. S., 163 Yurkevich, A. M., 215 Yurugi, S., 485 Yuryshev, N. N., 131 Yusuff, K. K., 191 Yustl, v. I., 553 Yzambart, F., 163 Zabik, M. J., 385, 433 Zabransky, V. P., 123 Zadorozhnyi, B. A., 276 Zady, M. F., 103, 456 Zafente, L., 149 Zafiriou, 0. C., 93 Zagorski, Z. P., 45 Zagusta, G. A., 224 Zahniser, M. S., 158 Zaikov, G. E., 158 Zajnal, H., 204 Zaitsev, S. V., 189

Zalesskii, I. E., 224 Zalessky, V. Yu., 135 Zalewski, E. F., 26, 30 Zamazova, L., 605 Zamie, P., 59 Zander, M., 93, 548 Zander, R., 152 Zanderighi, L., 277, 398 Zandstra, P. J., 23 Zanker, V., 426 Zannoni, C., 204 Zaraga, F., 57, 196 Zare, R. N., 24, 34, 57, 114, 147 Zasada-Parzynska, A., 289, 48 1 Zdunkowski, W. G., 150 Zecher, D. C., 276 Zechner, J., 68 Zeegers, P. J. Th., 145 Zeeman, P. B., 29 Zegarski, B. R., 155 Zeiger, H. J., 579 Zelenskaya, L. G., 489 Zeller, K. P., 401,493,494,512 Zellner, R., 46, 159 Zemel, J. N., 32, 152 Zener, C., 571 Zen’kevich, E., 597 Zentoh, T., 93 Zepp, R. G., 415 Zerger, R. P., 193 Zetsch, C., 126 Zhabotinskii, A. M., 226 Zhdanov, G. S., 551 Zheltvai, I. I., 193 Zherebtsov, 1. P., 550 Zhitnev, Y. N., 131 Zhitneva, G. P., 158 Ziebarth, M., 230 Ziegler, G. R., 347 Zimek, Z., 45 Zimmerer, G., 39 Zimmerman, I. H., 132 Zimmerman, W. T., 257, 534 Zimmermann, D., 142 Zimmermann, H. E., 67, 101, 284,295, 321,439, 506 Zimmermann, I., 379 Zimmermann, J., 27 Zinato, E., 172 Zinina, E. M., 193 Zink, J. I., 184, 191 Zink, M. P., 246, 287,420 Zipin, H., 31 Zittel, P. F., 57, 117 Zitter, R. N., 146 Zlotina, 1. B., 211 Zlotskii, S. S., 360 Znobina, G. K., 211 Zolin, V. F., 192 Zolotoi, N. B., 553 Zolotovitskii, J. M., 579 Zumbulyadis, N., 602 Zupan, M., 317,393 Zwanenburg, B., 512 Zwarich, R., 104, 118 Zweegers, F. P. A., 102 Zweig, A., 87, 553

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